Mendel studied seven contrasting traits in Pisum sativum, established pure-breeding lines, and conducted controlled cross-pollinations to derive three laws of inheritance.

The Law of Dominance states that in F1, only the dominant allele's phenotype is expressed; the Law of Segregation states that paired alleles separate during meiosis so each gamete carries one allele.

The Law of Independent Assortment states that alleles of different genes on different chromosomes assort freely during meiosis, producing a 9:3:3:1 F2 ratio in dihybrid crosses.

Incomplete dominance produces a blended intermediate F1 phenotype (e.g., pink snapdragons from red × white parents), with a 1:2:1 phenotypic ratio in F2.

Co-dominance produces both parental phenotypes simultaneously in the heterozygote (e.g., AB blood group showing both A and B antigens), also with a 1:2:1 F2 ratio.

The ABO blood group system demonstrates multiple allelism (three alleles: $I^A$, $I^B$, i), co-dominance ($I^A$ $I^B$ = AB), and dominance hierarchy ($I^A$ = $I^B$ > i), giving six genotypes and four phenotypes.

Pleiotropy describes one gene causing multiple unrelated phenotypic effects (e.g., HbS allele in sickle cell anemia affecting haemoglobin, RBCs, and multiple organs).

Polygenic inheritance describes many genes with additive effects controlling one trait (e.g., human skin colour and height), producing continuous variation in populations.

The Chromosomal Theory of Inheritance (Sutton and Boveri, 1902) established that genes are on chromosomes; Morgan demonstrated linkage in Drosophila, showing that linked genes violate Independent Assortment but can be separated by crossing over.

Gene interactions in dihybrid crosses produce modified F2 ratios: 9:7 (complementary), 9:3:4 (recessive epistasis), 12:3:1 (dominant epistasis), 15:1 (duplicate), and 13:3 (inhibitory) — all derived from the standard 16-combination dihybrid framework.