Mendel and one gene

Children resemble their parents, yet no two are exactly alike. Genetics studies both: inheritance, how characters pass from parents to offspring, and variation, how offspring differ from their parents and from each other.

Gregor Mendel, a monk, worked out the first rules by crossing garden peas for seven years (1856-1863). He chose peas wisely: they grow fast, can be cross-pollinated by hand, and come in clear-cut varieties. He used 14 true-breeding varieties (plants that always give offspring like themselves), forming seven pairs of contrasting traits, such as tall and dwarf stems, round and wrinkled seeds, green and yellow pods.

A monohybrid cross follows one trait. Write T for the tall allele and t for dwarf; each plant carries two copies.

• Parents: true-breeding tall TT × dwarf tt.
• F₁ (first generation): all tall, Tt. The dwarf trait has not vanished; it is hidden. There is no blending: no medium-height plants.
• Self the F₁ (Tt × Tt). Each parent gives T or t, each half the time, so the F₂ is 1 TT : 2 Tt : 1 tt.
• In appearance, that is 3 tall : 1 dwarf. The dwarf reappears unchanged.

A Punnett square (a grid with one parent's gametes on top and the other's down the side) is the easiest way to work this out.

Mendel explained his results with two laws:

• Law of Dominance: characters are controlled by units called factors (now: genes), which occur in pairs. When the pair is unlike, one (the dominant) shows and the other (the recessive) is hidden.
• Law of Segregation: the two alleles separate when gametes form, so each gamete gets only one. A heterozygous Tt plant makes two kinds of gametes, T and t, in equal numbers.

Some words: the genotype is the genetic make-up (TT, Tt, tt); the phenotype is what you see (tall, dwarf). Homozygous means the two alleles are the same (TT, tt); heterozygous, different (Tt).

The test cross. A tall plant may be TT or Tt: they look exactly the same, just as two people of the same height give no clue about their genes. To find out, cross it with the recessive parent, a dwarf tt. If all the offspring are tall, the plant was homozygous TT; if half are tall and half dwarf (1 : 1), it was heterozygous Tt.