You can thank your genes for your blue eyes and brown hair. Genes are small areas on your chromosomes that store the code for making proteins. You have 23 pairs of chromosomes, one pair member from each of your parents. Just about all of your traits can be traced back to your genes, sometimes in combination with your environment. The fact that you have two of each gene can make a big difference in the way you turn out.
Chromosomes and Genes
Each chromosome is a long molecule containing two strands of DNA, or deoxyribonucleic acid, mixed with proteins. The chromosomes are highly coiled and compact so that they fit into your cells. If you laid out a cell’s entire DNA end to end, it would stretch out to about six feet. Protein-coding genes occupy only about 2 percent of your chromosome’s real estate. Each gene contains the genetic code for one protein. It’s your proteins that give your body its shape and features. In additions, proteins in the form of enzymes control your body’s biochemical activities, such as respiration and metabolism.
Proteins determine traits, which are genetically determined characteristics. The pair of genes that code for the same trait are called alleles. Half of your alleles come from your mother, and the other half from your father. Alleles can relate to each other in several ways. In some cases, one allele is in charge. This allele is dominant. The paired allele can be another dominant gene, or it can be recessive. In either case, the protein expressed by the dominant gene will manifest physically. You can only experience the recessive trait if both alleles are recessive. For example, you can only have blue eyes if both alleles code for blue eyes. If even only one allele codes for brown, you'll have brown eyes, because brown eyes dominate blue eyes.
Codominance and Semidominance
Sometimes, both alleles are equally dominant, or codominant. In this case, both alleles express themselves equally. For example, the alleles that determine whether your hair is curly or straight are codominant. If you have both types of alleles, your hair will be a hodgepodge of straight and curly, giving you a wavy look. In incomplete or semidominance, the two genes result in a true mixture of traits. For example, a mix of semidominant alleles for red and white flower color would yield plants with pink flowers. If the genes were codominant, the flowers would have red and white spots.
Sometimes, different genes work together to express a trait, a condition known as epistasis. In this case, the two or more genes involved are not alleles. Depending on how the genes express themselves, the results can mimic dominant, codominant, semidominant and recessive relationships. For example, your genes for hair color and baldness are epistatic. If you have the gene for complete baldness, it dominates over your hair color gene, since you have no hair. Some genetic diseases are linked to epistasis and environmental effects.