Gene interactions is the phenomenon, where of two or more genes governing the development of a single character in such a way that they affect the expressions of each other in various ways. Thus, as a single character can be governed by two or more genes, they are also called as non – allelic or intergenic genetic interactions.

This part is continuous to Extension of mendelian inheritance

Alleles do not always behave in dominant and recessive patterns

• Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes.
• Codominance describes the simultaneous expression of both of the alleles in the heterozygote.
• Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles for a gene to exist in a population.
• In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome.
• Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two.

Types of gene interaction

Allelic/non epistatic gene interaction

• This type of interaction gives the classical ratio of 3:1 or 9:3:3:1. (Already discussed in Extension of mendelian inheritance)

Non allelic/ epistatic gene interaction

• In this type of gene interaction genes located on same or different chromosome interact with each other for their expression.

• Frequently, genes exhibit independent assortment but do not act independently in their phenotypic expression; instead, the effects of genes at one locus depend on the presence of genes at other loci. This type of interaction between the effects of genes at different loci (genes that are not allelic) is termed gene interaction.
• Whether or not they are sorting independently, genes may interact at the level of gene products, such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. 

Epistasis

• “Epitstasis is a phenomenon in which the expression of one gene is masked or prevented by another non-allelic gene.”
• The gene which prevents the expression of another gene is called epistatic gene, the gene whose expression is masked is called hypostatic gene.
• When epistasis is operative between two gene loci, the number of phenotypes appearing in the offspring from di-hybrid parents will be less than four.
• There are six types of epistatic ratios commonly recognized, three of which have 3 phenotypes and the other three having only 2 phenotypes.

• Epistasis should not be confused with dominance.
• Epistasis is the interaction between different genes (non-alleles) where as dominance is the interaction between different alleles of the same gene. The other differences given in table below

Dominant Epistasis (12:3:1 or 13:3)

• Fruit colour in squash, and there are three types of fruit color yellow, green and white.
• In this case, White is found dominant over yellow as well as green colour. When yellow is crossed with green, yellow is found to be dominant.
• Here, the character (colour of fruit) is governed by 2 pair of genes-
  • White x green → White dominant
  • Yellow x green → Yellow dominant
  • Here white is dominant factor and yellow is hypostatic factor.
  • White x Yellow → White dominant.
• When the hypostatic gene is also recessive “y”, the color of the fruit is green (wwyy).
• If white dominant is represented by ‘W’ and its recessive by ‘Y’, both non-allelic factors or genes may be represented as follows-

Recessive Epistasis / Supplementary Gene

• In recessive epistasis the recessive allele of ne locus masks the expression of both dominant and recessive alleles at another locus. It is known as recessive epistasis.
• Gene which by itself has no effect but qualitatively alters the effect of another gene is the supplementary gene.
• If the recessive genotype at one locus (aa) suppress the expression of alleles at the B-locus, the A-locus is said to exhibit recessive epistasis over the B locus.
• Only if the dominant allele is present at the ‘A’ locus can the alleles of the hypostatic B locus be expressed. The genotypes A-B- and A-bb produce two additional phenotypes. The 9:3:3:1 ratio becomes a 9:3:4 ratio.

Example:- Colour Coat in Mice (9:3:4)

• AC- Agouti/ Gray colour, aC- Black colour, Ac, ac- White colour
• In mice agouti (gray) colour is due to the dominant gene-’A’. The dominant gene-’C’ in absence of dominant gene-’A’ gives black colored mice and in presence gives agouti mice.
• But dominant gene-’A’ is unable to produce agouti colour in presence of recessive gene-’c’.
• Therefore recessive gene-’c’ acts as a epistatic over dominant gene-’A’.

Duplicate Recessive Genes (9:7) or Complementary Gene Interaction

• Non allelic genes that act together to produce a phenotype different from that produced by either alone.
• In the case where identical phenotypes are produced by both homozygous recessive genotypes, the F2 ratio becomes 9:7.
• The genotypes aa B-, A-bb and aabb produce one phenotype.
• Both dominant alleles, when present together, complement each other and produce a different phenotype.

