TOPIC 2: GENETICS - BIOLOGY NOTE FORM 4

TOPIC 2: GENETICS - BIOLOGY NOTE FORM 4

Genetics: Is a branch of biology which deals with the study of inheritance and variation. OR

Is the study of heredity and variations in organisms.

Heredity (inheritance)

Is the passing on of characteristics or traits from parents to offspring.

OR

Is the possession of characteristics similar to those of the parents.

Variations

Are the observable differences in organisms of the same species.

Common terms used in genetics

TRAIT

Is the characteristic shown by an organism

OR is a feature that can be passed on from parent to young.

Example of the characters

• • Tall or Short
  • Green or red
  • Black or white
  • Blood group A or blood group B
  • Smooth or wrinkled.

GENE

Is a part of chromosome that carries the genetic materials called DNA and RNA

Genes are responsible for transferring a hereditary trait from the parent to the offspring.

ALLELE

Is an alternative form of a gene controlling the same characteristic but producing a different effect.

Example

Agene that controls height of an organism may occur as T or t

An allele is also known as an allelomorph

GENOTYPE

Is the genetic makeup or constitution of an organism.

Example: RR, Rr, rr

PHENOTYPE

Is the outward or physical appearance of an organism.

Example of phenotypes

• Tall
• Short
• Brown
• Round

DOMINANT GENE

Is a characteristic or a gene that masks the expression of the other gene.

Example

In garden peas tallness (T) is dominant over shortness (t)

RECESSIVE GENE

Is a characteristic or gene that does not express itself when a dominant one is present.

It is masked characteristic or gene

Example

In garden pea shortness (t) is recessive to tallness (T).

HOMOZYGOUS

Is a state where the alleles in an individual are similar.

Example

TT for tallness

tt for shortness

NB: An individual with similar alleles in a corresponding locus on a pair of homologous chromosome is known as homozygote.

HETEROZYGOUS

Is a state where the alleles are dissimilar.

Example

Tt (T for tallness and t for shortness)

NB: An individual with similar alleles in a corresponding locus on a pair of homologous chromosome is known as heterozygote

PARENTAL GENERATION (P1)

Is the group of organisms that are used to make the first cross in breeding experiments

FIRST FILIAL GENERATION (F1)

Are the offspring produced after crossing the parental genotypes.

HYBRID

Is the offspring produced by crossing two individuals with contrasting characters.

SECOND FILIAL GENERATION (F2 )

Refers the offsprings produced after crossing the individuals of the F1 generation.

LOCUS

Is the exact position or location of a gene on a chromosome.

HAPLOID (N)

Is a state of having one set of unpaired chromosomes in the nucleus.

Haploid cells are produced by Meiosis in the gonads(ovaries and testes)e.g. sperms and ova(eggs)

Haploid state is denoted by the latter ‘n’

DIPLOID (2n)

Is a state of having two sets of homologous (similar) chromosomes in the nucleus.

Diploid cells are produced by Mitosis in the body e.g. Somatic cells.

Diploid state is denoted as “2n”

SELFING

Is the process of crossing offspring of the same pair of parents.

Example of selfing

The crossing of offspring of F1 generation to produce F2 generation

CHROMOSOMES

Are threads like structures found in the nucleus of the cell.

OFFSPRING (Progeny)

Are individuals produced after crossing the parents.

MONOHYDRID INHERITANCE

This is the passage of a height from the parents to the Offspring.

DIHYBRID INHERITANCE

This is the passage of two characteristics from the parents to the offspring.

MONOHYBRID CROSS

Is a cross between two individuals having one contrasting Characters e.g. A cross between Tall individual and Short individual.

DIHYBRID CROSS

Is a cross between two individuals having two contrasting characters

Example

The cross between Tall and White individual with short and black one.

BACK CROSS

Is a cross between F1 Offspring and any parent.

TEST CROSS

Is across between an individual of an unknown genotype and the homozygous excessive individual.

PURE LINE

Is a population that breeds true for the characteristics being investigated

Pure line is also called pure breeding

GENETIC MATERIALS

Are chemical substances present in the living cells that determine the genetic makeup of an individual

• Hereditary materials are also known as nucleic acids or hereditary materials
• Hereditary materials are named as nucleic acid because they were originally isolated from nucleus of an eukaryotic cell.

Function of genetic materials

• They are capable of carrying genetic information from one generation to another

TYPES OF GENETIC MATERIALS

There are types of genetic materials, namely:

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

CHEMICAL STRUCTURE OF DNA & RNA (NUCLEIC ACIDS)

• Both DNA and RNA are made up of building blocks called nucleotides

NUCLEOTIDE

Is a sub unit of nucleic acid

Components of nucleotide

Each nucleotide has three components, namely

1. Pentose sugar (five carbon sugar)
2. Nitrogenous base (iii)Phosphate group

A diagram of the Structure of nucleotide

DEOXYRIBONUCLEIC ACID (DNA)

Is a double stranded helical molecular chain of nucleic acid found within the nucleus of a cell

1. DNA is called the molecule of life because it is the one that determines the physical and behavioural characteristics of an organism.

2. The colour of your hairs, your eyes or skin, the shape of the ears or nose, your height, ability and inability to roll the tongue, hair texture are all determined by the DNA

STRUCTURE OF A DNA

1. DNA is double stranded helical which means the DNA consists of two strands which twist around each other in a spiral fashion.

2. DNA is made up of many nucleotides, forming a polynucleotide chain which runs in the opposite direction.

3. Each polynucleotide chain is joined to the other by pairs of bases called nitrogenous base

Structure of DNA showing spiral fashion

Components of DNA

DNA has three components, namely:

1. Deoxyribose sugar
2. Phosphate group (iii)Nitrogenous bases

DNA has four nitrogenous bases, namely

• Guanine (G)
• Cytosine (C)
• Adenine (A)
• Thymine (T)

NB: Guanine and adenine are called purines while cytosine and thymine are called pyrimidines Base pairing

Is the linking of bases across the two chains

• • Guanine pairs with cytosine
  • Adenine pairs with thymine
  • These bases are joined together by weak hydrogen bonds

Structure of DNA showing the complementary base pairing between antiparallel strands

ROLES OF DNA

1. It acts a template for the formation of RNA
2. It controls the transfer of characters from parents to offspring (iii)It determines the type of protein to be synthesized

ROLES OF DNA IN PROTEIN SYNTHESIS

DNA controls protein synthesis by; –

1. Assembling the amino acids to form a molecule.
2. Dictating the type of amino acid required to form Protein.

ADAPTATIONS OF DNA TO ITS FUNCTION

1. It has ability to replicate during mitosis and meiosis. This provides sufficient genetic materials that are to be passed on the next generation.
2. It can embryo changes and has genetic information on the characteristics of a species.

RIBONUCLEIC ACID (RNA)

Is a single stranded molecule made up of several nucleotides.

