The Hardy-Weinberg Principle

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Hardy–Weinberg Principle

From Wikipedia, the free encyclopedia


The Hardy–Weinberg principle states that both allele and genotype frequencies in a population remain constant--that is, they are in equilibrium--from generation to generation unless specific disturbing influences are introduced. Those disturbing influences include non-random mating, mutations, selection, limited population size, random genetic drift and gene flow. It is important to understand that outside the lab, one or more of these "disturbing influences" are always in effect. That is a Hardy Weinberg equilibrium is unlikely in nature. Nonetheless, the idea of genetic equilibrium is a basic principle of population genetics that provides a baseline for measuring genetic change.

In the simplest case of a single locus with two alleles: the dominant allele is denoted A and the recessive a and their frequencies are denoted by p and q;

freq(A)=p; freq(a)=q; p + q = 1.
If the population is in equilibrium, then we will have
freq(AA)=p2 for the AA homozygotes in the population
freq(aa)=q2 for the aa homozygotes
and freq(Aa)=2pq for the heterozygotes.


The overall equation for the Hardy-Weinberg equilibrium is expressed in this way:

p2 + 2pq + q2 = 1


Based on these equations, we can determine useful but difficult-to-measure facts about a population. For example, a patient's child is a carrier of a recessive mutation that causes cystic fibrosis in homozygous recessive children. The parent wants to know the probability of her grandchildren inheriting the disease. In order to answer this question, the genetic counselor must know the chance that the child will reproduce with a carrier of the recessive mutation. This fact may not be known, but disease frequency is known. We know that the disease is caused by the homozygous recessive genotype; we can use the Hardy-Weinberg principle to work backward from disease occurrence to the frequency of heterozygous recessive individuals.

This concept is also known by a variety of names: HWP, Hardy–Weinberg equilibrium, HWE, or Hardy–Weinberg law. It was named after G. H. Hardy and Wilhelm Weinberg.


Problem 1

In a population of mice, long hair (h) is recessive and short hair (H) is dominant. The population of the mice is 100, and there are 9 mice with long hair.
Question A: How many of the mice have short hair?
Answer: mice

Question B: What percentage of the alleles for hair length in this population are the long hair (h) allele?
Answer %

Question C: What percentage of the alleles for hair length in this population are the short hair (H) allele?
Answer %

Question D: How many of the mice are homozygous for the short-hair allele?
Answer mice

Question E: How many of the mice are heterozygous for the hair allele?
Answer mice



Problem 2

In snapdragons, there are two alleles for flower color, red flower color (R) and white flower color (r). The heterozygotes have pink flowers. In a particular population of snapdragons, 81% of the flowers are red.

Question A: What is the frequency of the red flower color allele in this population of snapdragons?
Answer

Question B: What is the frequency of the white flower color allele in this population of snapdragons?
Answer

Question C: What percentage of snapdragons in this population will exhibit white flowers?
Answer %

Question D: What percentage of snapdragons in this population will exhibit pink flowers?
Answer %



Problem 3

In humans, unattached earlobes are dominant, and attached earlobes are recessive. In China, it is reported that 64% of the population exhibit unattached earlobes.

Question A: What percentage of the Chinese population exhibit attached earlobes?
Answer %

Question B: What is the frequency of the recessive (attached earlobe) allele in the Chinese population?
Answer

Question C: What is the frequency of the dominant (unattached earlobe) allele in the Chinese population?
Answer

Question D: What percentage of the Chinese population are homozygous for the dominant allele (unattached earlobe)?
Answer %

Question E: What percentage of the Chinese population are heterozygous for the earlobe allele?
Answer %