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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.
Different genes often interact to influence the same trait. In the Blue-eyed Mary (Omphalodes verna), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its ...
These genes have maintained an unusually high level of allelic diversity throughout time and throughout different populations. This means that for each MHC gene, many alleles (or gene variants) consistently exist within the population, and many individuals are heterozygous at MHC loci (meaning they possess two different alleles for a given gene ...
Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. [2] The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of ...
Genes tell cells what to do by telling them which proteins to make and in what amounts. Genes are expressed by being transcribed into RNA, and this RNA then translated into protein. Proteins are made of a chain of 20 different types of amino acid molecules. This chain folds up into a compact shape, rather like an untidy ball of string.
The MHC genes are highly polymorphic; many different alleles exist in the different individuals inside a population. The polymorphism is so high, in a mixed population (non endogamic ), no two individuals have exactly the same set of MHC molecules, with the exception of identical twins .
At each gene tree, taxa that fixed the same allele will show closer phylogenetic similarity than taxa that fixed different alleles. Introgressive hybridization - hybridization resulting in the transfer of a gene or genetic tract from one species into the gene pool of another species by repeated backcrossing.
When a locus is identified as imprinted, two different classes express different alleles. [65] Inherited imprinted genes of offspring are believed to be monoallelic expressions. A single locus will entirely produce one's phenotype although two alleles are inherited. This genotype class is called parental imprinting, as well as dominant ...