This type of nucleic acid is synthesized from DNA replication

Components of RNA

Structurally, RNA is composed of three components;

• Ribose sugar
• Phosphate group
• Nitrogenous base

The structure of RNA

RNA has four nitrogenous bases, namely

• • Guanine (G)
  • Cytosine (C)
  • Adenine (A)
  • Uracil (U)

NB: Guanine and Adenine are called purines while Cytosine and Uracil are called pyrimidines

• • Guanine pairs with Cytosine
  • Uracil pairs with Adenine
  • These bases are joined together by weak hydrogen bonds

Structure of RNA showing the complementary base pairing between antiparallel strands

TYPES OF RNA

There are three types of RNA, namely:

1. Messenger RNA (mRNA)
2. Ribosomal RNA (rRNA) (iii)Transfer RNA (tRNA)

Messenger RNA (mRNA)

Is the type of RNA which is synthesized from DNA in the nucleus of a cell during the process of transcription

It copies the information stored in the strand of DNA

Function of Messenger RNA (mRNA)

It carries genetic information about the type of protein to be synthesized

Ribosomal RNA (rRNA)

Is the type of RNA which found in the cytoplasm attracted to the ribosomes

Function of Ribosomal RNA (rRNA)

Used to attract mRNA and tRNA towards the ribosome for efficient protein synthesis

Transfer RNA (tRNA)

Is the type of RNA synthesized from DNA molecules in the nucleus.

tRNA is always found throughout the cytoplasm

Function of Transfer RNA (tRNA)

1. It carries activated amino acid towards the ribosomes so that protein can be formed.
2. It is responsible for assembling the amino acids into the polypeptide chain.

DIFFERENCES BETWEEN DNA AND RNA

RNA	DNA
(i) It has a single strand	It has a double strand (double helix)
(ii) The pentose sugar is ribose sugar	The pentose sugar is deoxyribose sugar
(iii)It is located in the nucleus and cytoplasm	It is located in the nucleus, mitochondria and chloroplasts
(iv)Has organic base cytosine, guanine, adenine and uracil	Has organic base cytosine, guanine, adenine and thymine
(v) Can not replicate	It can replicate
(vi)It is chemically unstable	It is chemically stable

INHERITANCE

PRINCIPLE OF INHERITANCE

Is the passage or flow of characteristics from the parents to the Offspring.

MENDELIAN INHERITANCE

These are the inheritance that obey Mendelian fashion laws of inheritance.

• Gregor John Mendel was a first person to investigate about inheritance hence he was named as “Father of Genetics”
• In 1822 – 1884 Mendel investigated how characteristics are transmitted from one generation to another (from the parents to the offspring) by using the pea plants (Pisum sativum)

Reasons for Mendel to choose the garden pea plant (Pisum sativum)

i. It matures very fast

ii. It produces many seeds and hence many off springs

iii. It is self-pollinating but can be cross-pollinated

iv. It has several physical properties

Characteristics that Mendel studied

i. Height of the stem – tall or dwarfs

ii. Texture of seed – smooth or wrinkled

iii. Flower colour – purple or white

iv. Pod colour – green or yellow

v. Pod shape – Inflated or constricted

vi. Position of flowers – axial or terminal

The figure below showing the characteristics studied by Mendel

The table below showing the Mendel’s results

Character	Parental Phenotype	F1	F2	F2 ratio
Texture of seed	Smooth X Wrinkled	All smooth	5474 Smooth: 1850 Wrinkled	3:1
Colour of Pods	Yellow X Green	All yellow	6022 yellow : 2001 Green	3:1
Position of flower	Axial X Terminal	All axial	651 axial: 207 terminal	3:1
Height of the stem	Tall X Short	All tall	787 tall: 277 short	3:1
Pod shape	Inflated constricted	Inflated	882 inflated: 299 constricted	3:1

Reasons for Mendel to be very successful in his experiments

He chose to study a single character at a time (monohybrid inheritance) and he later studied two characters at a time (dihybrid inheritance).

Each character he chose was expressed in two clearly contrasting forms without intermediates. (iii)He quantified his results. He counted and recorded the number of offspring bearing each trait.

Mendel conducted his experiments as follows:

i. The pollen grain from pure short plant was donated on the stigma of tall pea plant.

ii. The stigma was dosed by a plastic bag to prevent further pollination.

iii. After that fertilization took place and a number of seed were produced and sown in the soil.

From the above experiment Mendel observed that:

All offspring pea plants were tall and he called them first filial generation (F1 generation).

After that Mendel conducted self cross (selfing of the obtained F1 offspring) as follows:

1. The pollen grains from F1 Pea plant were dusted on the stigma of another F1 Pea plant.
2. Then, the stigma was closed by a plastic bag to prevent further pollination. (iii)Fertilization took place and a number of seeds were produced and sown in the soil.

From the above experiment Mendel observed that:

¾ of all plants were tall and 1/4 was short.

Assumptions made by Mendel from this experiment were: –

The tall character is dominant over short character as when combined only the tall characters were expressed and short character disappeared.

The characters of living organism are controlled by a pair of factors that segregate during gamete formation.

From the assumptions, he created law of inheritance called Mendel’s first law of inheritance or law of segregation.

MENDEL’S FIRST LAW OF INHERITANCE (LAW OF SEGREGATION)

The law states that,

“Characteristics of an organism are controlled by internal factors (genes) which occur in pairs and only one of the factors can be carried in a single gamete”.

Concepts in law of segregation

There are four main concepts in law of segregation:

Genes can exist in more than one form.

• The alternative forms of genes re called alleles E.g. The gene determining height can be

(T) for tallness or (t) for dwarfism

1. An organism inherits two alternative forms of a gene for a particular trait, one from each parent
2. During the production of gametes pair of alleles separate. Thus, each gamete has one allele for each trait.
3. When the two alleles in a pair are different one is dominant while the other is recessive. Thus, one trait is expressed while the other is masked. This condition is called complete dominance.

NB: When inheritance of one pair of characteristics is studied at a time it is called Monohybrid inheritance.

MONOHYBRID CROSSES AND INTERPRETATION OF THEIR RESULTS AND RATIOS

Monohybrid Inheritance

Is the inheritance of one pair of contrasting characteristics.

For example, height as tall or dwarf, seed texture as smooth or wrinkled

GENETIC SYMBOLS

Genes are genetic factors that transmit genetic information from the parents to the offspring

Genes are represented using letters of the alphabet.

It is customary to use a capital letter for a dominant gene and small letter for a recessive gene.

MONOHYBRID CROSS

Is a cross that involves one pair of contrasting traits.

For example, when you cross a pure-line, round – seeded plant with a pure line, wrinkle – seeded plant.

In Mendel’s experiments there were two experiments done, one resulting to F1 and the other to F2

WAYS OF SHOWING GENETIC DIAGRAMS/GENETIC CROSSES

There are three ways showing genetic diagrams or genetic crosses, namely:

1. Branching method
2. Punnet square method (iii)Algebraic method

NB: In all methods of showing genetic Crosses the alphabetical letters are used to represent the genes (factors)

BRANCHING METHOD

Example: A pure breeding tall pea plant was crossed with a pure breeding short pea plant. Show the results of F1 and F2. State the genotypes and phenotypes of F1and F2. Work out the phenotypic and genotypic ratios of the F2.

Meiosis Gametes

Fertilization F1

Results:

F1 Phenotype: All are tall pea plants.