Example:- Flower color sweet pea, Lathyrus odoratus (9:7)

• When two white flowered varieties of sweet pea, Lathyrus odoratus are crossed, F1 progeny had all coloured flowers.
• When F2 progeny obtained from F1 was classified, plants with coloured flowers and those with white flowers were obtained in 9:7 ratio which was the modification of 9:3:3:1 ratio.
• A pea plant with white flowers (genotype =CCpp) is crossed to a plant with white flowers (genotype =ccPP), the F1 plant will have purple colored flowers and a CcPp genotype.
• The normal ratio from selfing dihybrid is 9:3:3:1, but interactions of the ‘C’ and ‘P’ genes give a modified 9:7 ratio.
• The following table describes the complementary gene interaction for each genotype and how the ratio occurs.

• Complementary gene interaction: Enzyme-C and enzyme-P cooperate to make a product, therefore they complement one another.

Duplicate dominant gene interaction (15:1)

• Duplicate genes are two pairs of alleles either alone or together produce the same effect.
• They are identical genes but are situated on two different pairs of chromosomes.
• Each gene is dominant to its allele but does not add to the effect of the other.

Example:-Kernel Color in Wheat

• For this type of pathway a functional enzyme ‘A’ or ‘B’ can produce a product from a common precursor.
• The product gives color to the wheat kernel. Therefore, only one dominant allele at either of the two loci is required to generate the product.
• Thus, if a pure line wheat plant with a colored kernel (genotype = AABB) is crossed to plant with white kernels (genotype =aabb) and the resulting F1 plants are selfed, a modification of the dihybrid 9:3:3:1 ratio will be produced.
• The following table provides a biochemical explanation for the 15:1 ratio.

• If we sum the three different genotypes that will produce a colored kernel we can see that we can achieve a 15:1 ratio. Because either of the genes can provide the wild type phenotype, this interaction is called duplicate gene action.

Example:- Fruit Shape of Shepherds Purse

• TV, Tv, tV gives triangular shape and tv- gives ovate shape

Duplicate genes with cumulative effects (9: 6: 1) or Additive gene interaction

• Two non-allelic genes have similar effect when they are separate, but produced enhanced effect when they come together. Such gene interaction is known as duplicate genes with cumulative effect.
• If the dominant condition (either homozygous or heterozygous) at either locus (but not both) produces the same phenotype, the F2 ratio becomes 9: 6: 1.
• Where the epistatic genes are involved in producing various amounts of substance such as pigment, the dominant genotypes of each locus may be considered to produce one unit of pigment independently.
• Thus genotypes A-bb and aaB produce one unit of pigment each and therefore have the same phenotype. The genotype aabb produces no pigment, but in the genotype. A-B- the effect is cumulative and two units of pigments are produced. The 9 : 3 :3 :1 ratio is modified into 9 : 6 : 1 ratio.

Example:- rains color

• In a Cross between two light purple grains i.e., P1 and P2 the F1 was with dark purple grains.
• The F2 segregated for 9 dark purple: 6 light purple: 1 white.
• Light purple of the grains is evidently due to the presence of a dominant gene P1 or another dominant gene P2.
• The two non-allelic dominant genes P1 and P2 possess an additive effect and the colour of the grain is dark purple when the genes P1 and P2 are present together.
• When both the dominant genes are absent, the colour of the grain is white.

Dominant and Recessive interaction (13:3) or Inhibitory gene interaction

• In this type a dominant allele at one locus can mask the expression of both alleles at second locus.
• Only two F2 phenotypes result when a dominant genotype at one locus (e g. A-) and the recessive genotype at the other (bb) locus produce the same phenotypic effect.
• Thus A-B-, A-bb and aabb produce one phenotype and aaB- produces another in the ratio of 13:3.

Example:-Maize Aleurone Colour (13:3 ratio)

• One dominant gene- ‘R’ produces concerned phenotype (red colour) and its recessive allele- ‘r’ produces contrasting phenotype (white colour).
• The second dominant allele- ‘I’ has no effect on the concerned phenotype (colour) but stops the expression of dominant gene- ‘R’, so when both dominant alleles are present, phenotype (white colour) as that of recessive homozygote is produced.
• ‘RI’- White, ‘Ri’- Red, ‘rI’- White, ‘Ri’- White.