Principles of Inheritance and Variation | Plus 2 Zoology| Exam Capsule Notes (Web and PDF)

CHAPTER AT A GLANCE

Gregor Mendel conducted experiments on garden peas (Pisum sativum). He selected 7 pairs of true breeding pea varieties. 

7 Characters	Contrasting Traits
	Dominant	Recessive
1. Stem height	Tall	Dwarf
2. Flower colour	Violet	White
3. Flower position	Axial	Terminal
4. Pod shape	Inflated	Constricted
5. Pod colour	Green	Yellow
6. Seed shape	Round	Wrinkled
7. Seed colour	Yellow	Green

INHERITANCE OF ONE GENE

Monohybrid cross:

A cross involving 2 plants differing in a character pair. 

Monohybrid phenotypic ratio = 3:1.

Monohybrid genotypic ratio = 1:2:1.

Backcross: Cross b/w a hybrid & its any parent.

Testcross: Cross b/w an organism with dominant phenotype & a recessive individual. 

Hence monohybrid test cross ratio= 1:1

Test cross is used to find out the unknown genotype of a character.

INHERITANCE OF TWO GENES

Dihybrid cross: Cross b/w two parents differing in 2 pairs of characters.

E.g. Cross b/w pea plant with round & yellow seeds (RRYY) and wrinkled & green seeds (rryy). 

F2:

Dihybrid Phenotypic ratio= 9:3:3:1

Mendel’s Laws of Inheritance 

First Law (Law of Dominance):

• Characters are controlled by factors.
• Factors occur in pairs.
• In a dissimilar factor pair, one factor dominates the other.

Second Law (Law of Segregation):

“During gamete formation, factors (alleles) of a character pair segregate each other such that a gamete receives only one of the 2 factors”.

3rd Law: Law of Independent Assortment: 

“When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.

Incomplete Dominance:

It is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics.

E.g. Flower colour in snapdragon (Antirrhinum).

Phenotypic ratio= 1: 2: 1

Genotypic ratio= 1: 2: 1

Multiple allelism:

More than two alleles of a gene govern same character.

E.g. ABO blood grouping (3 alleles: IA, IB & i).

Pleiotropy: 

A single gene exhibits multiple phenotypic expressions. E.g. Starch synthesis in pea, phenylketonuria.

Starch synthesis in pea plant:

BB gene: Effective starch synthesis, produce large starch grains.

bb gene: Lesser starch synthesis, produce small starch grains.

Starch grain size also shows incomplete dominance.

Co-dominance:

The inheritance in which both alleles of a gene are expressed in a hybrid. E.g. ABO blood grouping in human.

ABO blood groups are controlled by the gene I.

The gene I has three alleles IA, IB & i.

When IA and IB are present together, they both express (AB group). 

Alleles from parent 1	Alleles from parent 2	Genotype of offspring	Blood types (phenotype)
IA	IA	IA IA	A
IA	IB	IA IB	AB
IA	i	IAi	A
IB	IA	IA IB	AB
IB	IB	IB IB	B
IB	i	IBi	B
i	i	ii	O

CHROMOSOMAL THEORY OF INHERITANCE (Sutton & Boveri) 

• Chromosomes are vehicles of heredity.
• Two identical chromosomes form a homologous pair.
• Homologous pair segregates during gamete formation.
• Independent pairs segregate independently.

Genes (factors) are present on chromosomes. Hence genes and chromosomes show similar behaviours.

T.H Morgan proved chromosomal theory of inheritance using fruit flies (Drosophila melanogaster).

Morgan’s experiment to study sex linked genes 

Linkage: Physical association of two genes on a chromosome.

Recombination: Generation of non-parental gene combination.

Drosophila is suitable material for genetic study because,

• They can grow on simple synthetic medium.
• Short generation time (life cycle: 12-14 days).
• Breeding can be done throughout the year.
• Hundreds of progenies per mating.
• Male and female flies are easily distinguishable.

Cross 1: Yellow-bodied, white-eyed female X Brown-bodied, red-eyed male (wild type)

Cross 2: White-eyed, miniature winged female X Red eyed, large winged male (wild type)

Morgan intercrossed their F1 progeny. He found that

• The two genes did not segregate independently.
• Parental gene combinations was much higher than non-parental type. This is due to linkage.
• Genes of eye colour & body colour were tightly linked (only 1.3% recombination). Genes of eye colour & wing size were loosely linked (37.2% recombination).
• Tightly linked genes show low recombination. Loosely linked genes show high recombination.

