1. Epistasis occurs when one gene masks or hides the expression of another gene. The masking gene is called the epistatic gene and the masked gene is called the hypostatic gene.
2. There are different types of epistasis including recessive epistasis, where a recessive allele of one gene masks the effects of a dominant allele of another gene, and dominant epistasis, where a dominant allele of one gene masks the effects of other alleles at a second gene.
3. Epistasis was first described in 1909 as an interaction between genes where the phenotypic effect of one gene depends on the genotype at a second gene location. Examples of epistasis have been observed in mice coat
2. EPISTASIS
• One gene hides the effect of other gene .It is different from Mendel’s Dominance
which is meant for intragenic alleles (alleles of a gene)but here dominance works
at intergenic level (alleles of different genes).One gene masks the expression of
another non-allelic gene .
• A gene which masks (hides ) the action of another gene (non –allelic)is termed as
epistatic gene .The gene whose effects are masked is called hypostatic gene
3. • For example if two gene A and C with their alleles a and c take part in a cross then
epistasis can be of following types
• Recessive Epistasis – Recessive allele ( c) of one gene may hide the effect of
dominant allele ( A) of other gene
• Dominant Epistasis - Dominant allele (A) of one gene may hide the effect of
dominant allele ( C ) of other gene
4.
5. EPISTASIS
• When two or more genes influence a trait, an allele of one of them may have an
overriding effect on the phenotype. When an allele has such an overriding effect,
it is said to be epistatic to the other genes that are involved; the term epistasis
comes from Greek words meaning to “stand above.”
• This allele essentially nullifies the work of all the other genes, masking their
contributions to the phenotype.
• Mutant allele of one gene is epistatic to a mutant allele of another gene if it
conceals the latter’s presence in the genotype.
6. Epistasis is Greek word meaning standing over.
• It was first used in 1909 by Bateson to describe a masking effect.
• An interaction between a pair of loci, in which the phenotypic effect of one
locus depends on the genotype at the second locus.
• Genes whose phenotype are
Expressed-epistatic
altered or suppressed-hypostatic
7. Chemical interpretation
A gene is a chemical determiner.
Gene products interact with the environment and factors such as temperature,
light, hormones and enzymes.
If there is any problem or mutation in the intermediates, it can lead to another
phenotype and hence disturb the Mendelian ratios.
8.
9. Bateson and Punnett
Comb shapes in chickens of different
breeds.
(a) Rose, Wyandottes
(b) pea, Brahmas
(c) walnut, hybrid
from cross between chickens with rose
and pea combs
(d) single, Leghorns.
11. The work of Bateson and Punnett demonstrated that two independently
assorting genes can affect a trait. Different combinations of alleles from
the two genes resulted in different phenotypes, presumably because of
interactions between their products at the biochemical or cellular level.
12. Recessive Epistasis
1. Any genotype that includes ca
ca
will be albino, masking the A
gene, but as long as at least one
dominant C allele is present, the
A gene can express itself.
2. Mice with dominant alleles of
both genes (A-C-) will have the
agouti color, whereas mice that
are homozygous recessive at the
A gene (aaC-) will be black.
3. So, at the A gene, A for agouti is
dominant to a for black. The
albino gene (ca), when
homozygous, is epistatic to the A
gene; the A gene is hypostatic to
the gene for albinism
13. Bateson and Punnett obtained a ratio of 9 purple: 7 white plants in
the F2. They explained the results by proposing that two
independently assorting genes, C and P, are involved in
anthocyanin synthesis and that each gene has a recessive allele that
abolishes pigment production
A plausible explanation is that each dominant allele produces an
enzyme that controls a step in the synthesis of anthocyanin from a
biochemical precursor. If a dominant allele is not present, its step in
the biosynthetic pathway is blocked and anthocyanin is not
produced.
15. Dominant Epistasis
Dominant epistasis happens when the dominant allele of one gene masks the
expression of all alleles of another gene. If an organism inherits one or two copies
of the dominant allele, they will have the trait.
• Studied in summer squash (Cucurbita pepo)
• Common fruit colours-white, yellow &green
• White (W) is dominant over coloured squash Yellow (Y) is dominant over green
squash
• Pure breeding white fruited variety is crossed with the double recessive green
variety,F1 hybrids are all white
• When the hybrids are selfed-white, yellow &green fruited plants arise in the ratio
of 12:3:1
16.
17. George Shull using a weedy plant called the shepherd’s purse, Bursa bursa-pastoris.
The seed capsules of this plant are either triangular or ovoid in shape. Ovoid
capsules are produced only if a plant is homozygous for the recessive alleles of two
genes—that is, if it has the genotype aa bb. If the dominant allele of either gene is
present, the plant produces triangular capsules