F1 genotype: All are Tt or All are heterozygous tall

Selfing F1 generation to obtain F2 generation

F1 phenotypes: Tall pea plant x Tall pea plant F1 genotypes Tt Tt

Results:

F2 Phenotype: 3 are tall pea plants and 1 is short pea plant. F2 genotype: 1TT, 2Tt and 1tt

Phenotypic ratio: 3: 1

Genotypic ratio: 1: 2 :1

PUNNET SQUARE

Is a chequer board diagram used to illustrate the formation of a zygote.

• It is used to show the crosses.
• The female gametes are placed on the right side while the male gametes are placed on the left side.

NB: Male gametes and female gametes are represented by the following symbols:

 – represents female gametes

 – represents male gametes

Example: A pure breeding tall pea plant was crossed with a pure breeding short pea plant. Show the results of F1 and F2. State the genotypes and phenotypes of F1and F2. Work out the phenotypic and genotypic ratios of the F2.

Let

T– represent a gene for tall

t – represent a gene for short.

Parental phenotypes: tall pea plant x short pea plant Parental genotypes: TT t t

Let us assume the tall pea plant is the male and short pea plant is the female

Results:

F1 phenotype: All are tall Pea plants. F1 genotype: All are Tt.

Selfing F1 generation to obtain F2 generation

F1 phenotypes: Tall pea plant x Tall pea plant F1 genotypes Tt Tt

Meiosis

Gametes

Let us assume one of the tall pea plants is the male and another tall pea plant is the female

Results:

F2 Phonotypes: 3 – are tall pea plants

1 – is short pea plant F2 genotypes: 1TT, 2Tt and 1tt Phenotypic ratio: 3: 1

Genotypic ratio: 1: 2: 1

NB: All characters that Mendel investigated showed complete dominance

COMPLETE DOMINANCE

Is a condition in which a dominant gene completely masks the recessive gene.

The dominant gene expresses itself in both homozygous and heterozygous state.

Recessive gene expresses itself in homozygous state only.

EXAMPLE

1. Fur mice is determined by two alleles. The allele for black fur is dominant over the allele for brown fur. A homozygous black mouse is crossed with a homozygous brown mouse. What will be the result of the F1? If the offsprings are allowed to mate, what are the genotypes, phenotypes, genotypic and phenotypic ratios of the F2.

2. A pure breeding pea plant with smooth seeds was crossed with a pure breeding pea plant with wrinkled seeds. Show the results of F1 and F2. State the genotypes and phenotypes of F1and F2. Work out the phenotypic and genotypic ratios of the F2.

3. In an experiment, genetists crossbreed yellow maize with white maize. The ratio of yellow to white in the F2 generation was 3:1

(i) Which of the two characters do you thick was dominant

(ii) What colour of the maize in the F1generation.

(iii) Show the percentage of pure yellow in the F2 generation.

4. For the time a man has reared animal and grown crops. Give four (4) phenotypic features have been selected for modern agricultural purpose

5. In an experiment a variety of bean seed having smooth seed coat were crossed with wrinkled seed coat. All seed obtained in F1 generation had smooth seed coat. F2 contained 1200 seeds.

(i) Use appropriate symbols to work out the phenotype of F1 generation.

(ii) From the information above work out the following relying on F2 generation.

1. 1. 1. Genotypic ratio
      2. Phenotypic ratio
      3. Total number of smooth seeds

PATTERNS OF INHERITANCE THAT FOLLOW MENDEL’S FIRST LAW

Some conditions in human follow Mendelian monohybrid inheritance, such conditions are inherited in Mendelian fashion.

1. Albinism
2. Sickle Cell anemia
3. Blood group
4. Tongue rolling
5. Rhesus blood group
6. Haemophilia
7. Achondroplasia

I: ALBINISM

Is the hereditary condition where the body lacks melanin pigment in hair, skin and eyes.

In human, albino is characterized by lack of melanin which is responsible for the dark colour of the skin, hairs and iris of the eye.

Albino has light skin, white hairs and pink eyes.

Importance of melanin pigment

Melanin pigment is very important in the body because:

1. Melanin pigment determines the colour of the skin, hair and eyes.
2. It protects the body against harmful rays from the sun that cause sunburn and skin cancer.

NB: Albinism occurs in the both plants and animals.

• • In animals is due to the lack of melanin pigments.
  • In plants is due to lack of Chlorophyll (green pigments)

CAUSES OF ALBINISM

Albinism is an inherited disorder and is caused by a recessive gene “a”

NB: The allele for melanin production is dominant (A) while that for albinism is recessive (a)

• • Albinism occurs only in homozygous state (aa)
  • Normal skin occurs in both homozygous and heterozygous state
  • Individuals in heterozygous state are phenotypically normal and are called carries The following are phenotypes and genotype of albinism

Phenotype	Genotype
Normal skin (homozygous normal skin)	AA
Normal but carrier (heterozygous normal skin)	Aa
Albino	aa

Example 1: What will be the result if a heterozygous normal skin man marries heterozygous normal woman?

Let

A – represent a gene for normal skin a – represent a gene for albino

Parental phenotypes: Normal skin x Normal skin Parental genotypes Aa Aa

Results:

F1 Phonotypes: 3 – are normal skin

1. – is albino

F1 genotypes: 1AA, 2Aa and 1aa

Phenotypic ratio: 3: 1

Genotypic ratio: 1: 2: 1

NB: The result above shows that there is a probability of normal skinned people to give out albino child. This occurs when both parents are normal skinned and carries.

Example 2: What would be the results, if a normal skin but carrier man marries a homozygous albino female?

Let

A – represent a gene for normal skin a – represent a gene for albino

Parental phenotypes: Normal skin x albino Parental genotypes Aa Aa

Results:

F1 Phonotypes: 2 – are normal skin

1. – are albinos F1 genotypes: 2Aa and 2aa Phenotypic ratio: 2: 2

Genotypic ratio: 2: 2

NB: The cross above shows that there is a probability of 50% of having albino child.

II. SICKLE CELL ANAEMIA

Is a disorder of haemoglobin in in the red blood cells.

It is a genetic disorder that is inherited from parents.

Sickle cell anemia cause haemoglobin to have abnormal shape and as a result red blood cell developed a sickle shape hence the name sickle cell.

CAUSES OF SICKLE CELL ANAEMIA.

Sickle cell is caused by a recessive gene “HbS”

A gene for sickle cell anaemia is recessive “HbS”

A gene for normal is dominant “HbA”

The following are the phenotypes and genotypes of sickle cell anaemia

Genotype	Phenotype
HbA HbA	Normal person
HbA HbS	Normal but carrier
HbS HbS	Sickle cell anaemic

Example: A normal but sickle cell carrier marries a normal woman but sickle cell carries. Let:

HbA – represent a gene for normal

HbS – represent a gene for sickle cell anaemia.

Parental phenotypes: Normal x Normal

Parental genotypes HbA HbS HbA HbS Meiosis

Results:

F1 Phonotypes: 3 normal and 1 sickle cell anaemic F1 genotypes: 1AA, 2Aa and 1aa

Phenotypic ratio: 3: 1

Genotypic ratio: 1: 2: 1

RHESUS FACTOR

Is a protein which is found in the red blood cell.

