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9.3.2. Gene interaction between non-allelic genes that influence for the same trait

Gene interaction between non-allelic genes that influence for the same trait

The epistasis occurs when a character depends not only on a pair of homologous genes, it is regulated by more than one pair of alleles, so that one of them ( epistatic ) suppresses the action of the other (hypostatical).

When this occurs, the ratio 9 is not obtained: 3:3:1 obtained in F2 of the third law of Mendel.

In both epistasis and dominance, one of the genes outshines the other. The difference is that dominance occurs between allelic genes, while epistasis corresponds to non-allelic genes.

No modification of the 9:3:3:1 ratio or 9:6:1 ratio. Non-epistatic gene interaction

It occurs when new phenotypes appear in the second generation (F2), but obtaining a 9:3:3:1 ratio or a 9:6:1 ratio.

For example, there are four different types (phenotypes) of crest in chickens called rosette, pea, walnut and serrated, typical of some breeds. If you cross the pure races you get:

P: rrPP x RRpp

           rosette pea

F1: RrPp


           RrPp x RrPp

9/16 R_P_ 3/16 R_pp 3/16 rrP_ 1/16 rrpp

        plain pea rosette nut

The 9: 6: 1 ratio may also appear, as in some chicken breeds where they have three phenotypes: long feathers, short feathers, and medium feathers.

With modification of the ratio 9:3:3:1. Epistatic gene interaction

The typical 9:3:3:1 ratio obtained in Mendel's third law can vary due to the interaction between non-allelic genes. It occurs because one gene (epistatic) suppresses the action or prevents the manifestation of another gene (hypostatic). This phenomenon is called epistasis, and there may be several types:



3 phenotypes

Dominant 12:3:1
Recessive 9:3:4


2 phenotypes

Dominant 15:1
Recessive 9:7
Dominant and recessive 13:3

Simple epistasy

Simple dominant epistasy

When the dominant allele of one pair suppresses the action of the other allelic pair. The 9:3:3:1 segregation becomes 12:3:1.

For example, the case of the color of pumpkins that in the crossing of a dominant homozygous (AABB, white) with another recessive homozygous (aabb, green) an all white F1 offspring (AaBb) is obtained, with (A) being the allele responsible for the White color. Then, when crossing F1 with F1 we obtain an F 2 offspring with the phenotypes white (A_B_; A_bb), yellow (aaB_) and green (aabb), the allele (B) being responsible for the color yellow and (b) for the color green; a 12/16, 3/16, 1/16 ratio is obtained with respect to phenotypic expression.

Simple recessive epistasis (or supplemental genes)

It is a gene interaction produced by the action of a gene whose recessive alleles prevent the phenotypic expression of another gene. The segregation that appears is 9:3:4. This type of epistasis is also known as supplementary genes. One gene needs another to be able to manifest itself, the first being independent of the second.

An example is the color of Labrador retrievers, which in their homozygous and heterozygous condition are black (B_E_), in their heterozygous condition brown (bbE_) and yellow (B_ee; bbee) in F2, then proportions 9/16 are obtained; 3/16; 4/16 respectively in the phenotypic expression of the color of Labrador retrievers, but it only occurs at the level of a pair of alleles that mask the expression of another pair, either dominant or also recessive, as in the case of brown dogs.

Double epistasy

Double recessive epistasis (complementary genes)

It occurs when the simultaneous presence of the two dominant members of each allelic pair is necessary for a certain character to manifest. The phenotypic segregation of F2 is 9:7.

It is produced by the double action of the recessive alleles on any other allele, it is enough to coincide in the genotype of the recessive alleles in the form (aa) or in the form (bb) so that the double recessive masking occurs in this case.

For example, the flowers that after crossing a dominant homozygous (AABB) purple with a homozygous recessive (aabb) white, an F1 is obtained with all purple flowers (AaBb). When crossing F1 with F1 , a descent in the F2 of purple phenotypes (A_B_) and all the other whites (A_bb; aaB_; aabb) is obtained, having a ratio of 9/16, 7/16 of the phenotypic expression.

Double dominant epistasis (duplicate genes)

When the presence of at least one of the dominant alleles (A or B) is sufficient by itself to originate the final product. Segregation becomes the 15: 1 ratio.

For example, the case of two allelic pairs such that the dominant alleles of each determine the production of chlorophyll. Chlorophyll precursors where both (A) and (B) express the green pigment (chlorophyll). Plants with chlorophyll (A_B_; A_bb; aaB_) and plants without lethal chlorophyll (aabb) will be obtained in F2 in proportions 15/16, 1/16 in relation to the phenotypic expression of the plant.

Dominant and recessive double epistasis

It occurs when the dominant allele of one locus (for example A) and the recessive one of the other (b) respectively suppress the action of the other alleles. A 13: 3 phenotypic ratio is obtained.

For example, the dominant allele at one locus (eg A) and the recessive allele at the other locus (eg b) respectively suppress the action of the other alleles. For example, a locus (A, a) that inhibits pigmentation and another locus (B, b) that controls pigment production.