Principles of Inheritance and Variation Class 12 Notes

Gregor Johann Mendel, the father of genetics, conducted hybridization experiments on garden pea (Pisum sativum) for seven years (1856–1863) with 29,000 plants. He chose pea plants for their short life cycle, easily distinguishable contrasting characters, natural self-pollination (and ability to cross-pollinate manually), and pure-breeding lines.

Monohybrid Cross

Dihybrid Cross

• Test cross: crossing an F₁ individual with the homozygous recessive parent to determine its genotype. If all offspring show the dominant trait, the parent was homozygous; if 1:1 ratio appears, the parent was heterozygous.
• Back cross: crossing an F₁ individual with any of the two homozygous parental types.

Incomplete Dominance

Co-dominance

• Multiple alleles: More than two alleles exist for a gene in a population (e.g. ABO system has Iᵃ, Iᵇ and i). An individual still carries only two.
• Pleiotropy: A single gene affects multiple unrelated phenotypic characters (e.g. pea gene for seed shape also affects flower colour and pod shape).

Polygenic Inheritance

T.H. Morgan's experiments with Drosophila melanogaster demonstrated that genes on the same chromosome tend to be inherited together — this is linkage. Linked genes do not follow independent assortment. Recombination is the formation of new combinations of linked genes due to crossing over during meiosis, producing recombinant offspring that differ from parental types.

Sex Determination

• XX/XY system: Females are XX (homogametic), males are XY (heterogametic). The Y chromosome carries the SRY gene that triggers male development.
• ZW/ZZ system (birds): Females are ZW (heterogametic), males are ZZ (homogametic).
• Criss-cross (X-linked) inheritance: A trait on the X chromosome is passed from a carrier mother to affected sons, and from an affected father to carrier daughters.

Pedigree analysis is the study of inheritance of traits through generations of a family using standard symbols. Squares represent males, circles represent females, shaded shapes are affected individuals, and half-shaded are carriers. It helps determine whether a trait is dominant or recessive, autosomal or sex-linked, and predicts the probability of a trait appearing in offspring.

• Autosomal dominant: affected individuals appear in every generation; affected individuals have at least one affected parent.
• Autosomal recessive: affected individuals may skip generations; affected offspring can be born to unaffected (carrier) parents.
• X-linked recessive: more males affected; no male-to-male transmission; affected daughters have an affected father.

Chromosomal Disorders

• Down syndrome (Trisomy 21): 2n = 47, caused by the presence of an extra copy of chromosome 21. Symptoms: mental retardation, characteristic facial features (flat face, protruding tongue), short stature, single palmar crease.
• Turner syndrome: XO (2n = 45), absence of one X chromosome in females. Symptoms: sterile female, short stature, webbed neck, shield-shaped chest.
• Klinefelter syndrome: XXY (2n = 47), an extra X chromosome in males. Symptoms: sterile male, gynaecomastia (breast enlargement), long limbs.

Mendelian Disorders

• Sickle-cell anaemia: autosomal recessive; point mutation in β-globin gene (GAG → GUG); HbS polymerizes in low oxygen.
• Phenylketonuria (PKU): autosomal recessive; deficiency of phenylalanine hydroxylase; untreated causes intellectual disability.
• Thalassemia: autosomal recessive; reduced synthesis of α or β globin chains; results in anaemia of varying severity.
• Cystic fibrosis: autosomal recessive; mutation in CFTR gene; thick mucus in lungs, pancreas.

Example: In a pedigree, you observe that a trait appears in every generation and every affected individual has at least one affected parent. Affected males and females appear with equal frequency. What is the mode of inheritance?

Solution: The trait appearing in every generation and requiring only one affected parent indicates an autosomal dominant mode of inheritance. Equal frequency in males and females rules out X-linked inheritance. There are no skipped generations, which is characteristic of dominant traits. If the trait were recessive, affected individuals could have unaffected parents (carriers).

Example: A colour-blind man marries a woman with normal vision whose father was colour-blind. What proportion of their children will be affected? Show your work using a Punnett square approach.

Solution: The man is XᶜY (affected). The woman's father was XᶜY, so she must have inherited Xᶜ from him; since she has normal vision, she is a carrier XᶜX. Cross: XᶜY × XᶜX → daughters: XᶜXᶜ (affected, 50%), XᶜX (carrier, 50%); sons: XᶜY (affected, 50%), XY (normal, 50%). So 50% of sons and 50% of daughters will be affected — overall 50% of children affected.