NB: Rhesus factor was named after the rhesus monkey where the factor was firstly identified.

The individual who has a gene for rhesus factor is said to be rhesus positive (Rh+)

The individual who has lacks a gene for rhesus factor is said to be rhesus negative (Rh–)

CAUSES OF RHESUS FACTOR

Rhesus factor is caused by dominant gene (Rh+) that is inherited from the parents.

A gene for rhesus negative is recessive (Rh–)

The following are genotypes and phenotype of rhesus factor.

GENOTYPES	PHENOTYPES
Rh+Rh+	Homozygous Rhesus positive
Rh+Rh-	Heterozygous rhesus positive
Rh-Rh-	Homozygous rhesus negative

Example: What will be the results of the F1, if heterozygous rhesus positive man marries heterozygous rhesus positive woman.

Let

Rh+ – represent gene for rhesus positive Rh– – represent gene for rhesus negative.

Parental phenotypes: Rhesus positive x Rhesus positive. Parental genotypes: Rh+ Rh– Rh+ Rh–

Results:

F1 Phonotypes: 3 rhesus positive and 1 rhesus negative F1 genotypes: 1 Rh+ Rh+, 2 Rh+ Rh– and 1 Rh– Rh– Phenotypic ratio: 3: 1

Genotypic ratio: 1: 2: 1

EFFECTS OF RHESUS FACTORS ON PREGNANCY.

Rhesus factors may cause miscarriage/death of foetus before birth process.

Example: When a man of Rh+ marries a woman of Rh– result to a pregnancy having a foetus of Rh+

During pregnancy some of the rhesus factor diffuse to the maternal blood through the placenta.

Then the maternal body reorganizes the rhesus factor as an antigen and start to make the antibodies.

The antibodies diffuse from the maternal blood to the foetal blood and destroy all red blood cells of the foetus.

The condition in which all red blood cell of the foetes to be destroyed by antibodies called erythroblastosis fetalis or hemolytic diseases

NB: The problem above does not affect the first pregnancy but starts from the second pregnancy to the next pregnancies.

The first pregnancy is not affected because at this time the maternal body has few antibodies which are not enough to destroy all red blood cells to the foetus.

BUT

For the second and the next pregnancies the maternal body has enough antibodies which are enough to destroy all red blood cells of the foetus, hence causing death.

Modern ways used to prevent and treat erythroblastosis foetalis or haemolytic diseases

1. Transfusion of blood to the foetus while still in the uterus
2. Injecting the mother with anti – rhesus antibody during pregnancy, which prevents the antibody – antigen reaction.

NB: The presence or absence of rhesus factor in the red blood cell gives the blood groups the positive of negative signs.

Example

• Blood group A can be A+ or A–
• Blood group B can be B+ or B–
• Blood group AB can be AB+ or AB–
• Blood group O can be O+ or O–

The table below shows the compatibility and incompatibility of blood groups

Key:

• – compatible (no agglutination)

 – incompatible (agglutination) BLOOD TRANSFUSION

QUESTIONS

Why blood group AB is not always considered as universal recipient?

Answer

Because blood group AB– can not receives blood from blood groups with rhesus positive such as A+, B+, AB+ and O+

Question

Why blood group O is not always considered as universal donor?

Answer

Because blood group O+ cannot donates blood to blood groups with negative rhesus factor such as A–, B–, AB– and O–

Which blood group is said to be a universal donor and a universal recipient? Give reason to your answer.

ACHONDROPLASIA

Is a disorder that is characterized by a shortened legs and hands.

Achondroplasia is transmitted from the parents to the offsprings through a dominant gene “A”

The homozygous dominant AA and heterozygous Aa individual s show achondroplasia

Homozygous recessive aa individuals are perfectly normal. Examples

Qn: What would be the results of the F1 and F2 if a normal man marries achondroplasia woman?

Let

A – represent a gene for achondroplasia a – represent a gene for normal legs

Parental phenotypes: achondroplasia x normal Parental genotypes AA aa

Results:

F1 Phonotypes: all are achondroplasia F1 genotypes: all are Aa

Selfing the F1 generation to obtain F2 generation

F1 phenotypes: achondroplasia x achondroplasia F1 genotypes AA Aa

Results:

F1 Phonotypes: 3 achondroplasia and 1 normal F1 genotypes: 1AA, 2Aa and 1aa

MENDELIAN DIHYBRID INHERITANCE

Are part from monohybrid inheritance also Mendel investigate dihybrid inheritance.

DIHYBRID INHERITANCE

Is the flow of two characters from the parents to the offspring

DIHYBRID CROSS

Is the cross between two individuals having two traits with four contrasting characters. For example: Mendel extended dihybrid cross as follows:

Plants of yellow round seed were crossed with plants of green wrinkled seed. The observation made by Mendel show that:

All plants in F1 were yellow round seed. When F1 were selfed, the F2 were:

• 315 round yellow seeds
• 101 wrinkled yellow seeds
• 108 round green seeds
• 32 wrinkled green seeds

From the above results the F2 phenotypic ratio was 9:3:3:1

NB: From the experiment, Mendel conducted that two pairs of characteristics behave quite independent to each other and from wanted the second of inheritance or law of independent assortment.

MENDEL’S SECOND LAW INHERITANCE (LAW OF INDEPENDENT ASSORTMENT)

The law state that:

“Any character from a pair of character may combine with any one of another pair”.

NB: The ratio of Mendel’s second law inheritance 9:3:3:1

NON-MENDELIAN INHERITANCE

Is the inheritance that do not follow Mendel’s first law of inheritance.

They do not involve dominant gene and recessive character Types of non mendelian inheritance

1. 1. Incomplete dominance
   2. Codominance

INCOMPLETE DOMINANCE

Is a condition in which no allele is dominant or recessive compared to the other.

• The alleles express themselves equally when in a heterozygous state, resulting in an intermediate characteristic in the offspring.
• Incomplete dominance results an offspring with new character different from two parents.
• Incomplete dominance shows that Mendel’s principle of dominance does not apply to the inheritance of all traits because intermediate phenotypic forms arise when some crosses are do not resemble any of the parents.

Example: A red flowered rose plants was crossed with white flowered rose plants and all members of the F1 were pink, when the pink flowered rose plants were selfed, a mixture of red, pink and white flowered rose plants were obtained. Illustrate this using genetic diagram.

Let

R – represent a gene for red flowered rose plants

W – represent a gene for white flowered rose plants

Parental phenotypes:	Red flowers	x	white flowers
Parental genotypes	RR		WW

Meiosis Gametes Fertilization

Results:

F1 Phonotypes: all are pink F1 genotypes: all are RW

Selfing the F1 generation to get F2 generation

Results:

F2 phenotype:1Red, 2Pink and 1White. F2 genotype 1RR, 2RW and 1WW

F2 phenotypic ratio: 1:2:1 F2genotypic ratio: 1:2:1 QUESTION

(a) Explain the meaning of the following

1. Back cross
2. Test cross

(b) Plants with red flowers were crossed with plants with white flowers. The results of F1 generation had a pink flower.

