Allele

allelesallelicmultiple allelesBi-allelicgenetic copiesalelleallelAllele frequency dataallele repeatsallelic distribution
An allele is a variant form of a given gene.wikipedia
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Gene

genesnumber of genesgene sequence
An allele is a variant form of a given gene. The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.
Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population.

Ploidy

diploidhaploid2n
Most multicellular organisms have two sets of chromosomes; that is, they are diploid.
Ploidy is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes.

Homologous chromosome

homologshomologous chromosomeshomologous
In this case the chromosomes can be paired: each pair is made up of two chromosomes of the same type, known as homologous chromosomes.
The alleles on the homologous chromosomes may be different, resulting in different phenotypes of the same genes.

Phenotypic trait

traittraitscharacters
Sometimes, the presence of different alleles of the same gene can result in different observable phenotypic traits, such as different pigmentation.
Different phenotypic traits are caused by different forms of genes, or alleles, which arise by mutation in a single individual and are passed on to successive generations.

Locus (genetics)

locuslociq arm
If both alleles at a gene (or locus) on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene (or locus).
A variant of the similar DNA sequence located at a given locus is called an allele.

Dominance (genetics)

autosomal recessiveautosomal dominantrecessive
In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles.
Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus.

Genetics

geneticgeneticistgenetically
The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.
These different, discrete versions of the same gene are called alleles.

Wild type

wild-typewildtypewild
The term "wild type" allele is sometimes used to describe an allele that is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies (Drosophila melanogaster). Such a "wild type" allele was historically regarded as leading to a dominant (overpowering - always expressed), common, and normal phenotype, in contrast to "mutant" alleles that lead to recessive, rare, and frequently deleterious phenotypes.
Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "mutant" allele.

Genetic carrier

carrycarrierscarrier
It was formerly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative "mutant" allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases, and more frequently in heterozygous form in "carriers" for the mutant allele.
Carriers are, however, able to pass the allele onto their offspring, who may then express the genetic if they inherit the recessive allele from both parents.

Blood type

blood groupblood typingblood types
For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, I A, I B, and i, which determine compatibility of blood transfusions.
Several of these red blood cell surface antigens can stem from one allele (or an alternative version of a gene) and collectively form a blood group system.

William Bateson

BatesonBateson, William
The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.
Bateson and Edith Saunders also coined the word "allelomorph" ("other form"), which was later shortened to allele.

Edith Rebecca Saunders

Edith SaundersSaunders
The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes.
Many of her genetic experiments led to her and William Bateson defining important terms like "allelomorphs" (nowadays referred to as alleles), heterozygote and homozygote.

ABO blood group system

ABOABO blood groupABO blood groups
For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, I A, I B, and i, which determine compatibility of blood transfusions.
Felix Bernstein demonstrated the correct blood group inheritance pattern of multiple alleles at one locus in 1924.

Genotype

genotypesgenotypicgenotypically
Any individual has one of six possible genotypes (I A I A, I A i, I B I B, I B i, I A I B, and ii) which produce one of four possible phenotypes: "Type A" (produced by I A I A homozygous and I A i heterozygous genotypes), "Type B" (produced by I B I B homozygous and I B i heterozygous genotypes), "Type AB" produced by I A I B heterozygous genotype, and "Type O" produced by ii homozygous genotype.
So, typically, one refers to an individual's genotype with regard to a particular gene of interest and the combination of alleles the individual carries (see homozygous, heterozygous).

Hardy–Weinberg principle

Hardy–Weinberg equilibriumHardy-Weinberg Equilibriumbasic law
The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (see Hardy-Weinberg principle).
In the simplest case of a single locus with two alleles denoted A and a with frequencies

Zygosity

homozygousheterozygousheterozygote
If both alleles at a gene (or locus) on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene (or locus). In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles.
Zygosity is the degree of similarity of the alleles for a trait in an organism.

Null allele

null mutationnull allelesNull mutations
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.
4) One example of a null allele is the 'O' blood type allele in the human A, B and O blood type system. The alleles for the A-antigen and B-antigen are co-dominant, thus they are both phenotypically expressed if both are present. The allele for O blood type, however, is a mutated version of the allele for the A-antigen, with a single base pair change due to genetic mutation. The protein coded for by the O allele is enzymatically inactive and therefore the O allele is expressed phenotypically in homozygous OO individuals as the lack of any blood antigen. Thus we may consider the allele for the O blood type as a null allele.

Tay–Sachs disease

tay-sachs diseaseTay–SachsTay Sachs Disease
Recessive genetic disorders include albinism, cystic fibrosis, galactosemia, phenylketonuria (PKU), and Tay–Sachs disease.
Carriers of a single Tay–Sachs allele are typically normal.

Phenylketonuria

PKUlarge neutral amino acid-transporter 1phenylketonuria, maternal
Recessive genetic disorders include albinism, cystic fibrosis, galactosemia, phenylketonuria (PKU), and Tay–Sachs disease.
As an autosomal recessive disorder, two PKU alleles are required for an individual to experience symptoms of the disease.

Nucleic acid sequence

nucleotide sequencesequencegenetic information
The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence.
A nucleic acid sequence is a succession of letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule.

Huntington's disease

HuntingtonHuntington’s diseaseHuntington's Chorea
Other disorders, such as Huntington disease, occur when an individual inherits only one dominant allele.
Huntington's disease has autosomal dominant inheritance, meaning that an affected individual typically inherits one copy of the gene with an expanded trinucleotide repeat (the mutant allele) from an affected parent.

Heredity

hereditaryinheritedheritable
While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species, a process termed transgenerational epigenetic inheritance.
If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles.

Species

specificspecific namespecific epithet
A population or species of organisms typically includes multiple alleles at each locus among various individuals.
Reproductive isolation is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model.

Fragile X syndrome

fragile Xfragile-X syndromeFXS
Examples include red-green color blindness and fragile X syndrome.
Between 45-54 repeats is considered a "grey zone", with a premutation allele generally considered to be between 55 and 200 repeats in length.

Transgenerational epigenetic inheritance

epigenetic inheritancetransgenerationaltransgenerational epigenetic
While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species, a process termed transgenerational epigenetic inheritance.
The B' and B-I alleles are considered to be epialleles because they are identical at the DNA sequence level but differ in the level of DNA methylation, siRNA production, and chromosomal interactions within the nucleus.