Principles of Inheritance and Variation NEET Notes 2027: PDF

Principles of Inheritance and Variation is Chapter 4 of NCERT Class 12 Biology and part of Unit 7, Genetics and Evolution, of the NEET UG syllabus. These notes cover Mendel’s laws, incomplete dominance, co-dominance, multiple alleles, linkage, polygenic inheritance, pleiotropy, sex determination, mutation and Mendelian and chromosomal disorders, with a free notes PDF.

Principles of Inheritance and Variation NEET Notes

Section-wise key points from NCERT Chapter 4, ratio and disorder tables, high-yield facts and common mistakes for NEET 2027 revision.

Principles of Inheritance and Variation at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

Mendel’s Experiments and Laws of Inheritance

Genetics deals with inheritance, the passing of characters from parent to progeny, and variation, the degree by which progeny differ from their parents.

• Gregor Mendel did hybridisation experiments on garden peas for seven years (1856 to 1863). It was the first time statistical analysis and mathematical logic were applied to biology.
• He selected 14 true-breeding pea varieties, as pairs differing in one character with contrasting traits. A true-breeding line shows stable trait inheritance after continuous self-pollination.
• Mendel’s ‘factors’ are now called genes, the units of inheritance. Alleles are slightly different forms of the same gene that code for a pair of contrasting traits.

Inheritance of One Gene: Monohybrid Cross

In a cross between true-breeding tall (TT) and dwarf (tt) plants, all F1 plants are tall (Tt), and the F2 shows a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

• TT and tt are homozygous; Tt is heterozygous. TT, Tt and tt are genotypes; ‘tall’ and ‘dwarf’ are phenotypes.
• Traits do not blend at F1 or F2: plants are either tall or dwarf, never in between.
• The Punnett square, developed by British geneticist Reginald C. Punnett, shows all possible offspring genotypes of a cross.
• The ratio 1/4 TT : 1/2 Tt : 1/4 tt is the binomial expansion of (1/2 T + 1/2 t)2.
• Dwarf F2 plants self-pollinated gave dwarfs in F3 and F4, so dwarfs are homozygous tt.
• Test cross: an organism with the dominant phenotype is crossed with the recessive parent to find its genotype.

Incomplete Dominance

• In snapdragon (dog flower, Antirrhinum sp.), red (RR) x white (rr) gives pink F1 (Rr).
• F2 ratio is 1 red (RR) : 2 pink (Rr) : 1 white (rr), so the phenotypic and genotypic ratios are the same.
• A recessive trait usually appears because the modified allele makes a non-functional enzyme or no enzyme.

Co-dominance and Multiple Alleles: ABO Blood Groups

• ABO blood groups are controlled by gene I with three alleles: IA, IB and i. IA and IB make slightly different sugars on the RBC surface; i makes none.
• IA and IB are completely dominant over i, but when IA and IB are together both express (co-dominance), giving blood group AB.
• Three alleles give six genotypes and four blood groups. ABO is also an example of multiple alleles, which can be found only in population studies, since one individual carries only two alleles.

In pea seeds one gene (B, b) controls starch synthesis: BB seeds are round with large starch grains, bb seeds are wrinkled with small grains, and Bb seeds are round but with intermediate-sized grains. So dominance depends on which phenotype is examined.

Inheritance of Two Genes: Dihybrid Cross and Chromosomal Theory

Crossing round yellow (RRYY) with wrinkled green (rryy) peas gives round yellow F1 (RrYy), and the F2 shows a 9:3:3:1 phenotypic ratio.

• The F1 RrYy forms four types of gametes, RY, Ry, rY and ry, each with a frequency of 1/4.
• 9:3:3:1 = (3 round : 1 wrinkled) x (3 yellow : 1 green): 9 round yellow, 3 wrinkled yellow, 3 round green, 1 wrinkled green.
• Mendel published his work in 1865, but it stayed unrecognised till 1900, when de Vries, Correns and von Tschermak independently rediscovered it.
• Reasons it was ignored: poor communication, factors as discrete non-blending units were not accepted, mathematics in biology was new, and he gave no physical proof of factors.
• Walter Sutton and Theodore Boveri noted that chromosomes behave like genes. Sutton united chromosomal segregation with Mendelian principles as the chromosomal theory of inheritance.
• Thomas Hunt Morgan verified it experimentally using Drosophila melanogaster: easy to grow on synthetic medium, life cycle of about two weeks, many progeny from one mating, clear sex differences and many visible hereditary variations.

Linkage and Recombination

• Morgan’s dihybrid crosses with X-linked genes in Drosophila deviated very significantly from 9:3:3:1.
• Linkage is the physical association of genes on a chromosome; recombination is the generation of non-parental gene combinations. Both terms were coined by Morgan.
• Genes white and yellow were tightly linked (1.3 per cent recombination); white and miniature wing showed 37.2 per cent recombination.
• Alfred Sturtevant used recombination frequency as a measure of distance between genes to map their positions on a chromosome.