(c) Use genetic diagram, work out the genotype of the F2 generation offspring and determine the phenotypic ratio.

(d) Explain why a cross between red flowered plant and white flowered plants produce pink flowered plant?

ANSWER

The cross resulted into pink flowers because none of the gene successfully masked the expression of another gene a condition is known as incomplete dominance

CODOMINANCE

Is a condition in which genes from both parents are dominant and are phenotypically expressed in the offspring.

OR Is a state that occurs when all character or gene equally expressed phenotypically.

The character from two parental both appear on an offspring.

Example: A red cow was mated with a white bull. In F1 generation all the offspring had equal patches of red and white fur (roan). When the roans were selfed, a mixture of red, roan and white were obtained. Illustrate this using genetic diagram.

Let

R – represent a gene for red cow

W – represent a gene for white bull

Parental phenotypes: Red cow x White bull Parental genotypes RR WW

Results:

F1 Phonotypes: all are roan F1 genotypes: all are RW

Selfing the F1 generation to get F2 generation

Parental phenotypes: Roan x Roan Parental genotypes RW RW

Results:

F2 phenotypes:1Red, 2Roan and 1White.

F2 genotypes: 1RR, 2RW and 1WW F2 phenotypic ratio: 1:2:1 F2genotypic ratio: 1:2:1

1. When pure breed black guinea pigs were Crossed with pure breed white guinea pigs all offspring had a coat with black and white patches as shown below.

Let

B – represent a gene for red cow

W – represent a gene for white bull

Parental phenotypes: Black x White

Parental genotypes BB WW Meiosis

Results:

F1 Phonotypes: All are black and white patched pigs F1 genotypes: all are BW

Selfing the F1 generation to get F2 generation

Parental phenotypes: Black and white x Black and white Parental genotypes BW BW

Results:

F2 phenotypes:1Black, 2black and white and 1White.

F2 genotypes: 1BB, 2BW and 1WW F2 phenotypic ratio: 1:2:1 F2genotypic ratio: 1:2:1

INHERITANCE OF BLOOD GROUPS

The entire human population falls under four (4) blood groups which are blood group A, blood group B, Blood group AB and blood group O.

Inheritance of blood groups is controlled by one gene which occurs in three forms (alleles) which are:

• Allele A
• Allele B
• Allele O

NB: Allele A and B are codominant while allele O is recessive to both A and B

A person’s blood group is determined by the alleles inherited.

Blood groups and their alleles

BLOOD GROUP	ALLELE
A	AA or AO
B	BB or BO
AB	AB
O	OO

The following are genotypes and phenotypes of the blood groups

GENOTYPES	PHENOTYPES
AA	Homozygous blood group A
AO	Heterozygous blood group A
BB	Homozygous blood group B
BO	Heterozygous blood group B

AB	Heterozygous blood group AB
OO	Homozygous blood group OO

Example: What would be the phenotypes, genotypes, if a heterozygous blood groups A man crossed with a heterozygous blood group B woman?

Let

A – represent a gene for blood group A

B – represent a gene for blood group B

Parental phenotypes: Blood group A x Blood group B Parental genotypes AO BO

Results:

F1 Phonotypes: 1 blood group AB, 1 blood group A, 1 blood group B and 1 blood group O F1 genotypes: 1AB, 1AO, BO and OO

2. What will be the phenotypes and genotypes, if a man of blood group B marries of a woman of blood group A

Let

A – represent a gene for blood group A B – represent a gene for blood group B O – represent gene for blood group O

Case I: Let say a man is homozygous blood group B and a woman is homozygous blood group A

Parental phenotypes: Blood group B x Blood group A

Parental genotypes BB AA Meiosis

Results:

F1 Phonotypes: All are blood group AB F1 genotypes: All are AB

Case II: Let say a man is heterozygous blood group B and a woman is heterozygous blood group A

Parental phenotypes: Blood group B x Blood group A

Results:

F1 Phonotypes: 1 blood group AB, 1 blood group A, 1 blood group B and 1 blood group O F1 genotypes: 1AB, 1AO, BO and OO

NECTA QUESTIONS

1. A woman of blood group A claims that a man of blood group B is a legimate father of her baby. A man refused, through blood test a child was blood group O. By using genetic diagram show if a woman’s claim was correct.

Let

A – represent a gene for blood group A B – represent a gene for blood group B O – represent gene for blood group O

Case I: Let say a man is homozygous blood group B and a woman is homozygous blood group A

Parental phenotypes: Blood group B x Blood group A

Parental genotypes BB AA Meiosis

Results:

F1 Phonotypes: All are blood group AB F1 genotypes: All are AB

Case II: Let say a man is heterozygous blood group B and a woman is heterozygous blood group A

Parental phenotypes: Blood group B x Blood group A

Parental genotypes BO AO Meiosis

Results:

F1 Phonotypes: 1 blood group AB, 1 blood group A, 1 blood group B and 1 blood group O F1 genotypes: 1AB, 1AO, BO and OO

Therefore, from the crosses above, if a man is homozygous blood group B and a woman is homozygous blood group A, a man is not a legimate father of a child but if a man is heterozygous blood group B and a woman is heterozygous blood group A, a man is a legimate father of a child.

In hospital two young babies were mixed accidentally after birth. Through blood test, one baby had blood group O and another baby had blood group AB. The parents’ blood groups were:

• Mr. John blood group A
• Mrs. John blood group B
• Mr. Joseph blood group AB
• Mrs. Joseph blood group A.

By using symbols shows the parents of each baby.

Case I: Let say, Mr. John is heterozygous blood group A and Mrs. John is heterozygous blood group B.

Let

A – represent gene for blood group A B – represent gene for blood group B O – represent gene for blood group O

Parental phenotypes: Blood group A x Blood group B Parental genotypes AO BO

Results:

F1 Phonotypes: 1 blood group AB, 1 blood group A, 1 blood group B and 1 blood group O F1 genotypes: 1AB, 1AO, BO and OO

Case II: Let say Mr. Joseph is blood group AB and Mrs. Joseph is homozygous blood group A.

Let

A – represent gene for blood group A B – represent gene for blood group B O – represent gene for blood group O

Parental phenotypes: Blood group AB x Blood group A Parental genotypes AB AA

Results:

F1 Phonotypes: 2 blood group A, 2 blood group AB F1 genotypes: 2AA and 2AB

Therefore, from the above crosses, the baby with group O belongs to Mr. and Mrs. John while the other baby with blood group AB belongs to Mr. and Mrs. Joseph.

1. A man who has blood group O married woman who has blood group A
2. What is the probability of their first child having blood group O if both parents are homozygous?
3. Assuming the mother is heterozygous, what is the probability of their first child having blood group O?
4. Determine the blood groups for an offspring whose parents:
5. Having heterozygous blood group A.
6. Have blood group AB and heterozygous blood B (iii)Have homozygous blood group A and blood group O

SEX DETERMINATION

Is the determination of maleness or femaleness of an individuals. OR

Is the determining of an individual will be a female or male.

Human beings have 46 chromosomes (23 pairs of homologous chromosomes) in every body cell.

