Principles of Inheritance and Variation Class 12 Notes वंशागति तथा विविधता के सिद्धांत — कक्षा 12 जीव विज्ञान अध्याय 4

Revise these first (prerequisites)

• Basic meaning of gene, allele, chromosome
• Meiosis: homologous chromosomes separate (Class 11)
• The simple idea of dominant vs recessive (Class 10)
• Basic probability / multiplication (for the Punnett square)

1. Why do children look like their parents?

   Characters (such as height, eye colour) pass from parents to offspring — this is called inheritance / वंशागति. At the same time, siblings are not identical — this is variation / विविधता. This chapter explains both.

2. Gene and allele

   Each character is controlled by a gene, and each gene can exist in two forms — these are called alleles (विकल्पी), e.g., T (tall) and t (dwarf). Every organism has 2 alleles of a gene — one from the mother and one from the father.

3. Dominant and recessive

   When two different alleles are together (Tt), the one that is expressed is dominant (प्रभावी) (T), and the one that stays hidden is recessive (अप्रभावी) (t). A recessive trait shows only when both alleles are recessive (tt). It is like two people talking, where one voice is so loud that the other cannot be heard at all.

4. Mendel's experiment

   Gregor Mendel performed experiments on the garden pea for 7 years (1856–1863). When he crossed tall (TT) × dwarf (tt), all F1 plants were tall, and on selfing the F1, the F2 showed tall : dwarf = 3 : 1. This showed that the dwarf factor had not disappeared; it was only hidden.

5. Punnett square = a table

   A Punnett square is a simple table: gametes of one parent along the top, gametes of the other along the side, and their combinations in the boxes. Tt × Tt → TT, Tt, Tt, tt, i.e., a 1 : 2 : 1 genotypic and 3 : 1 phenotypic ratio. With a little practice, this becomes the easiest scoring part.

6. How is sex determined?

   In humans, females are XX and males are XY. Every ovum carries an X, whereas a sperm carries either X or Y — so the sex of the baby is decided by the sperm. This is a biological fact; it is not the mother's 'fault' in any way.

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As per NCERT 2026-27 — every point you need for the board exam.

1. Heredity & variation

   Inheritance = passing of characters from parents to offspring. Variation = difference of offspring from parents. Mendel experimented on garden pea (Pisum sativum) from 1856–1863; he chose 7 contrasting traits.

2. Monohybrid cross

   Tall (TT) × dwarf (tt) → F1 all Tall (Tt). F1 selfing → F2 phenotypic ratio 3:1, genotypic ratio 1:2:1. Punnett square: Reginald C. Punnett.

3. Mendel's laws

   Law of Dominance: factors occur in pairs, one dominant and the other recessive. Law of Segregation: alleles separate during gamete formation (law of purity of gametes). Law of Independent Assortment: two pairs of traits segregate independently (from dihybrid cross).

4. Test cross

   Crossing a dominant-phenotype individual (TT or Tt?) with a recessive parent (tt). If the result is 1:1, it is heterozygous; if all are dominant, it is homozygous.

5. Incomplete dominance

   Snapdragon / dog flower (Antirrhinum): red (RR) × white (rr) → F1 pink (Rr). In F2, both the phenotypic and genotypic ratios are 1:2:1.

6. Co-dominance & multiple alleles

   ABO blood group: gene I has 3 alleles — Iᴬ, Iᴮ, i. Iᴬ and Iᴮ are co-dominant (AB blood group), i is recessive. 6 genotypes, 4 phenotypes. 3 alleles = multiple alleles.

7. Pleiotropy & polygenic

   Pleiotropy: one gene, many effects (phenylketonuria). Polygenic: many genes together control one trait (human skin colour, height).

8. Dihybrid cross

   Round yellow (RRYY) × wrinkled green (rryy) → F2 9:3:3:1. This gave the Law of Independent Assortment.

9. Chromosomal theory

   Sutton and Boveri linked genes to chromosomes. Morgan worked on Drosophila → linkage (genes on the same chromosome are inherited together) and recombination. Alfred Sturtevant: gene mapping using recombination frequency.

