BIOL 112 Lab Exam 1 Lab 1

Deleterious Alleles

Alleles that negatively affect an organism; usually recessive and often lethal.

Recessive Refuge

A phenomenon allowing a deleterious recessive allele to persist by hiding in heterozygous individuals.

Why does recessive refuge occur?

The recessive trait is masked by the presence of a beneficial allele.

Example of recessive refuge

Albinism.

When is recessive refuge easier to maintain?

When the recessive allele becomes rarer in the population because it is not being selected against.

Heterozygote Advantage

A phenomenon where heterozygotes have a survival advantage over either homozygous genotype.

Example of heterozygote advantage

Sickle-cell anemia and malaria.

Gene

A unit of DNA that determines a given trait.

Allele

A different form of a gene.

Allele example

B = brown eyes; b = blue eyes.

Genotype

The genetic makeup of an organism.

How many genotypes are possible for one gene with two alleles?

Three possible genotypes: homozygous dominant, heterozygous, and homozygous recessive.

Phenotype

The physical appearance of an organism based on genotype; may be modified by the environment.

Population

A group of interbreeding individuals.

Population Genetics

The study of genes and alleles in specific populations.

Gene Pool

All alleles present in a population and their relative proportions or frequencies.

Evolution

A change in allele frequency from one generation to the next.

Malaria

A disease caused by infection with the single-celled Plasmodium parasite.

Plasmodium

A single-celled parasite that causes malaria and must infect human red blood cells to complete its life cycle.

Difference between malaria and Plasmodium

Malaria is the disease; Plasmodium is the parasite that causes the disease.

Malaria vector

The female Anopheles mosquito, which transmits Plasmodium between humans.

Vector

An organism that transmits a pathogen from one host to another; for malaria, the mosquito.

Host

The organism that harbors a pathogen; for malaria, humans are the host.

Where is malaria most prevalent?

Very wet regions because mosquitoes need standing water to breed.

Why is malaria unlikely in temperate regions?

Plasmodium does not survive temperatures below 20°C (68°F) for long.

Sickle-cell allele (HbS)

A mutated or alternate form of the normal human hemoglobin allele (HbA).

Normal hemoglobin allele (HbA)

The normal human hemoglobin allele responsible for producing normal hemoglobin.

Hemoglobin

A protein found in red blood cells that is responsible for oxygen transport.

How is the sickle-cell allele inherited?

Through standard Mendelian genetics.

Effect of HbS on hemoglobin

HbS causes hemoglobin to polymerize, forming long chains.

Effect of two HbS copies

Two HbS copies cause red blood cells to become stretched, spindle-shaped, or sickle-shaped.

HbA/HbA phenotype

Normal hemoglobin with no resistance to malaria.

HbA/HbS phenotype

Sickle-cell trait; provides resistance to malaria.

HbS/HbS phenotype

Sickle-cell anemia; provides no resistance to malaria.

Sickle-cell trait

Having one HbA allele and one HbS allele; provides resistance to malaria.

Why is HbS incompletely recessive?

HbA/HbS carriers produce some sickled red blood cells, but usually not enough to cause symptoms.

Natural Selection

The process in which environmental interactions cause individuals with different traits to reproduce with varying success.

Reproductive success

The ability to produce offspring that survive and reproduce, allowing genetic traits to be passed on.

How does natural selection change populations?

Individuals with greater reproductive success pass their genetic traits to the next generation in greater numbers.

Adaptation

Inherited characteristics obtained through natural selection that improve an organism's survival and reproduction.

Are natural selection and adaptation conscious processes?

No. Natural selection and adaptation are not conscious processes.

Long-term result of natural selection

Over enough time, populations can acquire many adaptations and develop into distinct new species.

Hardy-Weinberg Equilibrium

A mathematical model for a sexually reproducing population that is not undergoing evolution.

Allele frequencies in Hardy-Weinberg equilibrium

Allele frequencies do not change from one generation to the next.

Purpose of the Hardy-Weinberg model

A tool used to assess whether a population is evolving.

First assumption of Hardy-Weinberg equilibrium

Random mating.

Second assumption of Hardy-Weinberg equilibrium

No natural selection or artificial selection.

Third assumption of Hardy-Weinberg equilibrium

Large population size.

Fourth assumption of Hardy-Weinberg equilibrium

No immigration or emigration.

Fifth assumption of Hardy-Weinberg equilibrium

No mutations.

Sexual Selection

Occurs when an organism's phenotype affects its ability to reproduce.

Which HWE assumption does sexual selection violate?

The first assumption: random mating.

Natural Selection in HWE context

Occurs when an organism's genotype gives it a survival advantage.

Which HWE assumption does natural selection violate?

The second assumption: no natural or artificial selection.

Genetic Drift

A change in allele frequencies caused by random outcomes of mating.

When is genetic drift strongest?

In small populations.

Which HWE assumption does genetic drift violate?

The third assumption: large population size.

Effect of genetic drift in large populations

Its effects are generally minimal.

Common effect of genetic drift

It often leads to a loss of genetic variation.

Genetic Bottleneck

A reduction in population size that can compound the effects of genetic drift.

Founder Effect

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

Why are bottlenecks and founder effects important?

They are important when considering genetic rescue of endangered species.

Fixation

When an allele reaches a frequency of 1.0 and the other allele is lost from the population.

What does fixation mean for a population?

100% of the population is homozygous for the fixed allele.

Gene Flow

The movement of individuals into or out of a population that adds or removes alleles.

Which HWE assumption does gene flow violate?

The fourth assumption: no immigration or emigration.

Effect of gene flow on allele diversity

It may introduce entirely new alleles for a gene into a population.

Mutation

The spontaneous occurrence of a new allele within a population.

Which HWE assumption does mutation violate?

The fifth assumption: no mutations.

Effect of mutation on allele frequencies

Mutations usually alter allele frequencies slightly but can have major effects if advantageous.

p + q = 1

The equation showing that the frequencies of two alleles for one gene sum to 1.

p

Frequency of one allele in a population.

q

Frequency of the other allele for a single gene.

Allelic Frequencies

The frequencies of different alleles in a population.

p² + 2pq + q² = 1

The Hardy-Weinberg equation showing the frequencies of three genotypes sum to 1.

p²

Frequency of the homozygous genotype for one allele.

q²

Frequency of the homozygous genotype for the other allele.

2pq

Frequency of the heterozygous genotype.

Genotypic Frequencies

The frequencies of different genotypes in a population.