Principles of Inheritance and Variation Class 12 Notes

Mendel's Experiments and Laws of Inheritance

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.

Extensions of Mendelism

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

Linkage, Recombination and Sex Determination

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.

Sex-Linked Disorders

Pedigree Analysis

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 and Mendelian Disorders

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.

Solved Examples

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.