5: Genes in Populations Flashcards
What is genotype frequency
Freq of a particular one-locus genotype in a population
What is allele frequency
Freq of a particular allele in a population
Formula for the freq of M
Formula for the freq of m
M = P + 0.5*H = p
m = Q + 0.5*H = q
What are P, H and Q
P = freq MM
H = freq Mm
Q = freq mm
What are p and q
p = freq M
q = freq m
Formulas for freq of MM, Mm, and mm
MM = p^2
Mm = 2(p)(q)
mm =q^2
What is Hardy-Weinberg Equilibrium
a relationship between allele and genotype frequencies established by random mating that can remain constant from one generation to the next
How many generations before we’re in HWE
After just one generation of random mating
Equations for P, H, Q under hardy-weinberg
P = p^2
H= 2pq
Q = q^2
What do we mean by ‘forces that disrupt HWE’
Things that change allele or genotype frequencies across generations or prevent P=p^2, H=2pq, Q=q^2 from being a good approximation
What is HWE used for
Making predictions - not perfectly adhered to
If values deviate a lot from HWE can indicate errors in genotyping
How would we reduce frequency of the m allele
Selecting only MM and Mm to become parents
Slide 17
Good example
What forces can disrupt HWE
Selection
Migration
Random drift
Non-random mating (inbreeding and outbreeding)
What is selection
Favoring certain genotypes to become parents; changes allele and genotype frequencies across generations
What is migration
Adding or removing individuals of certain genotypes can change allele and genotype freq across generations, cause deviations from HWE equations
What is random drift
Changes in allele and genotype frequencies across generations in small populations due to chance
What is inbreeding
Mating individuals who are closely related. Tends to increase homozygous genotypes in progeny compared to what you get with random mating
What is outbreeding
Mating individuals who are very distantly related (unrelated). Tends to increase heterozygosity in progeny compared to predictions of HWE
Example of random drift
Small population of rabbits with .5 p (freq of A) and .5 q
Only five rabbits reproduce, and they all happen to be AA or Aa
In the next generation p will be .7 and q will be .3
Only two rabbits reproduce to make the third generation, now p is 1 and q is 0
What is fixation
One genotype is lost; population is ‘fixed’ at this locus
Particular allele becomes the only allele at its locus in a population
Random mating vs inbreeding
RM: any genotype mates with any genotype
Inbreeding: tendency for like genotypes to mate (=more homozygotes)
Random mating vs outbreeding
RM: any genotype mates with any genotype
Outbreeding: tendency for unlike genotypes to mate (= more heterozygotes)
How does inbreeding affect simply-inherited traits
Increases uniformity of these traits, helping to ‘fix’ alleles (e.g. coat colour)
BUT increases the risk of recessive disease alleles becoming homozygous
Effective way of doing outbreeding
Cross breeding (males of one breed, female of another)
How does outbreeding affect simply inherited traits
Might reintroduce unwanted alleles for some simply-inherited traits but reduces risk of recessive disease alleles becoming homozygous
Slide 24
Recessive disease alleles image
How do inbreeding and outbreeding affect polygenic traits
Through gene combination value = boost in performance of animals w lots of het genotypes, reduced in animals w lots of homo genotypes (inbreeding)
Outbreeding favourable
Boost in performance when outbreeding is called… Reduced performance from inbreeding is called…
Hybrid vigor and inbreeding depression
How can you use inbreeding and outbreeding in combination for polygenic traits
Inbreeding used to create diff populations of inbred animals that r homo at many loci
Inbred populations then crossed (outbreeding) so progeny are hets at many loci
= hybrid vigor
Go over example qs in slides
27, 16, 17, 13, 8
