Mendelian inheritance

Alleles, dominance, and predictable ratios.

The idea

A gene can come in different versions called alleles, and a diploid organism carries two of them, one from each parent; that pair is its genotype, and the trait it shows is its phenotype. A dominant allele masks a recessive one in the heterozygote, so AA and Aa look the same and only aa shows the recessive trait. Mendel’s law of segregation says the two alleles separate into different gametes (in meiosis I), and his law of independent assortment says alleles of genes on different chromosomes are sorted independently — which is why a dihybrid cross AaBb × AaBb yields a 9 : 3 : 3 : 1 ratio. A Punnett square simply lists every gamete combination to turn those laws into predicted ratios, and a test cross against a homozygous recessive partner reveals whether a dominant-looking individual is AA (all offspring dominant) or Aa (a 1 : 1 split).

Work through an example

In pea plants tall (T) is dominant over dwarf (t). Cross two heterozygotes, Tt × Tt: each parent makes T and t gametes, the square fills with TT, Tt, Tt, and tt, and the expected ratios are 1 : 2 : 1 by genotype and 3 tall : 1 dwarf by phenotype — 300 tall and 100 dwarf in 400 offspring. Mendel’s own count was 787 tall to 277 dwarf, a ratio of 2.84 : 1. A test cross Tt × tt would give half tall and half dwarf, whereas TT × tt gives all tall.

What to watch for

Dominant does not mean common, strong, or better — it only describes which phenotype appears in the heterozygote. Huntington’s disease and polydactyly are dominant yet rare, while blood group O, a recessive trait, is the most common group in many populations. And Mendelian ratios are probabilities: four children of Aa parents are not guaranteed to come out three to one.