Gregor Johann Mendel's experiments on Pisum sativum (garden pea) between 1856 and 1863 established the foundational principles of heredity. Mendel chose peas deliberately: the plant is naturally self-pollinating (facilitating controlled crosses), has a short generation time, produces large numbers of offspring suitable for statistical analysis, and displays seven pairs of clearly contrasting traits — including stem height (tall vs dwarf), flower colour (violet vs white), seed shape (round vs wrinkled), and seed colour (yellow vs green). Critically, these seven traits reside on different chromosome pairs, which — though unknown to Mendel at the time — allowed each trait to assort independently from the others.
From monohybrid crosses (involving one pair of contrasting traits), Mendel derived two laws. The Law of Dominance states that when two pure-breeding parents with contrasting traits are crossed, the F1 generation expresses only the dominant trait; the recessive trait is entirely masked. The Law of Segregation (First Law) states that the two alleles for any trait separate during gamete formation, so that each gamete carries only one allele. When the F1 heterozygotes (Tt) are self-crossed, the F2 generation produces a 3:1 phenotypic ratio (3 tall : 1 dwarf) and a 1:2:1 genotypic ratio (1 TT : 2 Tt : 1 tt). The test cross — crossing an individual of unknown genotype with a homozygous recessive (tt) — is the practical tool to determine whether a dominant phenotype individual is homozygous (TT, yielding all dominant offspring) or heterozygous (Tt, yielding a 1:1 ratio).
From dihybrid crosses involving two gene pairs, Mendel formulated the Law of Independent Assortment (Second Law): alleles of different genes assort independently during gamete formation, producing a 9:3:3:1 F2 phenotypic ratio. This law holds true exclusively when the two genes are located on different (non-homologous) chromosomes. When genes are physically linked on the same chromosome, they violate independent assortment and tend to be inherited together.
Inheritance is not always strictly Mendelian. Incomplete dominance produces a blended intermediate phenotype in the F1 heterozygote, with a 1:2:1 phenotypic ratio (equal to genotypic ratio) in F2. The classic example is snapdragons: red (RR) × white (rr) → pink F1 (Rr). Co-dominance differs fundamentally: both alleles are fully and simultaneously expressed in the heterozygote. The ABO blood group system is the canonical example — the genotype (blood group AB) expresses both A and B antigens on red blood cells, neither masking nor blending with the other.
The ABO system also demonstrates multiple allelism: three alleles (, , and i) exist for this single gene locus within the human population. These three alleles generate six possible genotypes and four phenotypes (A, B, AB, O). The dominance hierarchy is = > i — and are co-dominant with each other but both dominant over the recessive i allele. Blood group O (genotype ii) carries no antigens and is the universal donor; blood group AB has no plasma antibodies and is the universal recipient.
Pleiotropy describes the phenomenon whereby a single gene influences multiple, seemingly unrelated phenotypic traits. The HbS allele in sickle cell anemia is the textbook example: a single nucleotide mutation in the beta-globin gene causes abnormal haemoglobin polymerisation, leading to sickle-shaped RBCs, haemolytic anaemia, impaired oxygen transport, and progressive organ damage — all from one allele. Polygenic inheritance is the conceptual opposite: multiple genes, each contributing small additive effects, collectively determine a single trait. Human skin colour and height are polygenic traits, exhibiting the continuous bell-curve distribution characteristic of additive gene action.
Sutton and Boveri's Chromosomal Theory of Inheritance (1902) provided the physical basis for Mendel's laws by demonstrating that chromosomes behave during meiosis exactly as Mendel's alleles behave: they occur in homologous pairs, separate during meiosis (corresponding to Segregation), and non-homologous pairs orient independently at Metaphase I (corresponding to Independent Assortment). Thomas Hunt Morgan subsequently demonstrated gene linkage in Drosophila melanogaster: genes on the same chromosome are inherited together but can be separated by crossing over during Prophase I of meiosis. The frequency of recombinant offspring from a test cross measures the physical distance between linked genes, forming the basis of genetic mapping. One centimorgan (cM) equals 1% recombination frequency.
Dihybrid crosses involving gene interactions produce modified F2 ratios based on which classes of the standard 9:3:3:1 are merged: complementary gene interaction yields 9:7; recessive epistasis yields 9:3:4; dominant epistasis yields 12:3:1; duplicate gene interaction yields 15:1; and inhibitory interaction yields 13:3. All modified ratios sum to 16 (reflecting the 4 × 4 = 16 total combinations from a dihybrid cross).
For NEET preparation, the most frequently tested concepts from this topic are: Punnett square calculations for monohybrid and dihybrid crosses, the distinction between incomplete dominance and co-dominance, ABO blood group genetics and transfusion compatibility, pleiotropy versus polygenic inheritance, the limitation of Independent Assortment for linked genes, and the identification of modified dihybrid ratios with their gene interaction types. Mastery of Mendel's foundational framework and its principal exceptions provides a complete conceptual toolkit for 4–5 marks of NEET Biology per year.