Population Genetics

Dr. Arshad Ayyaz

What year did the world human population surpass 8 billion?

2022

What is the world human population estimated to be in 2025-2026? 2050?

• 8.2-8.3 billion in 2025-2026

• Projected to reach approximately 9.7-9.8 billion by 2050

Are humans distributed randomly across the world?

No, they are clustered in discrete populations (local groups of people sharing a common gene pool)

What is meant by a gene pool?

The set of genetic information (e.g. alleles of a gene) carried by the members of a sexually reproducing population

What can populations be described by?

Age structure, geography, birth and death rates, and allele frequencies

What is population genetics?

The study of genetic variation and how genes and genotypes are maintained or change in populations (NOT in individual matings)

Are populations or individuals more diverse? Why?

Populations

• only a group can carry all of the alleles for traits such as blood types A, B, AB, and O

• Gene pool = collection of all the alleles represented in a population

How does geography affect inferences?

Gene pools in two geographically distinct populations will likely be different. Therefore, inferences drawn for one population are not generally applied to other populations or the entire humanity

Why do we study population genetics?

1. Identification of genetic risk factors: helps identify individuals at higher risk for certain diseases

2. Enhancement of preventative healthcare: detect predispositions to diseases

3. Promotion of health equity: incorporating diverse genetic data ensures that all populations benefit

4. Optimization of resource allocation: allocating resources effectively

5. Advancements of personalized medicine: targeted therapies and interventions

• To achieve these, alleles and their frequencies must be identified

What is allele frequency?

The frequency with which alternate forms of a particular gene are present in a population

• allele frequencies may change in a population from generation to generation (and can cause genotype frequency changes)

What is meant by complete dominance?

Heterozygotes and homozygous dominant have the same phenotype

What is meant by incomplete dominance?

Homozygotes have a more severe phenotype than a heterozygote (“semi-dominant”)

• here we know the genotype based on the severity of the disease

• Haploinsufficiency

What is meant by co-dominant alleles?

Both alleles of a pair are expressed in the heterozygous state

• allele frequencies can be measured directly by counting phenotypes (phenotype = genotype)

  • MN blood group (three phenotypes M, N, MN)

What gene encodes a red blood cell membrane protein in the MN blood type example?

Glycophorin A

• 2 variants of protein can be easily detected with antibody testing in individuals

How can allele frequencies be determined when there is complete dominance

Hardy Weinberg Law

What are the assumptions of HWE?

1. The population is large enough that there are no sampling errors in measuring allele frequencies

2. All genotypes are equally able to reproduce

3. Mating is random

4. No migration in or out

5. No new mutations

6. No matings between different generations (22-35 year difference)

7. All matings reproduce same # of offspring who are equally fertile

Why is it 2pq?

Two parents, each can give each allele

What is genetic equilibrium?

When the allele and genotype frequency for a particular gene remains constant from generation to generation

• equilibrium in a population explains why dominant alleles do not replace recessive allleles

What happens when a population is not in equilibrium?

Allele frequencies may stay the same, however, the combinations of genotype frequencies could change… this can increase observed phenotype frequency

How do we statistically test HWE?

Chi-square test (requires actual allele frequencies and actual genotype frequencies