10. Sex determination

    Humans: XX female, XY male (male heterogametic — the sex of the child is decided by the father's sperm). Grasshopper: XO type. Birds: ZW female (female heterogametic), ZZ male. Honey bee: haplodiploid — fertilised egg = female (2n), unfertilised = male drone (n).

11. Mutation & pedigree

    Mutation = a change in the DNA sequence. Example of a point mutation: sickle cell anaemia. Frameshift: insertion/deletion of bases. Pedigree analysis is used to track the inheritance of a trait in a family.

12. Mendelian disorders

    Haemophilia (sex-linked recessive, blood fails to clot; Queen Victoria's pedigree). Colour blindness (sex-linked recessive, on X). Sickle cell anaemia (autosomal recessive; HbS; GAG → GUG, glutamic acid → valine, at the 6th position of β-globin). Phenylketonuria (autosomal recessive). Thalassemia (autosomal recessive; reduced synthesis of α or β globin).

13. Chromosomal disorders

    Aneuploidy (gain/loss of one chromosome), polyploidy (an entire set — common in plants). Down's syndrome: trisomy of chromosome 21 (47). Klinefelter: 47, XXY male (gynaecomastia). Turner: 45, XO female (sterile).

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. Dominance is not a 'property of the gene'; it depends on what is observed

   In pea, the gene for seed shape also controls starch grain size. Looking at seed shape, round (BB, Bb) appears dominant; but looking at starch grain size, BB = large, bb = small, Bb = intermediate — i.e. incomplete dominance. So dominance depends on which phenotype of the gene product we are looking at.

2. ABO blood group: one example, two concepts

   Gene I has three alleles — I^A, I^B, i → multiple allelism (3 in the population, but only 2 in one person). When I^A and I^B are together, both are expressed (AB) → co-dominance. This gives 6 genotypes and 4 phenotypes (A, B, AB, O). Children of I^Ai × I^Bi parents can have all four blood groups.

3. How did linkage cause deviation from Mendel's 9:3:3:1?

   Mendel's traits were on separate chromosomes, so independent assortment occurred. Morgan observed in Drosophila that genes located close together on the same chromosome mostly go together (linkage), and parental combinations appear more frequently. White eye–yellow body showed only 1.3% recombination, whereas white eye–miniature wing showed 37.2% — the farther apart the genes, the greater the recombination. Using this idea, Sturtevant made gene maps.

4. Trick for reading a pedigree

   If two normal parents have an affected child → the trait is recessive. If the trait appears mostly in males and all sons of an affected father are normal → suspect X-linked recessive (e.g. haemophilia, colour blindness). Pedigree analysis is essential because controlled crosses cannot be performed on humans.

5. Sickle-cell anaemia: one base, a major disease

   At the 6th position of the β-globin chain, glutamic acid is replaced by valine, because the GAG codon in the gene changes to GUG (point mutation). Under low oxygen tension, RBCs become sickle-shaped. It is autosomal recessive — Hb^AHb^S are carriers, Hb^SHb^S are affected. This links directly to the genetic code of Chapter 5.

6. Aneuploidy vs polyploidy

   Failure of chromatids to separate during cell division (non-disjunction) causes gain or loss of a chromosome → aneuploidy: Down's (trisomy 21), Klinefelter (47, XXY), Turner (45, XO). Failure of cytokinesis increases the whole set → polyploidy, which is seen more often in plants.

Common confusions — cleared up

1. Incomplete dominance vs Co-dominance

   Incomplete dominance: the heterozygote shows an intermediate phenotype (pink in snapdragon). Co-dominance: both alleles are fully expressed together (AB blood group).

2. Pleiotropy vs Polygenic inheritance

   Pleiotropy: one gene → many traits (phenylketonuria). Polygenic: many genes → one trait (human skin colour, height).

3. Test cross vs Back cross

   A test cross is always done with a homozygous recessive parent, to find out the unknown genotype (1:1 or all dominant).

4. Who determines sex in birds?

   In birds, females are heterogametic (ZW) and males are homogametic (ZZ), so the sex is determined by the ovum — the opposite of humans.

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