MECHANISMS OF SEX DETERMINATION

Sex chromosomes include X & Y.

Autosomes are chromosomes other than sex chromosomes.

1. XX-XO mechanism: Male heterogametic, i.e. XO (Gametes with X and without X) and female homogametic, i.e. XX (gametes with X-chromosomes). E.g. grasshopper.
2. XX-XY mechanism: Male heterogametic (X & Y) & female homogametic (X only). E.g. Human & Drosophila.
3. ZZ-ZW mechanism: Male homogametic (ZZ) and female heterogametic (Z & W). E.g. Birds.

Sex Determination in Humans (XX-XY)

Thus the sperm determines whether the offspring male or female.

MUTATION

Sudden heritable change in DNA. 2 types:

• Point mutation: Change in a single base pair. E.g. sickle cell anaemia.
• Frame-shift mutation: Deletion or insertion of base pairs resulting in the shifting of DNA sequences.

Mutagens: Agents which induce mutation. 2 types.

• Physical mutagens: UV radiation, α, β, γ rays, X-ray etc.
• Chemical mutagens: Mustard gas, phenol, formalin etc.

PEDIGREE ANALYSIS

Analysis of genetic traits in several generations of a family. It helps to understand whether a trait is dominant or recessive.

Representation of family genetic history is called family tree (pedigree).

Male: ⬜

Female: ⚪ 

Sex unspecified: ◇ 

Affected individual: 

Mating: 

Mating b/w relatives (consanguineous mating): 

Parents above & children below:

Parents with affected male child:

Five unaffected offspring:

Pedigree analysis of

(A) Autosomal dominant trait (E.g. Myotonic dystrophy)

(B) Autosomal recessive trait (E.g. Sickle-cell anaemia) 

GENETIC DISORDERS

1. Mendelian Disorders

Due to change in gene.

Haemophilia (Royal disease): 

• Sex linked (X-linked) recessive disease. A blood clotting protein is affected. 
• The disease is controlled by 2 alleles, H (normal) & h (haemophilic).
• In females, haemophilia is very rare because it happens only when mother is at least carrier and father haemophilic.

Sickle-cell anaemia: 

• Autosome linked recessive disease.
• RBC becomes sickle shape.
• Homozygous dominant (HbAHbA): normal
• Heterozygous (HbAHbS): carrier; sickle cell trait
• Homozygous recessive (HbSHbS): affected
• It is due to substitution of Glutamic acid by Valine at the 6th position of β-globin chain of haemoglobin.
• This is due to the single base substitution at the sixth codon of the β-globin gene from GAG to GUG.

Thalassemia: 

    Autosome-linked recessive disease. 

    

    Reduced synthesis of α or β globin chains of haemoglobin. 

    2 types:

• α Thalassemia: Reduced synthesis of a globin due to mutation of genes HBA1 & HBA2 on chromosome 16.
• β Thalassemia: Reduced synthesis of b globin due to mutation of gene HBB on chromosome 11.

    Thalassemia is a quantitative problem. Sickle-cell anaemia is a qualitative problem.

Colour blindness: 

• Sex-linked recessive disorder due to defect in red or green cone of eye. 
• Fails to discriminate red and green colour. 
• It occurs in 8% of males and 0.4% of females because the genes are X-linked.

Phenylketonuria: 

• Inborn error of metabolism. 
• Autosomal recessive disease. 
• Due to mutation of a gene coding for phenyl alanine hydroxylase enzyme (it converts phenylalanine to tyrosine).
• Affected individual lacks this enzyme. So, phenylalanine becomes phenyl pyruvic acid.
• They accumulate in brain causing mental retardation. These are also excreted through urine.

2. Chromosomal Disorders

Due to change in number or structure of chromosome.

a. Down’s syndrome:

Presence of an additional chromosome number 21 (21 trisomy).

Genetic constitution: 45 A + XX or 45 A + XY.

Features: 

• Short stature, small round head. Broad flat face.
• Furrowed big tongue and partially open mouth.
• Retarded physical, psychomotor & mental development.

Klinefelter’s Syndrome:

Presence of an additional X-chromosome in male.

Genetic constitution: 44 A + XXY

Features:

• Development of breast (Gynaecomastia).
• Sterile.

Turner’s syndrome:

Absence of an X chromosome in female.

Genetic constitution: 44 A + X0

Features:

• Sterile, Ovaries are rudimentary.
• Lack of other secondary sexual characters.
• Dwarf.

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