Mendel and the monohybrid cross

Children look like their parents, yet no two are exactly alike. Passing characters from parents to offspring is heredity; the differences between offspring are variation; and the science of both is genetics. You cannot change genes on purpose, as biotechnology does, without first understanding how they are inherited.

Gregor Mendel, an Austrian monk, worked with the garden pea (Pisum sativum). It was a good choice: it grows in one season, has flowers with both male and female parts, and shows many characters in two clearly different forms. He studied seven pairs of such contrasting traits (tall or dwarf plants, round or wrinkled seeds, green or yellow pods, and so on). He first made pure lines that always bred true, then crossed them by hand and counted large numbers of offspring.

Crossing tall × dwarf gave a first generation (F1) that was all tall. When the F1 plants were self-pollinated, the second generation (F2) had tall and dwarf plants in a 3:1 ratio. The dwarf form had not been lost, only hidden. Mendel proposed that each trait is controlled by two factors (now called genes), one from each parent. Tall (T) is dominant; dwarf (t) is recessive and shows only when both factors are t. The F1 plants are heterozygous (Tt).

A Punnett square (devised by Reginald Punnett) shows the F2: genotypes 1 TT : 2 Tt : 1 tt, appearance (phenotype) 3 tall : 1 dwarf. Two laws follow:

• Law of dominance: of two contrasting characters, only the dominant one shows in a heterozygote.
• Law of segregation: the two factors for a trait separate into different gametes, so each gamete carries only one.

To find out whether a tall plant is TT or Tt, cross it with a recessive (tt) plant: a test cross. If all offspring are tall, it was TT; if half are tall and half dwarf (1:1), it was Tt. Mendel published in 1866, but his work was ignored until 1900, when de Vries, Correns and von Tschermak rediscovered it.