Out of 46 chromosomes, 2 of them are sex chromosomes while 44 (22 pairs of homologous chromosomes) are known as autosomes

NB Autosomes determine physical characteristics of human being such as height and body size.

SEX CHROMOSOMES

Are chromosomes that determine sex of a child.

TYPES OF SEX CHROMOSOMES

There are two types of sex chromosomes that determine the sex of a child, namely:

1. X – chromosomes
2. Y – chromosomes

The female carries X and X chromosomes which are similar in shape and size and are said to be homogametic

• The female genotype is XX

The male carries X and Y chromosomes which are different in shape and size and are said to be heterogametic

• The male genotype is XY

A female gamete always contains the X – chromosome while Male gamete has either X or Y chromosome

Female secondary sexual characteristics are controlled by genes on the X – chromosomes while male secondary sexual characteristics are controlled by genes on the Y – chromosome.

MECHANISM OF SEX DETERMINATION

The sex of a child is a matter of chance and depends on the whether the sperm that fertilizes the ovum carries a Y or an X chromosome.

• If a Y – carrying sperm fuses with the ovum, the zygote formed will have 44 autosomes + XY chromosomes that will develop into a boy.
• If the X – carrying sperm fuses with the egg, the zygote formed will have 44 autosomes + XX chromosomes that will develop into a girl.

Therefore, it is the sperm that fuses with the egg that determine the sex of the baby born.

• Maleness depends upon the presence of Y chromosomes
• Femaleness depends upon the absence of the Y chromosome.
• The ratio of a baby being a girl or a boy is 1:1. This means that the probability of getting a boy or girl child is 50%

WORKED EXAMPLE OF SEX DETERMINATION QUESTIONS

1. A newly married couple expects a baby. Using genetic cross, work out the probability of their first born child being a boy.

Let

XY – represent gene for male XX – represent gene for female

Parental phenotypes: Male x Female

Parental genotypes XY XX

RESULTS

F1 Phenotypes: 2 girls and 2 boys F1 genotypes: 2XX and 2XY

The ratio of a baby being a girl or a boy is 1:1. This means that the probability of their first born child being a boy is 50%

SEX LINKED CHARACTERS

Are observable features of an organism which are controlled by gene located on the sex chromosome.

OR

Are characters which are influenced or determined by the genes located in the sex chromosomes/sex-linked gene.

SEX LINKED GENES

Are the gene located in the sex chromosomes.

• They influence sex-linked character.

Most of sex – linked genes are carried on the X – chromosomes and not on Y – chromosomes.

QUESTION

Why Most of sex – linked genes are carried on the X – chromosomes and not on Y – chromosomes?

ANSWER

Because:

• X – chromosomes have longer tail which has ability to carry more genes than Y – chromosomes.
• X – chromosomes are larger than Y – chromosomes which makes most of the sex-linked characters to be located on x-chromosomes.

Example: In Drosophila Melanogaster, the gene which determines eye colour is located on the X – chromosome. If homozygous red – eyed female is crossed with white – eyed male, all the offspring in F1 generation have red – eyes. Using genetic cross, work out the phenotypes and genotypes of the offspring in F2 generation.

RESULTS

F1 phenotypes: All have red eyes F1 genotypes: 2XRXr and 2 XR Y

Selfing F1 generation to get F2 generation

RESULTS

F1 phenotypes: 3 red eyes and 1 white eyes

F1 genotypes: 1XRXR, 1XR Y, 1XRXr and 1Xr Y

NB: Y – linked characters (traits) are passed only from father to son.

Example of genes that are carried on Y – chromosome

Holandric gene that determine the growth of hairs on the pinna of some men.

COMMON SEX – LINKED CHARACTERS

The following are most common and familiar sex – linked characters: –

1. Haemophilia.
2. Colour blindness.

HAEMOPHILIA

Is a hereditary trait in which an individual’s blood is unable to clot.

Individual with haemophilia is in danger of bleeding excessively even from minor injuries

Haemophilia is also known as Bleeder’s disease.

CAUSES OF HAEMOPHILIA

Haemophilia is caused by a recessive gene “h” located in X – chromosomes. A gene for normal blood clotting is dominant “H” over the gene for haemophilia.

A person who suffers from haemophilia is known as Haemophilic

The following are the phenotypes and genotypes of haemophilia

Phenotypes	Genotypes
Homozygous Normal female	XHXH
Heterozygous normal female (normal carrier)	XHXh
Homozygous haemophilic female	XhXh

Normal male	XHY
Haemophilic male	XhY

NB: Haemophilia affects males more than females because a single recessive allele in the male’s X – chromosome causes haemophilia while a female would be a carrier.

EXAMPLE 1: What would be the phenotypes, genotypes, if a normal man marries a normal but haemophilia carrier woman.

Solution

RESULTS

F1 phenotypes: 3 normal and 1 haemophilic

F1 genotypes: XHXH, 1 XH Y, 1 XHXh and 1Xh Y

EXAMPLE 2: A non- haemophilic man is married to a haemophilic woman. What will be the phonotypes of their offspring?

Solution

RESULTS

F1 phenotypes: 2 normal but carrier females and 2 haemophilic sons

QUESTION

A certain woman who is a carrier of haemophilia was married to a haemophilic man. Is there any possibility of having phenotypically normal children from the couple?

Solution

RESULTS

F1 phenotypes: 2 normal children and 2 haemophilic children The couple will have 50% chance of having a normal child.

Question: 

Why Hemophilia is very rare to female and very common to males?

Answer

Because:

1. A male suffers hemophilia by receiving only one recessive allele of haemophilia but female suffers haemophilia by receiving two recessive alleles of haemophilia from both parents which is not common.
2. Most of the females with haemophilia die during their first menstrual due to the excessive loss of blood.

NB: In most cases Haemophilia is transfused to replace the blood lost through bleeding.

COLOUR BLINDNESS

Is the inability to distinguish between certain colours.

• It is a sex – linked disorders in which a person fails to distinguish between certain colours.
• The most common is the inability to distinguish red and green colours called red – green blindness.
• It is also known as deuteranopia CAUSES OF COLOUR BLINDNESS
• Colour blindness is caused by recessive gene “b” located in the X – chromosome.
• Gene for normal vision is dominant “B”

The following are the phenotypes and genotypes of colour blindness

Phenotypes	Genotypes
Homozygous Normal female	XBXB
Heterozygous normal female (carrier female)	XBXb
Homozygous colour blind female	XbXb
Normal male	XBY
Colour blind male	XbY

EXAMPLE: 

A woman carrier of colour blindness was married to a normal man. What will be the probability of getting a normal colour visioned son?

Solution

RESULTS

F1 phenotypes: 1 normal daughter, 1 carrier daughter, 1 normal son and 1 colour blindness son. F1 genotypes: XBXB, 1 XB Y, 1 XBXb and 1Xh Y

The probability of getting a normal colour visioned son is 50%

Example 2: What would be the results, if a colour blind man marries a normal woman?

Solution

RSULTS

F1 phenotypes: 2 carrier daughters and 2normal colour visioned sons. F1 genotypes: 2 XBXb and XB Y

SEX-LIMITED CHARACTERS.

