How Does Having Two of Each Kind of Chromosome Affect the Genes a Person Has?

You can have blue eyes if both alleles code for it. Alternatively, you can buy blue contacts.
••• Pixland/Pixland/Getty Images

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.

Allele Domination

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.

Related Articles

What Is the Dominant Phenotype?
Dominant Physical Genes in Humans
Examples of Genetic Characteristics
How Are Genes on Sex Chromosomes Inherited?
Do All Humans Have a Unique Genotype & Phenotype?
What Is Expressed When Neither Copy of an Allele Completely...
What is a Gene?
What is an Example of a Recessive Phenotype?
The Differences Between Mendelian & Polygenic Traits
How Many Chromosomes Are Found in Human Body Cells?
What Makes an Allele Dominant, Recessive or Co-Dominant?
Cat Chromosome Information
What are the Causes of Genotype and Phenotype?
What Are Twisted Strands of DNA in the Nucleus of the...
What Are the Characteristics of a Homozygous Phenotype?
What Are the Different Variants of a Gene Called?
What Is a Trait Which Results From Two Dominant Genes?
Types of Genetic Crosses
Difference Between Homozygous & Heterozygous
What Is a Homologous Allele?