Multiple Alleles & Epistasis

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Multiple Alleles

Although only two alleles per gene are found in an individual diploid organism, there may be multiple alleles at the population level. This means many possible combinations of alleles can exist.

Homologous chromosomes

Chromosomes in diploid organisms exist in homologous pairs.
Homologous chromosomes consist of the same genes in the same order.
The only variation between homologous chromosomes is in the alleles.
Two alleles can be present for one gene.

Multiple alleles

Although only two alleles can be present in an organism, there could be multiple different alleles for a single gene.
Multiple alleles provide many different genotype combinations.
This creates large variation at the population level.

Rabbit coat colour

Four phenotypes exist for the c gene (coat colour):
- CC - brown fur.
- c^chc^ch - blacktipped white fur.
- c^hc^h - black fur on the extremities and white fur elsewhere.
- cc - white fur.

Dominance hierarchy

Multiple alleles have a dominance hierarchy.
E.g. rabbit coat colour.
- CC is dominant over all other allele combinations.
- c^chc^ch is incompletely dominant over c^hc^h and cc.
- c^hc^h is dominant over cc.

Epistasis

Multiple genes act in unison to produce the phenotype of an organism. Epistasis is when a gene at one locus inhibits the expression of a gene at another locus. Epistasis can be dominant or recessive.

Function of epistasis

A gene at one location can mask the expression of another gene. This is called epistasis.
The alleles that are being masked are hypostatic to the epistatic alleles.
Epistasis often involves a pathway where expression of one gene is dependent on the function of another gene.

Recessive epistasis

Recessive epistasis is where the epistatic allele (the allele that masks another gene) is recessive.
This means two copies of the epistatic allele must be present for expression of the hypostatic allele to be affected.

Mouse coat colour

Agouti (AA, Aa) coat colour is dominant to solid-coloured fur (aa) in mice.
The epistatic gene (C) is necessary for pigment production.
If a mouse is homozygous recessive (cc), no pigment is produced and the mouse coat colour is albino regardless of the A alleles.
If two heterozygotes (AaCc) are crossed, the offspring phenotypic ratio will be:
- 9 agouti; 3 solid-coloured; 4 albino.

Dominant epistasis

Dominant epistasis is where the epistatic allele is dominant.
This means only one copy of the epistatic allele must be present for expression of the hypostatic allele to be affected.

Squash fruit colour

Yellow fruit (YY, Yy) is dominant to green fruit (yy).
The epistatic gene (W) is necessary for pigment production in the fruit.
If a squash has one copy of the dominant W allele (WW or Ww), the fruit is white regardless of the Y alleles.
If two heterozygotes are crossed (WwYy), the offspring phenotypic ratio will be:
- 12 white; 3 yellow; 1 green.