Heredity

Children often resemble their parents: the same hair color, the same nose shape, the same eye color. That is no coincidence. Traits are passed on through genes, a kind of instruction set that you receive from your mother and your father. For each trait you inherit two versions, one from each parent.

Dominant and recessive

The two versions do not both come through equally strongly. A dominant version is stronger and shows up even when it is present only once. It is written with a capital letter, for example B. A recessive version is weaker and only shows up when both versions are recessive. It gets a lowercase letter, for example b. So in the combination Bb, the dominant B wins. A living thing with two identical versions, BB or bb, is pure. With one capital and one lowercase version, Bb, it is mixed.

The Punnett square

With the Punnett square you can predict which traits the offspring will probably show. You write the mother's versions along the top and the father's along the side. The four cells then hold the possible combinations. If you cross Bb with Bb, you get the four cells BB, Bb, Bb, and bb. Only that one bb cell shows the recessive trait, so 1 out of 4. With real offspring this ratio holds on average, but a single batch can turn out a little differently.

Mendel and the peas

These rules were discovered by the monk Gregor Mendel, who published his results in 1866, more than 150 years ago. In the monastery garden he crossed thousands of pea plants and counted exactly how many offspring had yellow or green seeds. Yellow turned out to be the dominant version, green the recessive one. That is how he found the fixed ratios by which traits are inherited. For single traits like Mendel's pea color this one-gene model works well. In humans, however, eye color is more complicated: several genes are involved, so a single dominant-recessive pair is not enough.