Hardy-Weinburg Equilibrium Learning Objectives/Vocab

Use the Hardy-Weinberg model to build a bridge between Mendelian and Population Genetics

1. Core Idea

• Mendelian Genetics: Focuses on individuals and single family crosses ($Aa \times Aa$).

• Population Genetics: Focuses on entire populations (the overall gene pool).

• Hardy-Weinberg (HW) Bridge: Takes basic Mendelian inheritance rules and applies them to an entire population to predict genotype frequencies.

2. Key Equations

• Allele Frequencies:

  $$\mathbf{p + q = 1}$$

  • p = frequency of dominant allele (A)

  • q = frequency of recessive allele (a)

• Genotype Frequencies:

  $$\mathbf{p^2 + 2pq + q^2 = 1}$$

  • p^2 = Homozygous dominant (AA)

  • 2pq = Heterozygous (Aa)

  • q^2 = Homozygous recessive (aa)

3. The Null Model (Baseline for Evolution)

• HW Rule: Mendelian shuffling (meiosis & fertilization) does not change allele frequencies by itself.

• HW Equilibrium: Allele frequencies stay constant unless evolutionary forces act on them.

5 HW Assumptions (No Evolution Occurring):

1. No mutation

2. Random mating

3. No gene flow (no migration)

4. Large population size (no genetic drift)

5. No natural selection

4. Main Takeaway

• HW Equilibrium = No Evolution (System is at rest).

• Deviation from HW = Evolution is Happening (Signals that selection, drift, or non-random mating is at work).

Know the assumptions of the Hardy-Weinberg model and the requirements for a population to be in Hardy-Weinberg Equilibrium

• Assumptions:

1. No mutation

2. No selection

3. No migration

4. Random mating

5. Infinite population size (no genetic drift)

• Nothing that can change allele frequencies —> NO evolution!

• Genotype frequencies are p2, 2pq, and q2

• AND genotype frequencies are NOT changing over time

Be able to predict genotype frequencies from observed allele frequencies

Look at practice problems/quiz questions

Be able to calculate allele frequencies from observed genotype frequencies

Look at practice problems/quiz questions

Perform a Chi-Square Test for evidence of evolution

X² = (Observed-Expected)²/Expected (Look at practice problems/quiz questions)

Modern Synthesis

Overarching evolutionary paradigm that took shape by the 1940s and scientists generally accept today

Microevolution

Changes in a population’s genetic structure

Macroevolution

Broader scale evolutionary changes that scientists see over paleontological time

Population Genetics

Study of how selective forces change the allele frequencies in a population over time

Allele Frequency

(Also, gene frequency) Rate at which a specific allele appears within a population

Gene Pool

All the alleles that the individuals in the population carry

Founder Effect

Event that initiates an allele frequency change in part of the population, which is not typical of the original population

Genetic Structure

Distribution of the different possible genotypes in a population

Genotype

An individual's genotype is the set of alleles the individual carries, usually referencing a single gene.

Allele

An allele is a version of a gene. A single gene may have different alleles, each of which may result in detectable differences in the gene product, usually a protein

Gene

A gene is a region of DNA that encodes either a specific protein or some other gene product, such as RNA

Population

In biology, a population is a group of individuals of one species living in the same area at the same time. Examples include all fish of one species living in a particular lake, or all pigs in an isolated farm pen

Phenotype

An individual's phenotype is any observable or measurable aspect of its appearance, structure, behavior, or physiological function

Trait

A trait is an specific characteristic of an individual. A trait may be determined by the organism's genes, environmental factors, or a combination of the two

Dominant

A dominant trait is one that is expressed in the phenotype when only a single copy of the associated allele is included in the genotype

Recessive

A recessive trait is one that is expressed in the phenotype only when two copies of the associated allele are included in the genotype. Recessive traits are masked when the genotype contains a dominant allele

Punnett Square

A Punnett square is an analytical device used to predict the genotypic ratios of offspring from a given cross. It was invented by Reginald C. Punnett

Gamete

Gametes are sex cells, which fuse to produce a zygote during sexual reproduction. Many organisms produce distinct male and female gametes, called sperm and ova (or egg cells), respectively

Zygote

A zygote is a fertilized egg, produced by the fusion of two gametes that each contribute DNA to the zygote

Calculating Genotype Frequencies

In a randomly mating population carrying exactly two alleles, A₁ and A₂, for a given gene, you can calculate expected genotype frequencies from allele frequencies. If the frequency of allele A₁ is p, and the frequency of A₂ is q, then:

• Frequency of A₁A₁ = p²

• Frequency of A₁A₂ + A₂A₁ = 2pq

• Frequency of A₂A₂ = q²

Hardy-Weinberg equilibrium principle

The Hardy-Weinberg equilibrium principle states that a population's allele and genotype frequencies will remain constant from one generation to the next, if the population experiences no outside evolutionary pressure. Moreover, for a population in Hardy-Weinberg equilibrium, genotype frequencies can be predicted from allele frequencies

Homozygous

Homozygous describes a genotype that has two identical alleles for a given gene.