These are characters that are only limited to a particular sex and cannot be found in the other sex.

• Sex limited characters are restricted to only one sex, either males or females.
• Sex limited traits are controlled by autosomal genes.

Examples of sex limited characters

• Beards in male
• Baldness in male.
• Soft voice in females
• Long hairs in male lions
• Comb plumage in hens.
• Lactation (milk production) in females
• Moustache in males
• Hairy pinna and nose in males
• Ovulation in females
• Semen production in males

SEX INFLUENCED CHARACTER

These are characters that tend to be more noticeable in one sex than the other.

• Sex influenced characters appear in organisms of both sexes but are expressed to a different degree in each sex
• Sex influenced characters could be caused by sex hormones e.g. all secondary sexual characteristics or environments.
• Although they may have identical genotypes in both males and females, they become more noticeable in one sex and almost lost in the other sex

Examples of sex influenced characters

• Breasts in females
• Long mane (hairs) of the lion
• Mammary glands enlarge and become functional only in females
• Comb and colourful plumage of a cock.
• Long horns of male goats, sheep and cows

SEX PREFERENCE

Is favouring a particular sex compared to the other.

OR is a tendency of people to like one type of sex than the other.

SEX SELECTION

Is the process of using various means to achieve a desired sex of the offspring.

• Sex preference and selection is more common in African countries and some parts of Asia
• Most of families prefer male than female children.

CONSEQUENCES OF SEXPREFERENCE AND SELECTION

1. It leads into discrimination among children.
2. It leads into inequality among children.
3. It leads into unfair treatment of individuals of a particular sex. E.g., Boys being educated and given ample time to play and learn while girls stay at home and do house chores.
4. It leads into family conflicts. E.g. some males blame their wives of not giving birth to the desired sex of the offspring

SOCIO – CULTURAL FACTORS INFLUENCING SEX PREFERENCE AND SEX SELECTION

Sex preference and sex selection are influenced by the following socio – cultural factors, these are:

1. Manpower generation
2. Generation and protection of wealth
3. Land ownership
4. Perpetuation of the lineage

MANPOWER GENERATION

Some societies especially pastoralists prefer boys over girls because boys help in animal grazing

GENERATION AND PROTECTION OF WEALTH

In some societies, girls are more preferred than boys because they generate wealth upon getting married. A family will get a lot of cattle or money as bride price.

LAND OWNERSHIP

In some societies, a woman cannot own land, thus families will prefer to have more sons than girls so that they can somehow benefit indirectly through their sons.

PERPETUATION OF THE LINEAGE

In some societies, boys are preferred in belief that they will perpetuate the lineage and take care of the parents when females are living far away with their husbands.

VARIATION

Is the differences between individuals of the same species.

These variations may be in terms of morphology, physiology, cytology and behavioural of individuals.

Examples of variations exist among members of the same family

• Acquired variations such as scars, body weight body height, possession of large muscles due to physical exercise, calluses in fingers
• Inherited variations such as blood groups, tongue rolling, sex

TYPES OF VARIATION

There are two types of variation namely:

1. Continuous variation.
2. Discontinuous variation.

CONTINOUS VARIATION

Is the type of variation which shows no clear cut differences from one organism to another.

• Continuous variation is also called quantitative variation
• These variations do not show a clear cut gap between the two extremes.
• Characteristics that show continuous variation are influenced by both genes and environment.

Characters in human being that shows continuous Variation

• Weight
• Height
• Skin colour.
• Intelligence
• Volume.

DISCONTINOUS VARIATION

Is a type of variation which shows clear cut differences from one organism to another.

• Discontinuous variation is also known as qualitative variation.
• This type of variation shows no intermediate i.e., person either has the character or not.
• Characteristics that show discontinuous are not influenced by the environment.
• White flower will remain white regardless of the environmental factor. Also blends of an individuals can not be affected by the environmental.

Example of the character that shows discontinuous variation.

• Tongue rolling
• Sex
• Blood group
• Albinism.
• Finger prints.
• Rhesus factor.
• Colour of flower.
• Sickle cell anaemia.

IMPORTANCE OF VARIATION

Genetic variations have a number of advantages in population as follows:

1. Helps to reduce the occurrence of recessive genetic disorder
2. Helps to increase resistance to certain disease-causing agents.
3. Improves the chance of survival of a particular species on the onset of unfavourable conditions.

DIFFERENCE BETWEEN CONTINUOUS VARIATION AND DISCONTINUOUS VARIATION

CONTINUOUS VATIATION	DISCONTINUOUS VARIATION
It has intermediate forms	It has no intermediate forms
It is quantitative	It is qualitative
It is influenced by genes and environment	It is influenced by genes
No clear cut differences from one organism to another	Shows clear cut differences from one organism to another

CAUSES OF VARIATION

Variation among organisms can be caused by a number of factors that are grouped into either genetic or environmental factors:

ENVIRONMENTAL FACTORS

Variation among organisms can be caused by environmental factors such as space, light, food, water, climate, population and diseases. Environmental factors cause acquired variations among organism which cannot be inherited or passed from one generation to the next since they affect only the phenotypes of an organism.

Example; the plant which lack light develop weakness character while plants receiving enough light look(stronger)

GENETIC FACTORS

These are factors that cause inheritable variations that can be transferred from one generation to another.

The following are genetic factors that cause variation among organisms:

1. Fertilization
2. Crossing over
3. Migration
4. Mutation
5. Independent assortment

FERTILIZATION

During fertilization the nuclei of the male gamete and female gamete fuse resulting to an offspring having the combination of the character from the mother and hence variation.

CROSSING OVER

Is a genetic recombination that occur during prophase I of meiotic cell division.

During crossing over, chromatids of homologous chromosomes exchange randomly their genetic materials leading to new recombinant chromosomes hence resulting to variation.

MIGRATION

Migration introduces alleles into the recipient population, if the migrant interbreeds with the individuals in the recipient population. The introduced alleles could lead to increased genetic variations.

MUTATION

Is a sudden and unpredictable change in the genes or chromosomes of an organism. When a gene or chromosome change may result an individual to have a character that is a different from other, hence variation.

INDEPENDENT ASSORTMENT

Independent assortment together with random fertilization produce more possibilities for genetic variations among organisms to occur since they produce new combination of alleles.

MUTATION

Is a sudden and unpredictable change in the genes or chromosomes of an organism.

Organism affected by mutation is called Mutant organisms.

The cell affected by mutation is called Mutant cell.

The chromosome affected by mutation is called a mutant chromosome

The gene affected by mutation is called a mutant gene

When mutation occurs in somatic cells (body cell) is called somatic mutation and cannot be inherited from one generation to another.

When mutation occur in sex cell is called germline mutation and can be passed from one generation to another.

CAUSES OF MUTATION

Mutation is caused by Mutagens or Mutagenic agents.

Mutagens: are factors that cause mutation

Examples of mutagens or mutagenic agents that cause mutation

Harmful rays such as x-rays, gamma rays, ultra violet rays and cosmic rays

High energic particles such as alpha particles (α-particles and Beta particles (β-particles)

Heavy metals such as Mercury.

