Mendelian inheritance

The essentials

Gregor Mendel performed hybridisation experiments on garden pea (Pisum sativum) for about seven years (1856–1863). He chose pea because it has many contrasting traits, is easy to grow, and can be self- or cross-pollinated. He studied seven pairs of contrasting characters, such as tall/dwarf stem, round/wrinkled seed and green/yellow seed colour. He used true-breeding lines, kept large sample sizes and applied statistics to his results.

In a monohybrid cross between tall (TT) and dwarf (tt) plants, all F₁ plants were tall. On selfing, the F₂ showed tall and dwarf in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio (TT : Tt : tt). From such crosses Mendel proposed:

• Law of dominance: characters are controlled by discrete units called factors that occur in pairs; in a dissimilar pair, one (dominant) expresses itself over the other (recessive).
• Law of segregation: the two alleles of a pair separate during gamete formation, so each gamete receives only one allele.

In a dihybrid cross (e.g. RRYY round yellow × rryy wrinkled green), F₂ showed a 9:3:3:1 ratio. This led to the law of independent assortment: when two pairs of traits are combined in a hybrid, segregation of one pair is independent of the other.

Key points

• Test cross: crossing an organism of dominant phenotype with a homozygous recessive to find its genotype. Monohybrid test cross of a heterozygote gives 1:1; dihybrid test cross of a double heterozygote gives 1:1:1:1.
• Back cross: crossing F₁ with either parent.
• The Punnett square was developed by Reginald C. Punnett.
• Mendel's work was rediscovered in 1900 by de Vries, Correns and von Tschermak.

Common mistakes

• A 1:1 ratio in a test cross shows the dominant parent was heterozygous, not homozygous.