Download 3-_epistasis

Interaction between gene loci
It is possible for different genes at different loci to
interact to affect the phenotype.
This can work in two ways:
1) The two genes may be antagonistic which means
they work against each other. If one gene masks
the effect of the other this is called epistasis.
2) They may work in complementary fashion.
Recessive epistasis
E.g. flower colour in Salvia (the herb sage)
B = purple flowers
b = pink flowers
BUT, a second gene (on a different chromosome) is also involved – A/a
Any plants with genotype aa have white flowers.
Neither B(purple) or b (pink) can be expressed if there is no dominant allele
(A) present.
The aa is epistatic to the B/b alleles (which are said to be hypostatic).
What colour?
aaBB
AaBb
white
purple
aabb
Aabb
AAbb
AABB
white
pink
pink
purple
Dominant epistasis
E.g. summer squash (looks like a marrow).
Two gene loci are involved - D/d and E/e.
E = yellow fruits
e = green fruits
BUT………..
If the D allele is present at the first locus then
the alleles at the second locus are not
expressed and the fruit is white.
Write down all the genotypes that produce:
a) white fruit.
b) yellow fruit.
c) green fruit.
More dominant epistasis
Feather colour in some chickens is another example.
This is controlled by alleles at two gene loci, I/i and C/c.
The dominant allele C codes for coloured feathers and the recessive allele c
for white.
BUT…………..
If the dominant allele I is present at the first locus then the alleles at the
second locus are never expressed and the feathers are always white.
Which allele is epistatic and which is hypostatic?
2) Genes at different loci working in
complementary fashion
Two genes interact to produce a different effect from that which
either gives separately.
E.g. Flower colour in sweet peas.
This is controlled by alleles at two loci, C/c and R/r.
Flowers are always white unless one dominant allele is present
at both loci, then they will be purple.
If cc or rr is present, it will mask the expression of the allele at
the other locus.
This is because C codes for an intermediate colourless
pigment (c = no intermediate compound).
R codes for an enzyme that converts the intermediate
compound to the purple pigment (r = no conversion).
Gene C
Precursor substance
(colourless)
Gene R



Intermediate compound
(colourless)

Final pigment
(purple)
Inheritance of flower colour in sweet peas