Summary
Chapter 4 of NCERT Class 12 Biology covers Principles of Inheritance and Variation, explaining how Mendel's laws of dominance, segregation, and independent assortment govern the passing of traits from parents to offspring, along with sex determination, mutation, and genetic disorders.
Genetics is the branch of biology dealing with inheritance and variation. Gregor Mendel conducted hybridisation experiments on pea plants (1856–1863) and proposed the laws of inheritance: Law of Dominance, Law of Segregation, and Law of Independent Assortment. The chapter explains monohybrid and dihybrid crosses, incomplete dominance, co-dominance, multiple alleles (ABO blood groups), polygenic inheritance, pleiotropy, sex determination (XY, XO, ZW types), mutation, and genetic disorders including Mendelian disorders (haemophilia, sickle-cell anaemia, thalassemia, colour blindness, phenylketonuria) and chromosomal disorders (Down's, Klinefelter's, Turner's syndromes).
Key points & formulas
- 01Mendel studied 7 contrasting traits in pea plants over 7 years (1856–1863) and proposed three laws: Law of Dominance, Law of Segregation, and Law of Independent Assortment.
- 02In a monohybrid cross, the F1 generation shows only the dominant trait; F2 produces a 3:1 phenotypic ratio and 1:2:1 genotypic ratio (TT:Tt:tt).
- 03In a dihybrid cross, F2 offspring appear in a 9:3:3:1 phenotypic ratio, demonstrating that allele pairs segregate independently of each other.
- 04ABO blood groups illustrate co-dominance (IA and IB both expressed) and multiple alleles (three alleles: IA, IB, i) controlling the same character.
- 05Sex determination in humans is XY type: females are XX, males are XY; the sex of the child is determined by whether the sperm carries an X or Y chromosome.
- 06Genetic disorders are classified as Mendelian (single-gene mutations — e.g., sickle-cell anaemia caused by Glu→Val substitution at position 6 of the beta globin chain) or chromosomal (e.g., Down's syndrome — trisomy of chromosome 21).
Frequently asked questions
01What are Mendel's three laws of inheritance?
Mendel proposed (1) the Law of Dominance — in a dissimilar pair of factors, one (dominant) masks the other (recessive); (2) the Law of Segregation — alleles separate during gamete formation so each gamete carries only one allele; and (3) the Law of Independent Assortment — when two pairs of traits are combined in a hybrid, segregation of one pair is independent of the other, producing a 9:3:3:1 ratio in dihybrid F2 offspring.
02How is sex determined in human beings?
Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females have XX and males have XY. During spermatogenesis, 50% of sperm carry an X chromosome and 50% carry a Y chromosome. All eggs carry an X chromosome. If a Y-bearing sperm fertilises the egg, the offspring is male (XY); if an X-bearing sperm fertilises the egg, the offspring is female (XX). It is therefore the sperm, not the egg, that determines the sex of the child.
03What is the difference between sickle-cell anaemia and thalassemia?
Both are autosomal recessive blood disorders. Sickle-cell anaemia is a qualitative problem: a single base substitution in the beta globin gene (GAG→GUG) replaces glutamic acid with valine at position 6, causing haemoglobin to polymerise under low oxygen and deform red blood cells into a sickle shape. Thalassemia is a quantitative problem: mutations or deletions reduce the rate of synthesis of alpha or beta globin chains, leading to abnormal haemoglobin and anaemia without a structurally incorrect protein.
04Is the NCERT Class 12 Biology Chapter 4 PDF free to download?
Yes. The NCERT Class 12 Biology Chapter 4 PDF — Principles of Inheritance and Variation — is free to download on cbseprepmaster.com.
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