Ciencias
Allele: One Version of a Gene
An allele is one version of a gene, sitting at a fixed spot on a chromosome. You carry two, one from each parent, even when a population holds more.
Allele
An allele is one of the different versions a gene can come in, sitting at the same place on a chromosome as the others.
- También llamado
- Gene variant
- Dónde lo encuentra el alumnado
- Grade 9 science, at the start of inheritance, immediately before genotypes and Punnett squares are introduced.
La respuesta corta
An allele is one version of a gene. The gene occupies a fixed position on a chromosome, and the allele is which form of it you carry there. Because chromosomes come in pairs, you hold two alleles of every gene, one inherited from each parent, and they may be the same or different.
Un ejemplo
B = brown-eye allele, b = blue-eye allele
In the school model of eye colour, one gene has two alleles: B for brown and b for blue. Everyone carries two of them, so the possibilities are BB, Bb and bb, and the capital letter marks the dominant version. Real eye colour is governed by several genes at once and does not divide neatly into two, so this is a simplification chosen because it makes the idea of alleles visible. The mechanism it teaches is genuine even though the example is tidied up.
The gene is the slot, the allele is what is in it
A gene is a place: a specific stretch of a specific chromosome, in the same position in everyone. An allele is which version of that gene sits in the place. Everyone has the gene for blood group and the gene that affects eye colour; what differs between people is the alleles they hold.
This distinction shows up in ordinary speech and is usually wrong there. Saying that somebody "has the gene for cystic fibrosis" is misleading, because effectively everybody has that gene — it codes for a protein that moves salt across cell membranes, and most people's copies work perfectly well. What some people carry is a faulty allele of it. Substituting the word allele for gene fixes the sentence.
Two per person, more than two in a population
You have exactly two alleles of each gene because chromosomes come in pairs and the gene sits at the same position on both members of the pair. One came in the egg, one in the sperm. Gametes are the exception: each carries one allele of each gene, which is why they combine to give the offspring two.
A population is not limited to two. The ABO blood group gene has three alleles in circulation — A, B and O — yet no individual holds more than two of them at once. That is exactly why four blood groups exist rather than three: A can pair with A or with O, B with B or with O, and A with B gives group AB.
Males are the other exception worth knowing. Most genes on the X chromosome have no counterpart on the much smaller Y, so a male carries one allele of them rather than two, with nothing to mask a faulty version.
When the two are the same, and when they differ
If both alleles are identical the organism is homozygous for that gene; if they differ it is heterozygous. That single fact is what the rest of inheritance is built on, because when the two alleles differ they may not both show. One can mask the other, and the one that shows is called dominant.
Alleles are also the thing meiosis separates. The pair is split when gametes form, so each gamete carries one of them and each parent passes on one at random. Everything a Punnett square does is bookkeeping on that.
Preguntas frecuentes
What is the difference between a gene and an allele?
A gene is a section of DNA that codes for a protein and sits at a fixed location on a chromosome. An allele is one of the alternative versions that section can take. Two people with different eye colours have the same gene in the same place; they differ in which alleles of it they carry.
How many alleles does a person have for each gene?
Two, one on each chromosome of a pair, inherited one from each parent. The exception is a male's X chromosome genes: the Y carries no matching copy for most of them, so he holds a single allele. That is why conditions such as red-green colour blindness show up far more often in males.
Can a gene have more than two alleles?
In a population, yes, and often many more. The ABO blood group gene has three well-known alleles. What stays fixed is how many any one individual carries, which is two, because there are only two places for them. School examples use two alleles because that keeps Punnett squares small, not because two is the natural limit.
Where do your alleles come from?
One from each parent, delivered by the gametes. When meiosis produces an egg or a sperm, each pair of chromosomes is separated, so each gamete carries one allele of every gene. Which one it gets is decided at random, which is why siblings with the same parents inherit different combinations.
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