Which alleles are recessive

The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.

Recessive alleles only show their effect if the individual has two copies of the allele also known as being homozygous. For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele.

Related Content:. What is a gene? What is inheritance? What is genetic variation? What are single gene disorders? What is a genetic disorder? Austin, M. Featured Content. Introduction to Genomics. Polygenic Risk Scores. An individual with one dominant and one recessive allele for a gene will have the dominant phenotype. Dominant and recessive inheritance are useful concepts when it comes to predicting the probability of an individual inheriting certain phenotypes, especially genetic disorders.

But the terms can be confusing when it comes to understanding how a gene specifies a trait. This confusion comes about in part because people observed dominant and recessive inheritance patterns before anyone knew anything about DNA and genes, or how genes code for proteins that specify traits. The critical point to understand is that there is no universal mechanism by which dominant and recessive alleles act.

Whether an allele is dominant or recessive depends on the particulars of the proteins they code for. The terms can also be subjective, which adds to the confusion. The same allele can be considered dominant or recessive, depending on how you look at it. The sickle-cell allele, described below, is a great example. However, these patterns apply to few traits. Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles. Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells.

The long, pointy blood cells get caught in capillaries, where they block blood flow. The disease has a recessive pattern of inheritance: only individuals with two copies of the sickle-cell allele have the disease. People with just one copy are healthy. In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos.

Malaria resistance has a dominant inheritance pattern: just one copy of the sickle cell allele is enough to protect against infection. This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease! Genotypes are described as homozygous if there are two identical alleles at a particular locus and as heterozygous if the two alleles differ.

Alleles contribute to the organism's phenotype, which is the outward appearance of the organism. Some alleles are dominant or recessive. When an organism is heterozygous at a specific locus and carries one dominant and one recessive allele, the organism will express the dominant phenotype.