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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.
X-linked recessive inheritance. X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation (see zygosity).
A hereditary carrier (genetic carrier or just carrier), is a person or other organism that has inherited a recessive allele for a genetic trait or mutation but usually does not display that trait or show symptoms of the disease. Carriers are, however, able to pass the allele onto their offspring, who may then express the genetic trait.
In the example on the right, both parents are heterozygous, with a genotype of Bb. The offspring can inherit a dominant allele from each parent, making them homozygous with a genotype of BB. The offspring can inherit a dominant allele from one parent and a recessive allele from the other parent, making them heterozygous with a genotype of Bb.
An individual that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the ...
In the process of substitution, a previously non-existent allele arises by mutation and undergoes fixation by spreading through the population by random genetic drift or positive selection. Once the frequency of the allele is at 100%, i.e. being the only gene variant present in any member, it is said to be "fixed" in the population. [1]
An amorphic allele elicits the same phenotype when homozygous and when heterozygous to a chromosomal deletion or deficiency that disrupts the same gene. [2] This relationship can be represented as follows: m/m = m/Df An amorphic allele is commonly recessive to its wildtype counterpart.
Achondroplasia is a skeletal system disorder caused by a recessive allele that can still result in a live birth in the homozygous state. [8] One mutant allele for achondroplasia can be tolerated, but having two results in death. In the case of homozygous achondroplasia, death almost invariably occurs before birth or in the perinatal period.