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A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive. For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, [13] classical genetics recognizes three alleles, I A, I B, and i, which determine compatibility of blood transfusions.
A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. [1] In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to generally be considered polymorphic. [2] Gene polymorphisms can occur in any region of the genome.
These classifications are still widely used in Drosophila genetics to describe mutations. For a more general description of mutations, see mutation, and for a discussion of allele interactions, see dominance relationship. Key: In the following sections, alleles are referred to as +=wildtype, m=mutant, Df=gene deletion, Dp=gene duplication.
Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number of protein-coding genes in a complete haploid set of 23 chromosomes is estimated at 19,000–20,000. [2] Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus.
Each autosomal gene is therefore represented by two copies, or alleles, with one copy inherited from each parent at fertilization. The expressed allele is dependent upon its parental origin. For example, the gene encoding insulin-like growth factor 2 (IGF2/Igf2) is only expressed from the allele inherited from the male. Although imprinting ...
Thus, allele R is dominant over allele r, and allele r is recessive to allele R. [4] Dominance is not inherent to an allele or its traits . It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, and co-dominant with a fourth.
A gene can acquire mutations in its sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. [4] Genes evolve due to natural selection or survival of the fittest and genetic drift of the alleles.
Some genes may have alleles with equal distributions. Often, the different variations in the genes do not affect the normal functioning of the organism at all. For some genes, one allele may be common, and another allele may be rare. Sometimes, one allele is a disease-causing variation while another allele is healthy. [citation needed]