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Lecture 16

BIOL299 Lecture Notes - Lecture 16: Epistasis

Biology (Biological Sciences)
Course Code
Darren De Lorey

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Lec16 BIOL207
Genetics of two loci, continued
9:3:3:1 phenotypic ratio expected if no gene interactions, i.e. phenotype of one
locus does not affect phenotype the other locus.
If the phenotype of one locus DOES affect the other locus, we get different ratios,
which are derived by summing various classes from within 9:3:3:1
a. 9:7 (complementary gene action) e.g. purple flowers
TWO phenotypic classes: genes act in the same pathway, and both
genes are required; loss-of-function (LOF) of EITHER gene (aa__
or __bb) blocks production of normal end product: intermediate
products can’t be distinguished phenotypically
b. 15:1 (redundant gene action) e.g. grain color
TWO phenotypic classes: genes act in the same pathway and
EITHER gene is sufficient; LOF of BOTH genes (aabb) is
required for loss of normal end product
c. 9:3:4 (recessive epistasis) e.g. retriever coat color
THREE phenotypic classes: genes act in the same pathway and
BOTH are required; LOF in EITHER gene blocks the production
of a normal end product; some intermediates CAN be
distinguished from each other, and e.g. aaB_ can be distinguished
from A_bb, but not from aabb
In retriever example, recessive alleles at one locus can be said to
be epistatic to the genotype of the other locus; i.e. recessive
homozygotes aa completely mask the phenotype of B i.e. (aaB_,
aabb have the same phenotype)
***BE SURE YOU DO NOT CONFUSE dominance (applies to
alleles of a single locus) with epistasis (applies to alleles at
different loci) ***
d. 12:3:1 (dominant epistasis) e.g. squash pigmentation
THREE phenotypic classes, but dominant allele in one gene masks
the phenotype of the other gene, e.g. dominant allele A_ masks
phenotype at B; so A_B_ and A_bb look the same
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