Allele

allelesallelicmultiple allelesBi-allelicepialleleMultiple allelismalelleallelAllele frequency dataallele repeats
An allele (, from German Allel and Greek ἄλλος állos “other”) is a variant form of a given gene, meaning it is one of two or more versions of a known mutation at the same place on a chromosome.wikipedia
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Gene

genesnumber of genesgene sequence
An allele (, from German Allel and Greek ἄλλος állos “other”) is a variant form of a given gene, meaning it is one of two or more versions of a known mutation at the same place on a chromosome. 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.

SNP genotyping

SNPDenaturing High Performance Liquid ChromatographyDynamic allele-specific hybridization
At the lowest possible end one can be the single base choice of an SNP.
A SNP is a single base pair mutation at a specific locus, usually consisting of two alleles (where the rare allele frequency is > 1%).

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.

Locus (genetics)

locuslociq arm
An allele (, from German Allel and Greek ἄλλος állos “other”) is a variant form of a given gene, meaning it is one of two or more versions of a known mutation at the same place on a chromosome. If both alleles of a gene at the locus on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene.
Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus.

Homologous chromosome

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

Phenotypic trait

traittraitscharacters
Sometimes, different alleles 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.

Dominance (genetics)

autosomal recessiverecessiveautosomal dominant
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 the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.

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.

Null allele

null mutationnull mutantnull alleles
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.
A null allele is a nonfunctional allele (a variant of a gene) caused by a genetic mutation.

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).
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.

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, WilliamBateson's rule
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 SaundersBecky SaundersEdith Rebecca "Becky" Saunders
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 principleHardy–Weinberg equilibrium
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

homozygousheterozygousHomozygote
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. If both alleles of a gene at the locus on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene.
Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") is the degree of similarity of the alleles for a trait in an organism.

Tay–Sachs disease

Tay-Sachs diseaseTay–SachsTay-Sachs
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

PKUphenylketonuria, maternalphenylketonurias
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

DNA sequencesnucleotide sequencegenetic 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 base-pairs signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule.

Species

specificspecific epithetspecific name
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.

Huntington's disease

HuntingtonHuntington diseaseHuntington’s disease
Other disorders, such as Huntington's 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

hereditaryinheritedinheritance
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.

Fragile X syndrome

Fragile Xfragile-X syndromeFragile-X
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.