Population Genetics

• What is population genetics?
• Frequencies of alleles in populations
• The Hardy-Weinberg Law
• Mechanisms that change allele frequencies
• Examples of human and animal population
genetics


What is a population?


• A species
– All members of the same species can
interbreed and produce fertile offspring
– not all have the opportunity


  • A population
    – A group of organisms within the same
    species that do have the opportunity to interbreed
    – Capable of gene flow🧬

Population Genetics

• Up to now considered genetics of individuals, but it is whole populations of individuals that mate

• Phenotypes appear with different frequencies in the population


-Up to now considered genetics of individuals, but it is whole populations of individuals that mate.

• Phenotypes appear with different frequencies in the population

• E.g. albinism – in humans and animals it is a relatively rare trait compared to the wild pigmented
phenotype

—

• Why are we interested in it?
• the probabilities of inheriting genetic traits – genetic counselling

  • factors that change the frequencies of genetic traits
    • gene flow between populations
    • effects of inbreeding

Allele Frequencies


• The variability of phenotypes in an individual population
(human or animal) is due to allele differences


• Differences in the frequency of phenotypes in a population must be due to differences in allele frequencies



• Usually the frequency of an allele is given as a % or decimal fraction:
– Frequency of A = 0.8 or 80%
– Frequency of B = 0.2 or 20%

1000-40 = 960%3= 320 A=160, a=20


IMPORTANT!!!

What happens to allele frequencies during genetic

crosses?


• If there are two alleles that determine a phenotype, the frequency of those two alleles will add up to 100%:

• Frequencies of alleles are designated by p and q

– p = frequency of one allele

– q = frequency of the other allele

• p + q = 1

Allele Frequencies

There are several assumptions under the HW Principle:

• Random mating must happen

• Populations must be large

  • No selection pressures

• Immigration or emigration

-Hardy-Weinberg Law Allele frequencies remain the same throughout generations


  1. albino tiger



  1. Phenylketonuria


Causes of evolutionary change in gene frequencies

-The HW principle indicates that allele frequencies do not change as a result of reproduction:

p = 0.5 and q = 0.5


• But there are processes that DO cause change in allele frequencies

Comparing populations


  • the frequencies are very different numbers which shows no gene flow, if numbers similar then there may be gene flow.






Mechanisms that cause allele frequency

changes

  • Factors causing Hardy-Weinberg law to break down:⭐️


1. Small populations - chance events

2. Founder effects

3. Gene flow between populations (migration)

4. Mutational changes

5. Non-random mating

– Inbreeding (e.g. Hapsburg Lip and Jaw)

– Selfing


Mechanisms that cause allele frequency changes

• Factors causing Hardy-Weinberg law to break down:⭐️



1.Small Populations, Chance events

a) Genetic drift (random loss or gain of alleles)

b) Bottleneck effect

P= frequency of A

q= frequency of a



-bottleneck effect 🍾

• Bottleneck effects: a sharp reduction in population size due to environmental effects resulting in a loss of genetic diversity


-Bottleneck in northern elephant seals \/

Mechanisms that cause allele frequency changes

• Factors causing Hardy-Weinberg law to break down:⭐


  1. Founder effects

-Founder effects: loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.


Founder effects example: Retinitis pigmentosa on Tristan de Cuhna (night blindness)


• Island was first colonised in 1810 by 15 people

– Just one person had night blindness

– Current population today has a high frequency of the condition

4.Gene flow between populations (migration)



Mechanisms that cause allele frequency changes

• Factors causing Hardy-Weinberg law to break down:⭐


5. Mutational change: before evolution can happen, genetic variation must exist within a population.


• Allele frequencies change over time because some alleles mutate into others

• Eventually, these alleles become fixed in the population



Mechanisms that cause allele frequency changes

• Factors causing Hardy-Weinberg law to break down:⭐


6. Assortive Mating-mate choice: a mating pattern and form of sexual selection where individuals with similar phenotypes mate with one another more frequently than expected.


• E.g.Bufo toads commonly choose mates based on size

– Large toads choose large toads

– Small toads choose small toads

7. Natural Selection IMPORTANT


The direction and magnitude of change depend on:

– Selection intensity

– Dominance relations of the alleles

– Allele frequencies