Exam 2: #2 Population Genetics

Population

Individuals that interbreed within the same area

Population Genetics

Concerned with genetic variation

• 90% of human genetic variation: is due to SNPs (single nucleotide polymorphisms)

  • SNPs: a change in a single nucleotide (A, T, C, or G) in the DNA sequence

• Population Geneticists: study genetic variation in a gene pool and how it changes from one generation to the next

Monomorphic

has only one allele in a population

• Everyone in the population has the same version of that gene

Gene pool

All alleles in a population

• Only individuals that reproduce contribute to the gene pool of the next generation

Allele Frequency

Proportion of a specific allele of a gene in a population

• (# copies of an allele) / (total # of alleles)

Example:

• Population: 10 individuals → each has 2 alleles → total alleles = 20.

• Genotypes: 3 AA, 4 Aa, 3 aa.

Step 1: Count the A alleles:

• AA → 3 × 2 = 6

• Aa → 4 × 1 = 4

• Total A alleles = 6 + 4 = 10

Step 2: Divide by total alleles:

Frequency of A= 10/20 = 0.5 

Step 3: Frequency of a = 1 − frequency of A = 0.5

Genotype frequency

Proportion of individuals in a population that have a specific genotype

• (# of individual with genotype) / (total # of individual) 

Hardy Weinberg Equilibrium

Describes a population in which allele and genotype frequencies remain constant from generation to generation

• Genotypic frequencies: p2 + 2pq + q2 =1

• Allele frequency: p + q = 1

Hardy Weinberg Assumptions

If all of these conditions occur, the population is in Hardy Weinberg equilibrium

1. No new mutation

2. No genetic drift: population so large, allele frequencies do not change

3. No migration

4. No natural selection: all genotypes equally likely to survive

5. Random mating

Directional Selection

Natural selection where one extreme of a trait is favored over the other

• Favors one extreme phenotype → increases its frequency in the population

• Reduces the other extreme → the population becomes skewed toward the favored trait.

Balancing Selection

Natural selection that maintains genetic diversity in a population

• keeps two or more alleles at higher frequencies than would be expected by chance

• Heterozygote advantage: Sickle cell (Aa) because it is resistant to malaria

Disruptive/ Diversifying Selection

Individuals with extreme traits at both ends of a spectrum have higher fitness than those with intermediate traits

• Increases genetic variation

• Common in diverse environments

• Bad for intermediates

Stabilizing Selection

Favors survival of individuals with intermediate phenotypes

• Against extremes

• Decreases genetic variation

Fitness (w)

Likelihood a genotype will reproduce

• Average Reproductive

Genetic Drift

Random changes in allele frequency due to random fluctuations

• Common in small populations

• or after population collapse like bottleneck

  • Ex: Natural Disaster Occurs: Blue randomly survives

  • Population now monomorphic, which decreases genetic diversity

Founder Effect

Small group of individuals separate from larger population and establish a colony in a new location

• Founding population: less genetic diversity

Migration- gene flow

The transfer of alleles from donor population to recipient population, changing gene pool

• Bidirectional Migration consequences:

  • Reduces allele frequency between populations

  • Increases genetic diversity within a population

Assortative Mating 

Non-random mating

Positive Assortative Mating

Individuals are more likely to mate due to similar phenotypic characteristics

Negative Assortative Mating

Individuals with dissimilar phenotypes mate preferentially

• Decrease in mating with same phenotype

Inbreeding

Mating between genetically related individuals

• Gene pool smaller bc parents are genetically related

• Increases proportion of homozygous and decreases heterozygous

  • Negative because many diseases are recessive and inbreeding increases homozygous recessive allele frequency

  • Inbreeding depression: reduced fitness

Outbreeding

Mating between genetically unrelated individuals