4: Dominance and Epistasis Flashcards
What is dominance
interaction between alleles at a single locus such that in heterozygotes one allele has more effect on phenotype that the
other. The allele with the greater effect is dominant over its recessive counterpart.
What is complete dominance
the phenotype of the heterozygous group is the same as one of the homozygous genotypes
What is a dominant allele
when the phenotype of the heterozygote is more similar to one of the homozygotes than the other, the allele in the more similar homozygote is called the dominant allele
What is partial dominance
the phenotype of the heterozygous group is between the two homozygotes but is more similar to one group than the other
Example of partial dominance
HYPP (hyperkalemic periodic paralysis) in horses
= episodes of muscle tremors (shaking-> trembling)
= having one copy of the mutation (n/H) leads to the condition
= but being homozygous for it (H/H) leads to more severe symptoms
What is no dominance
the phenotype of the heterozygous group is exactly midway between the phenotypes of the homozygous groups. There
is no dominant allele.
No dominance example
Gene that determines resistance to tuberculosis
= animals with two copies of a tuberculosis-resistant allele (TR) survive 100% of the time
= animals with two copies of a tuberculosis-susceptible gene (TS) survive only 40% of the time
= and heterozygotes (TRTS) survive 70% of the time
= each additional T^S allele adds 30% susceptibility
What is overdominance
the heterozygote exhibits a phenotype that is beyond the range of either of the homozygotes
Overdominance example
Gene that controls resistance to rat poison (warfarin) in rats
= AA and Aa are unaffected, survive more than aa
= AA also make less vitamin K, and are less healthy
= Aa are resistant and still make vitamin K
What is haplosufficiency
having one copy of the dominant allele is sufficient
e.g. in complete dominance
In overdominance, having two different alleles may give…
intermediate level of protein activity that is for some reason more optimal for phenotype, or having two alleles may give the protein a broader range of activities / capabilities
Slide 12
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What is epistasis
An interaction among alleles at different loci such that the impact
of alleles at one locus depends on which alleles are present at one or more other loci
Example of epistasis
Labrador retriever coat colour
Which loci are involved in lab coat colour? What do they do
MC1R and TYRP1
MC1R has alleles E and e
= if ee present, coat is yellow!!
TYRP1 has alleles B and b
= effect depends on what MC1R alleles are present
= if at least one E present, Bb or BB lead to black lab, bb leads to brown lab
