DLA 10a Population And evolutionary genetics part A Flashcards
Analyze a biallelic system
A-dominant allele (or more common allele)= p
a-recessive allele(or the less common allele)= q
We recognize that there can be 3 possible genotypes:
- AA
- Aa
- aa
What are the requirements under which binomial expansion may be applied to genetics for populations?
If alleles are randomly distributed in a large population
And there are no factors selecting for or against any particular genotype
This relationship will follow the mathematics of the binomial expansion
-(p+q)^2
What is the Hardy Weinberg equilibrium equation?
(P+q)^2= p2 +2pq+q2
What does hardy Weinberg equation describe?
This equation describes the distribution of genotypes in people
So “1”=the whole population, and each component of p2+2pq+q2 represents the fraction of the population
p^2= fraction of people who have genotype for dominant alleles
q^2= fraction of people who have genotype of recessive allele
2pq= fraction of carriers
Genotypes are _______ created
Randomly
Why are genotypes randomly created?
Randomness comes from meiosis, and independent assortment
So when 2 different populations (same species) meet and randomly mate, only 1 generation is needed to achieve a new Hardy Weinberg equilibrium for all alleles, at all loci
-randomness ensures that the mathematically described distributed will be followed
What does the Hardy Weinberg equation allow us to do?
Allows us to relate allele frequency to genotype frequency
Allele frequency (for a given locus):
- number of alleles in the population that are dominant(A)
- number of alleles in the population that are recessive(a)
- Can be obtained by counting the alleles directly, or back calculating from phenotype(as will be explained later)
- Alleles are found within people (if autosomes, each person has alleles)
The higher the frequency of a dominant allele …
As p/frequency of the dominant allele increases and q2 becomes more and more rare, the vast majority of recessive alleles are marked as carriers, and less are marked as homozygous
How is the frequency affected across generations?
The frequency of a is halved in two generations and halved again by the sixth generation
Subsequent reductions occur slowly because the majority of a alleles are carried by heterozygotes
We can’t eliminate all q*2 possibilities as 2pq allows it to be possible
Spontaneous mutation in gametogenesis allows for a steady state rate at low frequencies
What is the simplifying rule?
If q is small, then estimate that p is qapproximately = 1
This is because incidence values/q2 from real life are only rough estimates
General rule: if q is less than 5% of all alleles, p is close enough to 1 for our calculations
Why are deleterious alleles never eliminated?
There will always be heterozygous which allows the deleterious recessive allele to exist
In the absence of other evolutionary forces, genotype frequencies are…
Expected to remain constant and the population is said to be at hardy Weinberg equilibrium