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An allele [1] (or allelomorph) is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule. [2]Alleles can differ at a single position through single nucleotide polymorphisms (SNP), [3] but they can also have insertions and deletions of up to several thousand base pairs.
The alleles (different versions of the same gene) responsible for sexual dimorphism can be limited to expression in only one sex when they first appear, or the alleles could begin by being expressed in both sexes then become modified (repressed or promoted) in one sex by modifier genes or regulatory elements.
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 ...
To what extent a particular genotype influences a phenotype depends on the relative dominance, penetrance, and expresivity of the alleles in question. One experiences this in everyday life with monozygous (i.e. identical) twins. Identical twins share the same genotype, since their genomes are identical; but they never have the same phenotype ...
Pure consideration of alleles is often referred to as Mendelian Genetics, or more properly Classical Genetics, and involves the assessment of whether a given trait is dominant or recessive and thus, at what rates it will be inherited. [9] The color of one's eyes was long believed to occur with a pattern of brown-eye dominance, with blue eyes ...
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 risk alleles within such variants are exceedingly rare, such that their large behavioural effects impact only a small number of individuals. Thus, when assessed at a population level using the R 2 {\displaystyle R^{2}} metric, they account for only a small amount of the differences in risk between individuals in the population.
The kinship coefficient is a simple measure of relatedness, defined as the probability that a pair of randomly sampled homologous alleles are identical by descent. [12] More simply, it is the probability that an allele selected randomly from an individual, i, and an allele selected at the same autosomal locus from another individual, j, are ...