You cannot delve too deeply into genealogy without considering the genetics of the members of your family. It can be fun to try and figure out where a relative got their blue eyes from. After that, you can predict what relatives may have children who will have blue eyes. It’s all about dominant and recessive genes.
Eye color is one of the things that is determined by the presence or absence of dominant or recessive genes. First, you need to figure out the phenotype and genotype of each of the parents. The phenotype of a person is something you can observe. In this example, the phenotype will be seen when you look at the color of a person’s eyes. The genotype refers to the genetic makeup of a person, and may or may not be observable.
Brown eyes are dominant. Blue eyes are recessive. If one parent has blue eyes, this means that he carries two recessive genes for eye color. If the other parent has brown eyes, it either means that she has two dominant genes, or that she has one dominant gene and one recessive gene. The recessive gene will not be expressed if there is a dominant gene present (in the case of eye color).
One way to put this all together is to put the information about each parent into a punnet square. This system was designed by Mendel, and uses probability to determine what the offspring of two parents will be like. It sort of looks like a small tic-tac-toe board, with the genes of one parent going across the top, and the other parent’s genes going down the side. Drag one gene from each parent into the boxes. Now, you can see the potential eye color the offspring that these two parents would produce.
There is a one in four chance that these two parents will have a child with brown eyes, who carries two dominant genes for eye color. There is a two in four chance that they will produce a child who has brown eyes, but carries one dominant gene (for brown eyes) and one recessive gene (for blue eyes). There is a one in four chance that they will give birth to a child who has blue eyes, and carries two recessive genes for eye color. This same probability is true for each child that they have. It does not necessarily mean that they can only have one child with blue eyes.
This same process can be used to find out if you are at risk for certain diseases or cancers. The more you know about the genetic potential of your ancestors, the better you can predict your own health.