Lab 3 - Population Genetics and Changing Allele Frequencies

Population genetics

• The study of how allele frequencies vary within populations and change over time.

Allele frequency

• The proportion of all copies of a gene in a population that are a particular allele.

• It can change through mutation, migration, genetic drift, natural selection, or nonrandom mating.

Population

• A group of individuals of the same species that live in a particular area and can reproduce with one another.

Hardy-Weinberg equilibrium

• A model in which allele and genotype frequencies remain constant across generations when specific assumptions are met: random mating, a very large population, no mutation, no migration, and no natural selection.

Nonrandom mating

• When individuals do not choose mates randomly, genotype frequencies can change.

• Inbreeding increases homozygosity and can increase the chance that recessive alleles are expressed, while not necessarily changing allele frequencies by itself.

Consanguinity

• Mating between biologically related individuals increases the chance that offspring inherit the same allele from both parents, raising homozygosity and the likelihood of recessive genetic conditions.

Identical by descent (IBD)

• Two copies of DNA are identical by descent when they were inherited from a common ancestor.

• Shared DNA segments can provide evidence of common ancestry.

Runs of homozygosity (ROH)

• Long stretches of DNA in which the two copies of a chromosome are identical.

• They can arise when both copies were inherited from a shared ancestor.

• Longer runs can indicate more recent shared ancestry.

Endogamy

• Marriage or reproduction within a particular social, cultural, or other defined group.

• It can contribute to distinctive allele frequencies within that group and increase genetic differences between groups when mating between them is restricted.

Clines

• Gradual geographic changes in allele frequencies across space.

• They can develop through migration, environmental differences, and natural selection.

• Geographic barriers and cultural boundaries can interrupt these patterns.

Assortative mating

• Individuals choose mates based on similarity or difference in particular traits.

• Positive assortative mating tends to increase homozygosity for the traits involved; disassortative mating can increase heterozygosity.

Genetic load

• The collection of harmful or deleterious alleles present in a population.

• Harmful recessive alleles can remain hidden in heterozygous individuals but may be expressed when two copies are inherited.

Migration

• When individuals move between populations and reproduce, they introduce or remove alleles, potentially changing allele frequencies and making populations more genetically similar.

Gene flow

• The movement of alleles between populations through migration and reproduction.

• It can introduce new variation into a population and reduce genetic differences between populations.

Migration and human variation

• Patterns of human migration help explain geographic differences in allele frequencies.

• For example, migration and historical settlement have influenced the distribution of traits such as lactose tolerance and genetic diseases.

Genetic drift

• Random changes in allele frequencies from one generation to the next, especially influential in small populations.

• It can cause alleles to become fixed or disappear regardless of whether they are advantageous.

Founder effect

• A type of genetic drift that occurs when a new population is established by a small number of individuals.

• The founders’ alleles may be unrepresentative of the original population, causing unusual allele frequencies in descendants.

Population bottleneck

• A sharp reduction in population size that leaves a smaller group of survivors.

• The survivors may carry only part of the original genetic variation, so later generations can have different allele frequencies and reduced diversity.

Genetic drift and small populations

• Chance has a stronger effect on allele frequencies in small populations.

• Rare alleles may disappear, while others may become common or fixed simply because of which individuals reproduce.

Mutation

• A change in DNA that can introduce a new allele into a population.

• Mutation is the ultimate source of new genetic variation, although changes in allele frequency from mutation alone are often slow.

Mutation and evolution

• Mutations can create alleles that are harmful, neutral, or beneficial in a particular environment.

• Their effects on a population depend on factors such as natural selection, genetic drift, and how often the mutation occurs.

Balancing selection

• A form of selection that maintains multiple alleles in a population rather than allowing one allele to replace all others.

• Examples include situations in which heterozygotes have an advantage or different alleles are favored under different conditions.

Sickle-cell trait and malaria

• In regions where malaria is common, people heterozygous for the sickle-cell allele can have greater protection against severe malaria.

• This heterozygote advantage helps maintain the allele despite the health risks associated with two copies.

Antibiotic resistance

• When antibiotics kill susceptible bacteria, resistant variants are more likely to survive and reproduce.

• The frequency of resistance can increase rapidly, especially when antibiotics are overused or misused.

Forces that change allele frequencies

1. Mutation.

2. Migration

3. Genetic drift.

4. Natural selection.

5. Nonrandom mating primarily changes genotype frequencies, particularly homozygosity and heterozygosity.

Population bottlenecks and conservation

• A small surviving population may lose genetic diversity through drift and inbreeding.

• Low diversity can reduce a population’s ability to respond to environmental change and can increase the risk of inherited disorders.

Eugenics

• A movement that sought to influence human reproduction in order to change the genetic composition of populations.

• Historical programs included coercive sterilization and discriminatory policies, often justified by false assumptions about heredity, disability, and social worth.

Positive and negative eugenics

• Historically, positive eugenics encouraged reproduction by people considered desirable, while negative eugenics sought to prevent reproduction by people deemed undesirable.

• Both relied on value judgments and often caused serious human-rights abuses.

Founder effect and inherited disorders

• If a small founding group carries a disease-associated allele at a relatively high frequency, descendants may show an elevated frequency of that condition.

• This can occur by chance and does not mean the allele is beneficial.