Class 12 Genetic Gene Interaction Recessive Genes

Recessive Epistasis Genes (9 : 3: 4 Ratio)

A gene is said to be epistatic when its presence supers the effect of a gene at another locus. According to Bateson (1901), the gene which masks the effect of other genes is termed epistatic gene.

l      In mice agouti colour is due to a colour combination present in-band along with each hair with yellowish tip. Numerous genes, not a single gene, may modify this colour pattern.

l      In a homozygous recessive condition such genes make the mice white albino.

l      Albino mice having pink eye are true-breeding.

l      Black colour in mice is also true-breeding and is hypostatic to agouti.

l      When black mice crossed into albino mice, only agouti progeny was produced. But, in the F2 generation, 9 agoutis, 3 black and 4 albinoes were produced.

l      Coat colour in mice is controlled by two dominant genes C and A.

l      Mice colour will be black if only gene C is dominant.

l      The agouti colour pattern is characterised by coloured banded hairs in which the part nearest the skin is grey, then a yellow band and finally the distal part is either black or brown.

ü    Recessive albino is produced when both recessive genes C and a are homozygous recessive.

 The above facts are clear from the following crosses-

Dominant Epistatic Gene (12 : 3: 1 Ratio)

l   When out of two genes, the dominant allele (e.g., W) of one gene masks the activity of allele of another gene (e.g., Y) and expresses itself phenotypically, W gene locus is said to be epistatic to Y gene locus. Because the dominant allele W can express itself only in the presence of either Y or y allele, the epistasis is termed dominant epistasis.

l   In common gourd, Cucurbita pepo, the colour of fruits can be green, white or yellow.

l   White gourd colour is dominant over both yellow and green, but yellow is dominant over green only.

l   White colour is determined by dominant gene W. Thus this gene (w) is epistatic to other fruit colour genes.

l   Plants with fruit (WWYY) crossed to plant with green fruit (wwyy) produced only white fruit in the F1 generation.

l   In the F2 generation, plants with white, yellow and green fruits are produced in 12 : 3: 1 ratio.

All the above facts of dominant epistasis genes can be explained by the following cross –

l   Epistasis genes are sometimes called inhibiting genes because of their effect on the other genes which are described as hypostatic.

Complementary Genes (9: 7 Ratio)

l   Complementary genes are also known as duplicate recessive genes.

l   If both gene loci have homozygous recessive alleles and both of them produce identical phenotypes, the F2 ratio 9 : 3 : 3: 1 would modify into 9 : 7.

l   Bateson and Punnett made a cross between two white-flowered varieties of sweet pea (Lathyrus odoratus) in which F1 progeny were purple-flowered.

l   In Lathyrus odoratus, the flower colour is determined by two genes C and P.

l   F2 progeny segregated into 9 purple and 7 white-flowered plants. Undoubtedly, this result obtained is a modification of 9 : 3 : 3: 1 ratio in which the last three classes (3 : 3: 1) have the same phenotype, thereby producing a phenotypic ratio of 9: 7.

l   Genes C and P are responsible for the production of certain chemical-anthocyanin. These genes are complementary to each other.

l   If either or both the dominant genes are absent, the flowers become white.

l   Purple colour of the flower is the result of the complementary effect of dominant alleles at two different loci which segregate independently of each other.

l   It is evident that both dominant alleles, when presented together, complement each other and are called complementary genes and produce a different phenotype.

            The above facts can be seen in the following cross –