Heterozygous

Heterozygous describes a genotype comprised of two different alleles for a given gene. An individual is heterozygous if the allele inherited from one parent is different than the allele inherited from the other parent.

Evolution by Natural Selection

Natural selection is a mechanism of evolution. If a trait in a population is variable, is heritable, and results in differential fitness, then evolution by natural selection will cause the genetic composition of the population to change from one generation to the next.

Differential Fitness

Differential fitness occurs when some individuals in a population either are more likely to survive and reproduce or have more offspring than other individuals, due to differences in trait values

Genetic Drift

Genetic drift is a mechanism of evolution that results when random events change the underlying genetic composition of a population

Mutation

A mutation is a copying error within an individual's genetic code. Mutations that occur during reproduction (usually during the formation of egg and sperm cells) cause an offspring to be genetically different than its parent(s) in unique ways. Mutations are the source of new genetic variation

Migration

Migration is the process of individuals either leaving a population (emigration) or entering a population (immigration), in the physical sense (i.e., not via births and deaths). Migration is one way that alleles move from one population to another (thus is a mechanism of evolution).

Random Mating

Random mating is a term used to describe populations in which every individual is an equally likely mate for every other individual. That is, there are no genetic or behavioral mating restrictions. Offspring produced by random mating will effectively receive their alleles at random from the parental population

Probability

Probability describes the likelihood of an event or occurrence. Mathematically, a probability is expressed as a number between 0 and 1.

Probabilities refer to events or occurrences that are unknown, either because they haven't happened yet or because they haven't been measured or examined yet. Because of this, a probability is an expected value.

The probability of an event can be estimated by observing its frequency.

The Hardy-Weinberg Equation

The Hardy-Weinberg equation describes expected allele and genotype frequencies for a population that is in Hardy-Weinberg equilibrium. The equation applies to a single locus with two possible alleles. The frequency of the first allele is p and of the second is q = 1 − p. Then, at equilibrium:

Where:

• p² is the frequency of individuals homozygous for the first allele.

• 2pq is the frequency of heterozygous individuals.

• q² is the frequency of individuals homozygous for the second allele.

Heterozygote Advantage

A heterozygote advantage occurs when, for a particular gene, a heterozygous genotype confers greater relative fitness than either of the homozygous genotypes comprising the alleles in the heterozygous genotype

Null Model

A null model describes a scenario (such as allele and genotype frequencies in a population) that results from an absence of influencing factors. That is, it describes what you expect if "nothing is going on".

Null models are useful for comparison. In fact, without a null model, it's difficult to attribute interesting observations or results to any particular factor. Null models are thusly related to both the null hypothesis and alternative hypothesis for any given research study.

The Hardy-Weinberg equilibrium model is an example of a null model

Null Hypothesis

A null hypothesis is a statement of the default position that the phenomenon of interest is not occurring.

Usually the null hypothesis posits that two or more entities are not related. It provides a framework for comparison, often when doing some sort of statistical analysis of collected data. The null hypothesis often comes directly from a null model.

Alternative Hypothesis

An alternative hypothesis (also called the research hypothesis) is a statement of relationship or explanation that forms the basis of an experimental or observational study. It is an "alternative" to the null hypothesis. One or the other of these rival hypotheses is supported by the study.

Alternative and null hypotheses underlie all statistical analyses from which inferences are made (i.e., inferential statistics).

Sickle-Cell Disease

Sickle-cell disease (also known as sickle-cell anemia) is a human genetic disease, resulting in malformed red blood cells that are sickle-shaped instead of disc-shaped. It is caused by the sickle-cell allele for hemoglobin (HbS), occurring when individuals are homozygous HbS/HbS.

Malaria

Malaria is a mosquito-borne infectious disease of humans (as well as other animals) caused by parasitic protozoa of the genus Plasmodium. Malaria symptoms typically include fever, fatigue, vomiting and headaches, and hundreds of thousands of people die every year from the disease. People contract malaria by being bitten by mosquitoes that carry Plasmodium.