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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.
However, in gene conversion, a ratio other than the expected 2A:2a is observed, in which A and a are the two alleles. Examples are 3A:1a and 1A:3a. In other words, there can, for example, be three times as many A alleles as a alleles expressed in the daughter cells, as is the case in 3A:1a.
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 ...
A diploid organism is heterozygous at a gene locus when its cells contain two different alleles (one wild-type allele and one mutant allele) of a gene. [3] The cell or organism is called a heterozygote specifically for the allele in question, and therefore, heterozygosity refers to a specific genotype. Heterozygous genotypes are represented by ...
The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric alleles. [citation needed] The shuffling of genes brought about by genetic recombination produces increased genetic variation.
Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus. Diploid and polyploid cells whose chromosomes have the same allele at a given locus are called homozygous with respect to that locus, while those that have different alleles at a given locus are called heterozygous. [3]
The alleles on the homologous chromosomes may be different, resulting in different phenotypes of the same genes. This mixing of maternal and paternal traits is enhanced by crossing over during meiosis , wherein lengths of chromosomal arms and the DNA they contain within a homologous chromosome pair are exchanged with one another.
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.