Transcript 4_-_epistasis_2

Interactions between gene loci
(part deux)
Learning objectives:
To draw a genetic diagram to show inheritance
of 2 alleles (dihybrid inheritance).
To understand the ratios of phenotypes
obtained when epistasis is involved.
Coat colour in mice
This is another example of interaction between gene loci.
Coat colour in mice is controlled by two gene loci B/b and A/a.
If there is a dominant B allele at the first loci then the mouse is
able to produce black pigment from an initial colourless precursor.
If there is a dominant A allele present at the second locus then the
black pigment is converted in to one that produces an agouti coat
colour.
Gene B
Gene A

Precursor substance
(colourless)


Black pigment

Agouti pattern
What are the possible genotypes of white, black and agouti mice?
white
bbAA
bbAa
bbaa
black
agouti
Bbaa
BbAA
BBaa
BbAa
BBAA
BBAa
Combs of domestic chickens
Again, this is controlled by two gene loci that
interact with each other, P/p and R/r
The effect of the P/p allele depends on which of
the R/r alleles are present
Comb shapes in domestic
chickens
P-R- =
ppR- =
P-rr =
pprr =
walnut comb
rose comb
pea comb
single comb
What do we get when we cross 2
heterozygous walnut combed chickens?
Parents phenotype:
Parents genotype:
gametes:
Now put this into a punnett square.
What phenotypes do you get, and in what
ratio?
How does epistasis affect these
ratios?
• Show the results of a cross between 2 purple
heterozygous sweet pea plants
Inheritance of flower colour in sweet peas
Predicted ratios – learn these!
How to work out what is going on!
Dihybrid inheritance of two unlinked genes produces a
ratio of 9:3:3:1
9:3:4 ratio suggests recessive epistasis.
12:3:1 ratio or a 13:3 ratio suggests dominant epistasis.
9:7 ratio suggests complementary gene action.
Does epistasis increase/decrease variation?
A genetic explanation for one of Mendel’s dihybrid
crosses