What determines how a genotype is expressed to create a particular phenotype? This question has not yet been fully answered (Moore, 2015). However, a number of genetic principles have been discovered, among them those of dominant and recessive genes, sex-linked genes, and polygenically determined characteristics.
Dominant and Recessive Genes In some cases, one gene of a pair always exerts its effects; in other words, it is dominant, overriding the potential influence of the other gene, which is called the recessive gene. This is the dominant-and-recessive genes principle. A recessive gene exerts its influence only if the two genes of a pair are both recessive. If you inherit a recessive gene for a trait from each of your parents, you will show the trait. If you inherit a recessive gene from only one parent, you may never know that you carry the gene. Brown hair, farsightedness, and dimples override blond hair, nearsightedness, and freckles in the world of dominant and recessive genes. Can two brown-haired parents have a blond- haired child? Yes, they can. Suppose that each parent has a dominant gene for brown hair and a recessive gene for blond hair. Since dominant genes override recessive genes, the parents have brown hair, but both are carriers of blondness and pass on their recessive genes for blond hair. With no dominant gene to override them, the recessive genes can make the child’s hair blond.