Hardy-Weinberg

Hardy-Weinberg in Nature

-It is rare for all conditions to be met. -Often, not even one condition is met. -If all conditions are not met, the population evovles.

Determining if Evolution is Occurring Are the following populations likely to be undergoing evolution? Read the description of the population and select yes or no from the drop-down menu.

A laboratory population of beetles is kept under conditions ideally suited for that species. All the beetles' needs are met. The space is very large, but contained. -No A large natural population of sea stars have lived near shore and relatively unchanged for many generations. A hurricane causes waves that sweep large numbers of sea star larvae into the area. -Yes

Conditions for Hardy-Weinberg Equilibrium

Condition for Hardy-Weinberg equilibrium: -Large population -Random mating -No migration -No natural selection -No mutations

Real-World Connection Hardy-Weinberg in the Human Population

Most human populations do not meet all the conditions for Hardy-Weinberg equilibrium, so human population are evolving. -Mating is generally not random -Considerable migration occurs

Purpose of the Hardy-Weinberg Equation

The Hardy-Weinberg equation can be used to: -Determine probable frequencies of genotypes in a population. -Track changes of genotypes from generation to generation. Genotype frequency: the percentage of a specific genotype within a population. -Genotype frequencies can be calculated from allele frequencies. Allele frequencies: the percentage of a specific allele within a population.

A Closer Look at the Hardy-Weinberg Equation

p^2+ 2pq + p^2 = 1 -p= frequency of A ( alleles) -q=frequency of a (alleles) -p2= predicted frequency of AA (genotypes) -q2= predicted frequency of aa (genotypes) -2pq= predicted frequency of Aa (genotypes) -At any point, the 3 genotypes frequencies add up to 1 (see graph).

Dominance vs. Frequency

-Before Hardy and Weinberg, scientist thought dominant alleles increased through each generation, overrunning recessive alleles and causing them to decrease. -Hardy and Weinberg showed that gene frequencies in populations remained stable unless affected by environmental change. -Their equation showed that dominant alleles can decrease as well as increase. They increase if they are adaptive and decrease if they are not.

The Hardy-Weinberg Equation

-If a population has two alleles(A and a) the sum of the frequencies, or probabilities, for the 2 alleles, p and q are 1 (100% of the alleles). -The 2 alleles types result in 3 genotypes (AA, Aa, and aa). Each genotype has 2 alleles, so the number of alleles in the population is twice the population size. -Each term represents a genotype. The AA genotype =p^2, aa genotype= q^2, and Aa genotype= 2pq. p+1= -p= frequency of A allele in population -q= frequency of a allele in population Equation: p^2 + 2pq + p^2 = 1

Real-World Connection Sickle-Cell: A Recessive Gene with Staying Power

-In sickle-cell disease, red blood cells have a sickle shape that blocks blood flow. -It has three genotypes: AA normal, Aa carrier, aa affected. -In West Africa, carriers (Aa) are more protected from malaria than AA or aa individuals. -Although harmful, the recessive gene stayed in the population.

Process of Predicting Genotypes from Phenotypes

-To determine if a population is in Hardy-Weinberg equilibrium, compare: 1. actual genotypes of the current and next generations 2. expected genotypes with actual genotypes in a single generation -Genotype frequencies stay the same= population in equilibrium -Genotype frequencies changes= population evolving

How Research Leads to New Questions

-When scientist assumed dominant traits should have the highest frequencies and recessive traits the lowest, they began to ask why this did not always appear to be true. -Research(including the development of the Hardy-Weinberg equation) eventually showed that frequencies are not based on dominance, but instead on how adaptive a trait is. Recessive traits persist when they help a population survive. -Now, scientist asks other questions. They might ask what environmental factors(selective pressures) can cause gene frequencies to change. Answers help them understand which traits (dominant or recessive) are adaptive and which are not

How to used the Hardy-Weinberg Equation

-You can use a known term or allele frequency to solve both allele frequencies and genotype frequencies by plugging it into the proper equation. -If q = 0.4, then p= 0.6, because p + q = 1 and therefore p=1-q

Defining Conditions for Hardy-Weinberg Equilibrium For Hardy-Weinberg equilibrium to occur (that is, for evolution not to occur), several conditions that would cause evolution must be absent. Which of the following conditions are required for Hardy-Weinberg equilibrium? Check all that apply.

-mating is random -the population is large -there is no migration into or out of the population

Population Genetics Term

Evolution: is a process of change in a population through genetic variation over time. Natural selection is a process in which organisms better adapted to their environment tend to survive, reproduce, increase in number, and pass traits to succeeding generations. Gene flow: the transfer of genes from one population to another. Genetic drift: the random change in the frequency of an allele in a population.

Evolution and Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium: the condition in which both allele and genotype frequencies in a population remain constant from generation to generation unless specific disturbances occur. -A population in Hardy-Weinburg equilibrium is not changing genetically, not evolving. -Comparing gene frequencies in two succeeding generations can show if evolution is occurring(it is occurring if the frequencies change) and can determine which direction and rate of evolution. -It cannot determine which factor is causing the genetic changes.

Identifying Dominance and Frequency

Which statement best describes the relationship between an allele's frequency and its dominance? -Frequency results from environmental stresses, not dominance. Which of the following explains why the allele for sickle-cell disease remains at a relatively high level in the population? -It confers protection from malaria to carriers.