As we saw in the previous chapter, every gene on a chromosome in our DNA makes a different protein. There are 20,500 of these genes in every human. Almost every single one of these genes are the same from person to person, which means that the vast majority of these genes contain information for our lungs, heart, liver, kidneys, bones, brain and more. There is only a handful of genes that contain information for the color of our skin, hair, eyes, and the shapes of our faces, hands and feet.
|Allele from mother||Allele from father||Gene that turns into a protein|
Since we have two copies of (almost) every gene in our body, we call these copies alleles. We get one allele from our father and one from our mother. Since only one of those alleles can turn into a protein, it is the more dominant allele that gets turned into a protein by our cells. If both alleles are dominant, then it is clear that the dominant protein is made. If one allele is dominant, then the other trait, the recessive trait, is ignored and the dominant protein is made. Only if both alleles are recessive then the recessive protein is made.
In order to try and figure out what the chances are of having a child with a dominant or recessive trait for a particular gene, something can be done called a Punnett square. A Punnett square is used to predict the probabilities and possibilities of traits in offspring. Along the top of the Punnett square, the alleles that could come from the father are listed, and the alleles from the mother are listed along the side. In the middle of the Punnett square, the possibilities for offspring are listed. Each square represents a 25% chance of getting that type of offspring. If an offspring has two of the same allele, it is called homozygous. If it has two different alleles, it is called heterozygous.