Predicting a child's eye color can be exciting for prospective parents, but not easy. Human eye color is determined by genes which come in pairs, one from the father and one from the mother, and are carried on chromosomes. Melanin, a pigment that is also found in the skin, gives us our eye color determined by the amount and placement of the melanin.
The basics of eye color are that the nucleus of each cell contains 46 chromosomes and these are divided into 23 pairs of chromosomes. In each pair of chromosomes, there is one chromosome that has been inherited from each parent. A gene, the basic unit of heredity, is a piece of DNA located on a chromosome and determines a baby's characteristics. Genes contain alleles that determine the appearance of any characteristic. For every trait a baby inherits there are two alleles. Homozygous means that the two alleles are the same. If the alleles are different, then they are called heterozygous.
Genes can be either dominant (expressed) or recessive (hidden). Also, the parents' egg and sperm each carry one of each chromosome, and they carry only one copy of each gene. For example, let's use two genes, one with dominant brown and recessive blue and the other with dominant green and recessive blue. The dominant brown in the first gene will likely override green or blue from the second gene, and the dominant green from the second gene will likely over power the blue from the first gene. The remaining gene is the brown one. So even though there may be two colors in a gene, the dominant color will have more influence over what your baby's eye color will be.
A very simple way to visually explain how the genetics of eye color work is to try a Punnett Square. Genes contain alleles that determine the appearance of a characteristic. If both the baby's alleles are the same they are homozygous. If each parent is carrying a dominant and recessive allele (Bb), they are homozygous.Draw something like a tic-tac-toe board with only four squares. In each square you are going to put a pair of genes, either two dominant genes BB, or two recessive genes bb, or one of each Bb. Above your squares you will put the possible eggs for the mother, for instance Bb, one dominant and one recessive. Along the left side of your squares do the same for the father's sperm, B above and b below.
Now, in the first square, upper left, take the mother's allele above the square, B and match it with dad's allele on the left, B. So in your first square you have BB. Next to it, the upper right square, do the same and you should have bB. In your lower left square you have the combined bB alleles and on the lower right will be bb. These combinations represent the possibilities for your baby's eye color. So, now try the Punnett puzzle for yourself and predict your child's, or children's, eye color. You can find a sample at athro.com.