Exercises (NCERT)

1. 1
   Mention the advantages of selecting pea plant for experimentation by Mendel.
2. 2
   What is the law of dominance? Explain with an example.
3. 3
   Why did Mendel choose pea plant for his experiments?
4. 4
   A man with blood group A marries a woman with blood group O. Work out a cross to show the possible blood groups of the offspring.
5. 5
   Explain the law of independent assortment with a dihybrid cross.
6. 6
   What is a test cross? How is it useful?
7. 7
   Differentiate between incomplete dominance and co-dominance.
8. 8
   What are multiple alleles? Give an example.
9. 9
   Explain the chromosomal theory of inheritance.
10. 10
    What is linkage? Who discovered it?
11. 11
    Describe the mechanism of sex determination in humans.
12. 12
    What are sex-linked disorders? Give two examples.
13. 13
    Differentiate between haemophilia and colour blindness.
14. 14
    What is a pedigree analysis? What is its use?
15. 15
    What are Mendelian disorders? Give two examples.
16. 16
    What are chromosomal disorders? Give two examples.
17. 17
    What is Down's syndrome? How is it caused?
18. 18
    What is Turner's syndrome? Give its symptoms.
19. 19
    What is Klinefelter's syndrome?
20. 20
    What is sickle-cell anaemia? How is it inherited?

Answers at a glance

1. 1
   Pea plant has a short life cycle, self-pollinates, has clear contrasting traits, produces many seeds and is easy to cross.
2. 2
   The law of dominance states that in a heterozygous pair of contrasting factors, only the dominant factor is expressed; e.g. Tt shows tall.
3. 3
   Mendel chose pea because it is self-pollinating, has true-breeding varieties and shows clear-cut contrasting characters.
4. 4
   Possible blood groups of the offspring: A and O (in a 1:1 ratio).
5. 5
   The law of independent assortment: the alleles of different genes segregate independently; a dihybrid cross gives a 9:3:3:1 ratio.
6. 6
   A test cross is a cross with the recessive parent to determine the genotype of the dominant individual.
7. 7
   Incomplete dominance: the heterozygote shows an intermediate phenotype (e.g. pink snapdragon). Co-dominance: both alleles express fully (e.g. AB blood group).
8. 8
   Multiple alleles: more than two alleles for a gene in a population; e.g. the ABO blood group system (I^A, I^B, i).
9. 9
   The chromosomal theory of inheritance states that genes are located on chromosomes and their behaviour parallels that of chromosomes during meiosis.
10. 10
    Linkage is the tendency of genes on the same chromosome to be inherited together; discovered by Bateson and Punnett (and explained by Morgan).
11. 11
    In humans, sex is determined by the sex chromosomes: females are XX and males are XY; the male gamete determines the sex of the child.
12. 12
    Sex-linked disorders are caused by genes on the sex chromosomes; e.g. haemophilia and colour blindness.
13. 13
    Haemophilia is a recessive X-linked bleeding disorder; colour blindness is a recessive X-linked defect of red/green colour vision.
14. 14
    Pedigree analysis is the study of the inheritance of a trait across generations using a family tree.
15. 15
    Mendelian disorders are caused by a single gene; e.g. haemophilia and sickle-cell anaemia.
16. 16
    Chromosomal disorders are due to changes in chromosome number or structure; e.g. Down's syndrome and Turner's syndrome.
17. 17
    Down's syndrome is due to trisomy of chromosome 21 (47 chromosomes).
18. 18
    Turner's syndrome (45, XO) causes a sterile female with short stature.
19. 19
    Klinefelter's syndrome (47, XXY) causes a sterile male with some feminine features.
20. 20
    Sickle-cell anaemia is an autosomal recessive disorder caused by a substitution of glutamic acid by valine in the beta-globin chain.

MCQ Questions

2 Mark Questions

1. 1
   State the law of dominance.
2. 2
   What is a test cross?
3. 3
   Differentiate between incomplete dominance and co-dominance.
4. 4
   Why is colour blindness more common in males?
5. 5
   What is a pedigree analysis?

3 Mark Questions

1. 1
   Explain the law of independent assortment with an example.
2. 2
   Describe sex determination in humans.
3. 3
   What is linkage? How did Morgan explain it?
4. 4
   Explain sickle-cell anaemia.
5. 5
   Describe Down's, Turner's and Klinefelter's syndromes.

5 Mark Questions

1. 1
   State and explain Mendel's three laws of inheritance.
2. 2
   Describe the various extensions of Mendelism with examples.
3. 3
   Explain the chromosomal theory of inheritance and sex-linked inheritance.