Genetics (6 Major Types of Epistasis)

[Arosa Hya]

6 Most Important Kinds of Epistasis


The epistasis is in between two genes, that is at least a dihybrid and the phenotypes are less than 4.

(a) Dominant epistasis (12: 3: 1):

When dominant allele ‘A’ masks the expression of ‘B’ ‘A’ is epistatic gene of ‘B’. A can express itself only in the presence of ‘B’ or b allele. Therefore it is called dominant epistasis. B expresses only when ‘aa’ is present. Therefore, in 9: 3: 3: 1 ratio both 9 and 3 are expressing the ‘A’ gene the ratio is now 12: 3: 1.
Image Courtesy : upload.wikimedia.org/wikipedia/commons/2/2b/blackandchocolate.jpg


Example:
Complete dominance at both gene pairs, but one gene, when dominant, epistatic to the other.
Fruit colour in summer squash:
Gene pair A: White dominant to colour 12/16 white
Gene pair ‘B’ – Yellow dominant to green 3/16 yellow
Green – 1/16 both recessive aabb
Dominant white hides the effect of yellow or green.

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[Arosa Hya]

b) Recessive epistasis (9:3:4):

Recessive alleles at one locus (aa) mask the phenotypic expression of other gene locus (BB, Bb or bb) such epistasis is called recessive epistasis. The alleles of ‘B’ locus express themselves only when epistatic locus ‘A’ has dominant allele like AA or Aa. The phenotypic ratio is 9: 3: 4.Example:Complete dominance at both gene pairs, but one gene, when homozygous recessive, is epistatic to the other.In Mouse coat colour.Gene pair A: colour dominant over albino.Gene pair ‘B’ agouti colour dominant over black.Interaction: homozygous albino is epistatic to aguoti and black.Agouti 9/16Black 3/16Albino 4/16

[Arosa Hya]

(c) Duplicate recessive gene (9: 7):

If both gene loci have homozygous recessive alleles and both of them produce identical phenotype the F2 ratio 9:3:3:1 would be 9:7. The genotype aaBB, aaBb, AAbb, Aabb and aabb produce same phenotype. Both dominant alleles when are present together only then they can complement each other. This is known as complementary gene.Examples:Complete dominance at both gene pairs, but either recessive homozygote is epistatic to the effect of the other gene.In sweet pea flower colour:Gene pair ‘A’ – Purple dominant over white.Gene pair ‘B’ – Colour dominant over colourless (white).

[Arosa Hya]

d) Duplicate genes with cumulative effect (9:6:1):

Certain phenotypic traits depend on the dominant alleles of two gene loci. When dominant is present it will show its phenotype. The ratio will be 9: 6: 1.Example:Complete dominance at both gene pairs, interaction between both dominance to give new phenotypes.Fruit shape in summer squashGene pair ‘A’ sphere shape dominant over long.Gene pair ‘B’ sphere shape dominant overlong.

Interaction at ‘AB’ when present together, form disc-shaped fruit (Fig .39.1)
Disc shaped fruits 9/16
Sphere shaped fruits 6/16
Long shaped fruit 1/16

[Arosa Hya]

e) Duplicate dominant genes (15:1):

If a dominant allele of both gene loci produces the same phenotype without cumulative effect, i.e., independently the ratio will be 15:1.Example:Complete dominance at both gene pairs, but either gene when dominant, epistatic to the other.Seed capsule of shepherd’s purse (Caps ell a bursa-pestoris)Gene pair ‘A’: Triangular shape dominant over ovoidGene pair ‘B': Triangular shape dominant over ovoid (double recessive)

[Arosa Hya]

(f) Dominant and recessive interaction (13: 3):

Sometimes the dominant alleles of one gene locus (A) in homozygous and heterogygous (AA, Aa) condition and homozygous recessive alleles bb of another gene locus (B) produce the same phenotype. The F2 ratio will become 13: 3. The genotype AABB, AaBB, AAbb, Aabb and aabb produce one type of phenotype and genotype aaBb, aaBB will produce another type of phenotype.Example:Complete dominance at both gene pairs, but one gene when dominant epistatic to the other, and the second gene when homozygous recessive, epistatic to the first.Feather Colour of FowlGene pair ‘A’: colour inhibition is dominant to colour appearance.Gene pair ‘B’: colour is dominant to white.Interaction:Dominant colour inhibition prevents colour even when colour is present, colour gene, when homozygous recessive prevents colour even when dominant inhibitor is absent.

[Arosa Hya]

Non epistatic Inter allelic genetic interaction:In some cases two pairs of genes determine same phenotype but assorted independently, produce new phenotypes by mutual epistatic interaction. The F2 ratio remains the same as 9:3:3:1.Example:Each gene pair affecting the same character complete dominance at both gene pairs, new phenotypes resulting from interaction between dominants, and also from interaction between both homozygous recessives.

Gene Pair A: Rose comb dominant over non rose.Gene Pair B: Pea comb dominant over non pea.Interaction:Dominant of rose and pea produce walnut comb. Homozygous recessive for rose and pea produce single comb.

According to Bateson and Punnett such result is obtained because by combination of homozygous recessive genes (bb) and a homozygous or heterozygous dominant AA or Aa the Rose comb is produced; and by combination of homozygous recessive (aa) and homozygous or heterozygous dominant condition BB or Bb produce pea comb while the single type comb is produced by double recessive aabb genes. ‘A’ gene determines the shape of rose comb and ‘B’ gene determines the shape of Pea comb, but when both genes combine a new shape Walnut appears. In the cross between two types of Walnut chickens 4 phenotypes appear. The genes here do not determine themselves in the development of character but modify the character determined by a basic gene. These genes are called supplementary or modifying genes.