Recessive Allele: Definition and Example

Recessive Allele Definition

A recessive allele is a variety of genetic code that does not create a phenotype if a dominant allele is present. In a dominant/recessive relationship between two alleles, the recessive allele’s effects are masked by the more dramatic effects of the dominant allele. An allele is a specific variation of a gene, or specific segment of DNA. Different alleles produce slightly different proteins, which function in different ways.

In dominant/recessive relationships, the recessive allele produces a non-functional protein. The dominant allele produces a functioning protein. A heterozygous individual will appear the same as a homozygous dominant individual. This means that an organisms with two dominant alleles appear the same as an organism with only one functioning allele. Many enzymes are able to process their substrates so fast that one functional allele can produce enough enzyme to service the needs of the organism. Most enzymes are regulated anyway through a variety of methods, and are not allowed to convert substrate to their full potential.

A common misconception is that a recessive allele is somehow worse, evolutionarily, than a dominant allele. Natural selection may select for a recessive allele if the products of a dominant allele are unfavorable in the environment that a population is living in. While genes only code for proteins, these proteins interact in ways that build and create the entire structure of an organism. Sometimes, not having an enzyme functioning is more beneficial than having it.

Examples of Recessive Allele

Coat Color

In many animals with hair, coat color is in part determined by a gene that controls melanin production. Melanin is an important pigment that colors the hair black or brown, depending on its concentration. The gene partially responsible for melanin production produces the enzyme that converts melanin from an intermediate molecule to a functional pigment molecule. The recessive allele for melanin production produces an enzyme that does not function in converting the intermediate molecule.

Suppose a population of black rabbits is set adrift in a small boat. The boat crashes on a cold island, covered almost year-round in snow. There are only a few predators on the island, so they can’t eat all the rabbits at once. Now suppose that two black rabbits, both heterozygous for the melanin gene, start to reproduce. According to a Punnett square, a homozygous recessive individual will be created between the parents at least 25% of the time. These individuals will not be able to produce melanin and will be white. In this generation, the black rabbits get eaten much more than the white rabbits. As such, white rabbits get more opportunities to reproduce.

If they produce successful offspring, the white recessive allele will increase in frequency in the population. Eventually, the predator will eat all or most of the black rabbits. Although the black allele will decrease in frequency, it will still be the dominant allele. This means it will still only take one black allele to produce a black phenotype, although the total number of black alleles in the population will be severely decreased.

Tay-Sachs Disease

Tay-Sachs disease is a recessive disorder, caused by non-functioning alleles in the HEXA gene. The gene codes for several lysosomal enzymes. The enzymes it codes for, beta-N-acetylhexosaminidase A and others, are responsible for the breakdown of certain lipids. If these lipids cannot be broken down, they rapidly accumulate in the brain and interfere with many processes. The mutations that cause Tay-Sachs can arise in several areas of the gene, and create enzymes with different functionalities.

People with Tay-Sachs disease experience different onset timeframes for their symptoms, as a result of the complex action of the recessive alleles. Recessive alleles caused by different mutations produce enzymes that vary in their level of functioning. Although two recessive alleles mean you will get the disease, when the disease will strike is still a mystery. Heterozygous individuals show no disease symptoms because one functioning enzyme is enough to rid the cells of excess lipids. Because it only takes one dominant allele to produce a functioning phenotype, Tay-Sachs is considered a recessive disorder.

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