Polygenic Inheritance and Pleiotropy

Polygenic traits are controlled by three or more genes, while a pleiotropic gene shows more than one phenotypic effect.

Sex Determination

Henking (1891) traced an ‘X body’ in spermatogenesis of some insects; it was later shown to be a chromosome and named the X-chromosome.

• In humans 22 of the 23 pairs are autosomes. Half the sperms carry X and half carry Y; all ova carry X.
• The sperm decides the sex of the child, and each pregnancy has a 50 per cent chance of a male or a female child.
• In honey bees a fertilised egg develops into a female and an unfertilised egg into a male (drone) by parthenogenesis. Males produce sperms by mitosis, have no father and cannot have sons, but have a grandfather and can have grandsons.

Mutation

Mutation is an alteration of DNA sequences that changes the genotype and phenotype; along with recombination, it leads to variation.

• Loss (deletion) or gain (insertion or duplication) of a DNA segment alters chromosomes. Chromosomal aberrations are commonly seen in cancer cells.
• A change in a single base pair is a point mutation; sickle-cell anaemia is the classic example.
• Deletions and insertions of base pairs cause frame-shift mutations.
• Chemical and physical factors that induce mutations are mutagens; UV radiation is one.

Genetic Disorders: Mendelian and Chromosomal

Pedigree analysis, the study of a trait across generations of a family tree, is used in humans because controlled crosses are not possible.

• Mendelian disorders come from alteration or mutation in a single gene and may be dominant or recessive. Examples: haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia.
• NCERT’s pedigree examples: myotonic dystrophy (autosomal dominant) and sickle-cell anaemia (autosomal recessive).

Chromosomal Disorders

• Caused by absence, excess or abnormal arrangement of chromosomes. Failure of chromatid segregation gives aneuploidy; failure of cytokinesis after telophase gives polyploidy, often seen in plants.
• A normal human cell has 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes. An extra copy is trisomy and a missing one is monosomy.

Most-asked Facts for NEET: Principles of Inheritance and Variation

Revise these one-line facts from the NCERT chapter just before the exam.

• Mendel worked on garden peas for seven years (1856 to 1863) and studied seven pairs of contrasting traits.
• Monohybrid F2: 3:1 phenotypic, 1:2:1 genotypic; dihybrid F2: 9:3:3:1 phenotypic.
• Test cross: dominant phenotype crossed with the recessive parent.
• Snapdragon flower colour shows incomplete dominance (1:2:1 in F2).
• ABO blood groups show co-dominance (IA, IB) and multiple alleles: six genotypes, four phenotypes.
• Mendel’s work was rediscovered in 1900 by de Vries, Correns and von Tschermak.
• Chromosomal theory of inheritance: Sutton and Boveri; verified by Morgan on Drosophila.
• Morgan coined linkage and recombination; Sturtevant made the first genetic maps.
• Human skin colour is polygenic; phenylketonuria shows pleiotropy.
• Henking (1891) named the X body; grasshopper has XO sex determination.
• Birds: ZW female, ZZ male (female heterogamety).
• Honey bee: females diploid (32), drones haploid (16) by parthenogenesis.
• Sickle-cell anaemia: Glu to Val at position 6 of beta globin, GAG to GUG; a point mutation.
• Colour blindness affects about 8 per cent of males and 0.4 per cent of females.
• Alpha thalassemia: HBA1 and HBA2 on chromosome 16; beta thalassemia: HBB on chromosome 11.
• Down’s syndrome: trisomy 21; Klinefelter’s: 47, XXY; Turner’s: 45, X0.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Assuming the F2 genotypic ratio of a dihybrid cross is 9:3:3:1. That is the phenotypic ratio.
• Mixing up incomplete dominance and co-dominance. In incomplete dominance the F1 is intermediate (pink); in co-dominance both alleles express fully (blood group AB).
• Saying a person can carry all three ABO alleles. An individual has only two; multiple alleles are seen in a population.
• Writing that the mother decides the sex of a human child. The sperm (X or Y) decides it.
• Applying male heterogamety to birds. In birds the female (ZW) is heterogametic.
• Saying honey bee drones produce sperms by meiosis. Drones are haploid and produce sperms by mitosis.
• Calling thalassemia sex-linked. It is autosomal recessive; haemophilia and colour blindness are X-linked.
• Treating thalassemia and sickle-cell anaemia as the same kind of defect. Thalassemia is quantitative (too few globin chains); sickle-cell is qualitative (an incorrectly functioning globin).
• Swapping Klinefelter’s and Turner’s karyotypes. Klinefelter’s is 47, XXY (male); Turner’s is 45, X0 (female).
• Calling aneuploidy polyploidy. Aneuploidy is gain or loss of chromosomes; polyploidy is an increase in whole sets.

Principles of Inheritance and Variation Notes PDF and Related Pages

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• Source text: NCERT Chapter 4 (NCERT, Class 12 Biology)

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