Alleles of a gene

Since diploid organisms have two copies of each chromosome, they have two of each gene. Since genes come in more than one version, an organism can have two of the same alleles of a gene, or two different alleles. This is important because alleles can be dominant, recessive, or codominant to each other.

Let's return to the pea plant and say that the red allele for flower color is dominant to the white allele for flower color. We can equivalently say that the white allele for flower color is recessive to the red flower-color allele. This means that if a pea plant has two red alleles, its flowers will be red. If it has to white alleles, its flowers will be white. And, if it has a red allele and a white allele, its flowers will be red. If the red and white alleles are codominant, then a plant with to red alleles will have red flowers, a plant with two white alleles will have white flowers, and a plant with a red allele and a white allele will have pink glowers.

Now, let's turn to a couple examples of dominant and recessive alleles in humans. A good way to demonstrate these concepts are through genetic diseases. A common genetic disease in humans is cystic fibrosis (CF). The normal version of the gene contains information for an important cellular protein. When this protein is missing or disfunctional, people secrete large amounts of mucus into their lungs, have great difficulty breathing, and are very prone to infections in the lungs. People who have two normal or wild-type CF alleles do not have the disease Cystic Fibrosis. Neither do people who have both one wild-type and one abnormal or mutant allele. However, people who have two mutant alleles do have CF. This disease is recessive because you need two mutant alleles to have the disease.

Another human disease is Huntington's disease. People with this disease start undergoing neurodegeneration as adults, eventually leading to death. In this case, people with two wild-type alleles of the HD gene do not have the disease. However, people with one mutant and one wild-type allele, do develop the disease, as do people with two mutant alleles. This disease is dominant because you only need one copy of the mutant allele to develop the disease.