Interaction of Genes

According to Mendel’s Law of Independent Assortment, the segregation of one gene is independent of segregation of another gene. It is also important that these genes must also have been functioning (exerting their effects on the phenotype) independently of one another.

Usually a single gene controls one character. However, it should not be surprising that the expression of one gene sometimes alter the expression of one or more of the alleles of a second (non-allelic) gene. In such a situation two or more than two genes may interact to give rise to a completely novel phenotype.

The phenomenon of two or more genes affecting or influencing the expression of each other in development of a single character of an organism is known as gene interaction.

Gene interactions lead to variations in the classical Mendelian mono and di-hybrid ratios.

Supplementary Genes (9:3:4 ratio)

Supplementary genes are two independent pairs of dominant genes which interact in such a way that one dominant gene will produce its effect, irrespective of the presence or absence of the other gene, while the second gene can only produce its effect in the presence of the first gene.

This is known as the supplementary gene interaction and the phenotypic ratio becomes modified into 9:3:4.

In Mice, Black, albino and agouti patterns of coat colour are seen. The wild body colour is known as agouti, characterised by banding of individual hairs. It is characterised by colour banded hairs in which the part nearest the skin is gray, then yellow band and finally the distal part is either black or brown. The agouti colour is controlled by a gene ‘A’. The dominant allele ‘C’ in absence of gene ‘A’ gives coloured mice. However, in the presence of dominant allele ‘C’, ‘A’ gives rise to agouti. A

In absence of gene ‘C’ , ‘A’ is unable to express itself and mice with genotype ccAA and ccAa and ccaa are albinos. The albino lacks pigments

Complementary Genes (9:7 ratio)

A classical example of interaction of genes is the complementation between two genes meaning that both genes are necessary for the protection of a particular phenotype.

W. Basteson and R.C Punnet observed that , when two white flowered varieties of sweet pea, Lathyrus odoratus were crossed, F

On selfing, the white flowered plants were not true-breeding. Some of them produced purple and white flowered plants in the ratio of 3:1. Obviously, the results obtained were a modification of 9:3:3:1ratio in which the last three classes have the same phenotype, there by producing a phenotypic ratio of 9:7

The above results are easily explained if we assume that purple colour of flowers in Lathyrus odoratus is determined by two dominant genes C and P. If either or both of the dominant genes are absent the flowers becomes white. The phenotype of white parents, therefore were CCpp or ccPP. Purple colour is the result of a complementary effect of dominant alleles at two different loci which segregates independently of each other.

Metabolic process in living organisms takefunctional enzyme, pigmentation does not occur.

 Enzyme products of genes C and P are both necessary for the production of anthocyanins. When the genotype is cc, the first enzyme is not produced, consequently, reaction to the intermediate product (chromogen)is stopped. Likewise, if the genotype is pp, the lack of the second enzyme halts the second metabolic step. In other words, if the genotype is cc or pp, the synthesis pathway is blocked and no pigments are produced, resulting in a white flowers .

Such an interaction of genes to jointly

Duplicate genes
When dominant alleles of two genes produce the same phenotype with out cumulative effect, the 9:3:3:1 ratio becomes modified into a 15:1 ratio. If different genes determine the same or nearly same phenotype, such genes are called as duplicate genes.
A classic example of duplicate genes occurs in shepherd's purse belonging to the genus Capsella (Capsella bursa pastoris).
Two kinds of fruit phenotypes with respect to the genus were known - (I) triangular capsules and (II) top shaped capsules.
When plants with these phenotypes were crossed, in F1 generation only triangular capsules were observed.
When such d individuals with triangular capsules were intercrossed among themselves, in F2 progeny,  plants with triangular capsules and top shaped capsules were obtained in 15:1 ratio.

Obviously the top shaped capsules results from double recessive genotype. If 'A' and'B' are two genes, top shaped capsules will be obtained on plants with the genotype aabb. Plants with triangular capsules can be AABB, AAbb or aaBB and other genotypes with heterozygosity.
It shows that even a single dominant gene is enough to give rise to triangular capsules.

The biochemical basis for the 15:1 ratio can be understood by examining one step pathway in which a dominant allele at either of two genes is enough to produce enzymes for the catalysis of a given reaction.

Only when there is a double recessive, say aabb, is the pathway blocked and the aparrent phenotype expressed.
When either of two genes can function to produce the dominant phenotype, this type of gene Interaction is called as duplicate gene interaction.
Pressumbly, the two different genes produce similar gene product, and one of them may have arisen by duplication from the other gene.

Epistasis
One important type of functional interaction between different genes occurs when an allele or genotype at one locus"masks" or " inhibits" the expression of a non-allele or genotype at a distinct locus, such an interaction is known as epistasis ( Gr. - standing upon).
Any gene that masks the expression of another non- allelic gene is known as epistatic gene. And the gene whose expression is prevented or masked is known as hypostatic gene.
Therefore, while dominance involved intragenic or inter- allelic gene suppression, the epistasis involves intergenic suppression.
Epistasis is of following types:- (I) Dominant epistasis and (II) Recessive epistasis.

Dominant Epistasis:
When the dominant allele of one gene (e.g. A) masks or inhibits the expression of alleles of another gene ( e.g. B) and expresses itself phenotypically, then gene A is said to be epistatic to the gene B.
Since a dominant gene exerts its influence by suppressing the expression of gene B or b, it is known as dominant epistasis.
Fruits of Cucubita pepo ( summer squash) can be white, yellow and green. White is dominant over both yellow and green. Yellow is dominant over green only.
White colour is determined by the dominant gene W and no other gene for fruit colour is expressed in its presence. Thus, the dominant W is epistasis to other fruit colour genes.
In the presence of homozygous recessive (ww) gene, another gene Y determines yellow colour of the fruit. Homozygous recessive for both genes ( wwyy) bear green fruits.
Plants with white fruits (WWYY) crossed to plants with green fruits (wwyy) produce F1 progeny which bears only white fruits. F2 progeny plants segregated in 12 white, 3 Yellow and 1 green i.e., 12:3:1 ratio.

Thus, fruit colour in Cucurbita pepo is determined by two different genes and the first two classes of a typical dihybrid F2 ratio(9:3:3:1) are phenotypically similar.

Recessive Epistasis
Sometimes the recessive alleles of one gene (aa) masks the phenotypic expression of alleles of another gene (BB, BB or bb alleles).
This type of epistasis due to a recessive gene is called recessive epistasis. Due to recessive epistasis the phenotypic ratio 9:3:3:1  becomes modified to 9:3:4 ratio.
Recessive epistasis is same as supplementary genes.

Recessive  Lethals:
In 1905, L Cuenot reported the inheritance of mouse body colour, which did not fit the expected Mendelian segregation pattern. It was shown that yellow body colour was dominant over normal brown colour.
Yellow colour was controlled by a single gene which is designated as Y, while y determine the normal brown colour.
It was found that yellow mice could never be obtained in homozygous condition. When yellow mice were crossed among themselves, segregation for yellow and brown body colour was obtained in 2:1 ratio. The brown individuals were pure, therefore, homozygous and yellow individuals were heterozygous.

These results could be explained if we assume that allele Y which is dominant for yellow body colour in heterzygous condition was recessive for lethality in homozygous condition since in the homozygous condition YY, it kills the individual in early embryonic state ( I.e., during gastrulation). Consequently, when ever a homozygous individual for Y is produced, the lethal character will express itself and the individual will die. Thus, a homozygous yellow will never be produced.