Inheritance patterns

Children often resemble their parents: the same eye color, the same hair color, sometimes the same shape of nose. That is no accident. Traits are passed from parents to children, and this process follows fixed rules. Once you know these rules, you can even predict how often a trait shows up.

Two versions for each trait

For every trait a living thing carries two versions, one from the mother and one from the father. Such versions are called alleles. During reproduction each parent passes on only one of its two versions. In this way the child again receives two alleles, one from each parent.

Dominant and recessive

When two different versions come together, one often prevails over the other. The version that shows is called dominant, the hidden one is called recessive. So a recessive trait only becomes visible when both versions are recessive. We write the dominant version with a capital letter, for example B, and the recessive one lowercase, for example b. Not every trait follows this pattern: sometimes the two versions blend, for example when a red and a white version for flower color together give a pink flower. Here we look at the traits with a clear winner.

The cross grid

To predict which versions the children can get, people use a square with two rows and two columns. Along the top you write the two versions of one parent, along the side those of the other. In each of the four boxes you enter what the top and the side bring together. Crossing Bb with Bb, the boxes hold BB, Bb, bB, and bb, and on average three out of four children show the dominant trait and one shows the recessive one.

Traits across generations

A recessive trait can skip a generation. Two parents may both show the dominant trait, yet each secretly carries one recessive version. In that case a child can show the recessive trait even though it stayed hidden in the parents. This is how some family traits reappear only in the grandchildren.