Biology today

The independent genes do not have cumulative effect. They produce the same phenotype whether present in homozygous or

heterozygous state. as a result, dominant phenotype is more abundant. f 2 ratio is 15:1.

Example: In Shephered’s purse there are two types of seed capsules i.e., triangular and elongated or top-shaped. These when
crossed, give triangular and elongated fruits in the ratio of 15 : 1 in f 2 generation.

The triangular character of weeds is controlled by two pairs of genes (T and D) which are present at two different loci on two different
chromosomes. both of them either collectively or singly produce the triangular character of pods. elongated pods appear only when
both these dominant genes are absent. Such genes are called duplicate genes or pseudoalleles.

epistasis

Epistasis is the interaction between genes present at two separate loci in which one gene suppresses or masks the expression of

other gene. The gene that masks the effect of other gene is known as inhibiting or epistatic gene/factor and the one which is

being prevented from expressing itself is known as hypostatic factor. It is similar to dominance and recessiveness but the two

factors occupy two different loci. While dominance works at inter-allelic but intragenic level, epistasis works at intergenic level.

Table: Differences between epistasis and dominance

Epistasis Dominance

(i) It is a gene interaction that involves two genes (two

pairs of alleles).

It involves a single pair of alleles, hence no gene interaction.

(ii) a gene suppresses the expression of another gene. an allele inhibits the expression of other allele.

(iii) expression of both dominant and recessive genes is

suppressed by epistatic gene.

expression of only a recessive allele is suppressed by a dominant

allele.

(iv) It reduces the number of phenotypes in f 2 generation. It does not reduce the number of phenotypes in f 2

generation.

epistasis reduces the number of phenotypes appearing in f 2

G

G

generation of dihybrid cross. Thus, instead of normal

9:3:3:1 dihybrid ratio, epistasis may result in the ratio

of 9:3:4 (recessive epistasis), 12:3:1 (dominant

epistasis), etc.

epistasis is of two types– dominant and recessive.

Dominant epistasis

In dominant epistasis, out of two pairs of alleles the
dominant allele (i.e., gene a) of one gene masks the
activity of other non-allelic pair (bb). Since the dominant
epistatic gene a exerts its epistatic influence by suppressing
the expression of gene b or b, it is known as dominant
epistasis.
Example: f ruit colour in summer squash (Cucurbita pepo) is
governed by a gene which produces yellow colour in dominant
state (y) and green colour in recessive state (y).
There is a non-pigment forming epistatic gene which in
dominant state (W–) produces white colour (W– y–, W – y)
while in recessive state it allows expression of the yellow
(wwy+) colour and green (wwyy) colour.