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Meiosis and variation
Lesson 6
Interactions between gene loci
part 1
Learning objectives
You should be able to:
•
Describe the interactions between loci
(epistasis). (Production of genetic
diagrams is not required)
•
Predict phenotypic ratios in problems
involving epistasis
Interaction between
alleles for different genes
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
1) Working
antagonistically
Epistasis may be dominant or recessive. In
other words, the alleles at the first locus
that masks the action of the second may be
dominant or recessive.
The alleles at the first locus are said to be
epistatic
The alleles at the second locus, the action of
which is being masked, are referred to as
hypostatic
Recessive epistasis
Flower colour in Salvia (the herb sage) is a good example
Two gene loci are involved. These are described as A/a and B/b.
In other words, both loci may have a dominant or a recessive
allele
At the second locus the B allele produces purple flowers and the
b allele pink if present in homozygous form
BUT…………..
If both recessive alleles are present at the first locus (aa) then
neither allele at the second locus can be expressed (bb, Bb or
BB) and the flowers will be white
Dominant epistasis
This is where the dominant allele at one locus masks the
expression of the alleles at the second locus
Summer squash (looks like a marrow) is a good example
Two gene loci are involved, D/d and E/e
The E allele produces yellow fruits and the e allele 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
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
Chicken breeding
Pure breeding White Leghorn chickens have
the genotype IICC
Pure breeding white Wyanodotte chickens
have the genotype iicc
If these two breeds are mated then all
progeny will be white with genotype IiCc
If these progeny interbreed they will produce
white and coloured feather chickens in a
13:3 ratio
2) Genes at different loci
working in complementary
fashion
Sweet peas may have white and purple flowers
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
This is because the first gene codes for an
intermediate colourless pigment, if the dominant C
allele is present
The second gene codes for an enzyme that converts
the intermediate compound to the purple pigment,
if the dominant R allele is present
Summary of mechanism by
which primrose flower
colour is determined
Gene C/c
Precursor substance
(colourless)
Gene R/r



Intermediate compound
(colourless)

Final pigment
(purple