An allele is one of two or more forms of the DNA sequence of a particular gene. Each gene can have different alleles. Sometimes, different DNA sequences (alleles) can result in different traits, such as color. Sometimes, different DNA sequences (alleles) will have the same result in the expression of a gene.
Most organisms have two sets of chromosomes, that is, they are diploid. Diploid organisms have one copy of each gene (and one allele) on each chromosome. If both alleles are the same, they are homozygotes. If the alleles are different, they are heterozygotes.
A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes (heterozygosity) in the population.
The word is a short form of allelomorph ('other form'), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.
Dominant and recessive alleles
In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous genotypes the phenotype of the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele involved is said to be dominant to the other, which is said to be recessive to the former. The degree and pattern of dominance varies among loci.
A number of genetic disorders are caused when an individual inherits two recessive alleles for a single-gene trait. Recessive genetic disorders include albinism, cystic fibrosis, galactosemia, phenylketonuria (PKU), and Tay-Sachs disease. Other disorders are also due to recessive alleles, but because the gene locus is located on the X chromosome, so that males have only one copy (that is, they are hemizygous), they are more frequent in males than in females. Examples include red-green color blindness and Fragile X syndrome. Other disorders, such as Huntington's disease, occur when an individual inherits only one dominant allele.
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