BI111 Lecture Notes - Lecture 2: Mendelian Inheritance, Heterozygote Advantage, Selective Breeding
BI111 – L2
Population Genetics (Continued)
Assumption 1: Random Mating
What Happens if we violate the assumptions?
No random mating: Assortative mating (mating with same as other dog)
oIf mated with another red dog → always would result in homozygous recessive (red) dogs
so no change
oFor black dogs: ⅔ of black dogs were heterozygous: when 2 heterozygous black dogs
mated, 50% of the time it would result with 2 black dogs still, but the other 50% would
result in one black dog and one red dog (heterozygous)
oTherefore the population would not be at HWE because the frequencies would change
Disassortative mating (mating with someone opposite to you)
oAfter this we would see an increase in heterozygote dogs showing the black phenotype
and a decrease in red dogs
oTherefore again the population would not be at HWE because there is selective mating
not random mating
If random mating starts again:
HWE will begin again after only one round of random mating
Assumption 2: Large Population Size
In a large population size variation is representative of genetic variation
In a small population, the genotypes may not be representative of the variation of the actual/total
population which is why HW falls apart
Assumption 3: No Migration or Emigration
If some dogs leave they take their alleles with them and if some dogs come into the population
they bring their alleles with them
Assumption 4: No Selection
Some dogs with certain alleles (different phenotypes) are more likely to reproduce than others
Ex. red dogs are selected against (people want black dogs not red)
Black dogs are mated with black dogs only = increase in black dogs
This will decrease the frequency of the b recessive allele
If those red dogs created from the first selected mating are again shipped off, and selected
mating between only black dogs occurs again we will see an increase again in black dogs (and
decrease in homozygous recessive)
Everytime this happens, the fate of the recessive alleles is that the frequency of b alleles will
decrease, but will not be eliminated from the gene pool
oDeleterious allele - an allele that causes death/health problems
Most of these are hidden because are recessive - the only time selection can act
on this allele is in the rare event that an individual has homozygous recessive
Selection act on phenotypes
Most genetic diseases are recessive in nature
Why are homozygous lethal recessive alleles not removed by selection? →
because selection can’t see them, hidden in heterozygous individuals
Population Genetics (Continued)
Clicker: In a population with 2 alleles (B & b), where the frequency of the b allele is 0.6 what is the
frequency of the genotypes at HWE?
Since only 2 alleles and 60% are b (recessive) therefore the other 40% must be showing
dominant
Take the small b frequency and multiply it by itself
oq x q = (0.6 x 0.6) = 0.36
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