Heavy chemicals such as mustard gas, food preservatives, tobacco.

Temperature – an increase in temperature increases the rate of mutation. Mutation increases five times with every rise of 10°C

Viruses – some viruses generate tumours in animals. These viruses are known as retroviruses. The human papilloma virus is a retrovirus that causes cervical cancer in woman.

TYPES OF MUTATIONS

Basically, there are two types of mutation namely: –

1. Gene mutation.
2. Chromosomal mutation.

GENE MUTATION.

Is the type of mutation that occurs in the gene when there is a change in the base sequences in nucleotide of DNA.

Gene mutation is also called point mutation.

The following are gene mutations, these includes:

1. Deletion
2. Insertion
3. Substitution
4. Inversion

DELETION

Is a gene mutation that involves the absence of one nucleotide in the gene.

This can be exemplified by a message SEND written wrongly as END.

Normal gene Mutant gene

INSERTION

Is the gene mutation in which extra nucleotide is added in the nucleotide sequence.

This can be exemplified by a message SAY written wrongly as SAYS.

SUBSTITUTION

Is a gene mutation in which the correct nucleotide replaced by another incorrect nucleotide in the nucleotide sequences.

This can be exemplified by a message NOW written wrongly as NOT.

 

INVERSION

Is a gene mutation occurs when the nucleotides representing a gene is inverted.

This can be exemplified by message MOTHER written wrongly as THERMO.

CHROMOSOMAL MUTATION

Is a type of mutation that occurs in the chromosomes where by a chromosome may change in structure or number.

Chromosomal mutation involves changes in chromosomal structure.

The following are chromosomal mutations, these includes:

1. Deletion
2. Duplication
3. Inversion
4. Translocation

DELETION

Is a chromosomal mutation which occurs when a part of a chromosome breaks off.

The broken part of the chromosome is not reconnected to any part of the chromosome.

DUPLICATION

Is a mutation which occurs when a segment of a chromosome is represented twice in a genome.

Genome are genes contained in a single set of chromosomes.

INVERSION

Is a mutation that occurs when a segment of a chromosome breaks and becomes reattached with the genes in the reverse order.

TRANSLOCATION

Is the chromosomal mutation that occurs when a segment of a chromosome breaks off and becomes attached to another non – homologous chromosome.

The figure below shows translocation between two pairs on non – homologous chromosomes.

NUMERICAL CHROMOSOMAL MUTATION

Is a chromosomal mutation that involves changes in number of chromosomes.

Chromosomal mutation that involves the change in the number of chromosomes include:

1. Euploidy
2. Polyploidy
3. Aneuploidy

EUPLOIDY

Is a chromosomal mutation that involves extra addition or loss of one chromosome.

• Examples (2n+1), (2n-1)

POLYPLOIDY

Is a condition that occurs when an organism has more than two sets of chromosomes.

ANEUPLOIDY

Is a condition in which the diploid state of the organism may change, resulting in loss or gain of a single chromosome.

Aneuploidy is also called non-disjunction

It occurs when homologous chromosomes fail to separate during meiosis I or when sister chromatids fail to separate in meiosis II of the egg.

After fertilization chromosome 21 will contain 3 (the 2 which failed to separate plus the one from the father) instead of the normal two (each from one parent).

NB: The best example of aneuploidy is the Down’s syndrome

GENETIC DISOSERS

Genetic disorder: is a condition that occurs as a result of mutation.

Examples of genetic disorders

1. Down’s Syndrome or Mongolism/Mongolia
2. Turner Syndrome
3. Super male and Super female
4. Haemophilia
5. Colour blindness
6. Albinism
7. Sickle cell anaemia

DOWN’S SYNDROME(MONGOLISM)

Is the genetic disorder caused by the presence of the extra copy of chromosome number 21, making an individual to have 47 chromosomes instead of 46.

• This is caused by failure of a pair of chromosome number 21 to separate during anaphase I of meiosis.
• Down’s syndrome is likely to occur in babies born from mothers and fathers who are over 40 and 55 years old respectively.

Symptoms of Down ’s syndrome

1. Low immunity in the body
2. Mental retardation. (iii)Thick tongue
3. Slit eye appearance
4. Short body with stubby fingers.
5. Round and flat face.

Effects of Down’s syndrome (Mongolism)

1. It leads to malformation of the heart which make them very susceptible to disease such as heart diseases.
2. They suffer discrimination in the society that consider Down’s syndrome something unusual.
3. They die young mostly not more than 30 years.

TURNER’S SYNDROME

Is a genetic disorder that affects females by missing an X – chromosome.

• In this disorder, an individual’s genotype will be XO instead of XX. O indicates the absence of an X – chromosome.
• Therefore, a female with Turner’s syndrome has 45 chromosomes instead of 46.

Causes of Turner’s syndrome

• Turner’s syndrome is caused by failure of the sex chromosome to separate during meiosis

Symptoms of Turner’s syndrome

1. Low intelligence (IQ)
2. Heart abnormalities
3. Abnormal genitals
4. Short stature and skeletal abnormalities.
5. Infertility due to failure of ovaries to develop.
6. Kidney problems

KLINEFELTER’S SYNDROME

Is a genetic disorder that occurs when a boy is born with an extra copy of the chromosome.

Causes of Klinefelter’s Syndrome

Klinefelter’s syndrome is caused by the failure of X – chromosome in one of the parents to separate during meiosis making an individual to have XXY condition.

Symptoms of Klinefelter’s syndrome

1. Smaller than normal testicles
2. Produce less testosterone
3. Produces less or no sperm
4. Smaller than normal penis

Effects of Klinefelter’s syndrome

• The individual with Klinefelter’s syndrome usually has low intelligence
• Male with Klinefelter’s syndrome is typically tall and may have small testes and some breast development,

SUPER MALE AND SUPER FEMALE

Is a genetic disorder caused by non – disjunction of sex chromosomes leading to male to have an extra Y – chromosome (XXY) and female having an extra X – chromo some (XXX).

Super male and super female have 47 chromosomes instead of 46 chromosomes

Effects of Super male and super female

1. They are taller than average
2. Increased risk for learning difficulties

ALBINISM

Is the genetic disorder where the body lacks melanin pigment in hair, skin and eyes.

Causes of albinism

It is caused by defects in one of several genes that codes for production of melanin.

Effects of Albinism

Albinos have no melanin hence suffer the following problems: –

1. Albino is susceptible to sunburn
2. Albino may be at risk to develop skin cancer.
3. Individuals have vision problem.

PRACTICAL APPLICATION OF GENETICS

The knowledge acquired in genetics is practically used in the following fields:

1. Blood transfusion
2. Genetic counseling
3. Genetic engineering
4. Agriculture (plant and animal breeding).

PEDIGREE

This is the tree diagram or chart which shows the passage of characteristics from one generation to another generation in a certain family.

FEATURES OF A PEDIGREE.

1. Individual which are joined horizontally are of the same generation.
2. The individuals which are joined vertically are of the next generation. (iii)The sphere represents females and the square as back represents males.

For example.

=>The following flow chart shows the inheritance of colour blindness