How to Answer Hardy-Weinberg Questions - A-level Biology - with question pack

Had a request for some Hardy Weinberg Questions.

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Remember to look out for questions where they give you a dominant phenotype frequency -for instance Huntingdons disease, where the frequency of the sufferers is 1 in 100000 - therefore q squared (recessive allele phenotype) is 99999 in 100000.

Also if they ask you to calculate allele frequency without HW - then all individuals in a population have 2 alleles of each gene. In a population of 600 rats there are 1200 alleles in total.

If 400 are AA, 100 are Aa, and 100 are aa then the frequency of a is (100+200)/1200 = 0.25.

Remember that the HW depends on some very unlikely assumptions !

Assumption 1: No Genetic Drift

Sexual reproduction recombines genetic information in a random pattern. In a small population, it is possible that few individuals carry an allele and simple chance could make them more or less successful in passing on that allele. This is known as genetic drift, and the Hardy-Weinberg assumes that it does not happen. In practical terms, a population at Hardy-Weinberg equilibrium has to be large enough that the frequency of an allele is not impacted by random events.

Assumption 2: A Closed Population

Emigration and immigration -- that is, transfer of individuals into and out of a population -- can change the frequency of alleles. Emigrating individuals might take more of one allele out of a population, and immigrating individuals might come from a population with a different proportion of alleles. This is known as genetic flow, and it is common for most populations, with notable exceptions such as remote islands, deep caves or mountaintops. A population at Hardy-Weinberg equilibrium is assumed to have no genetic flow, or to be a completely closed population.

Assumption 3: Mutations Don’t Happen

Mutations are errors that happen when DNA is being copied. They can be nonsense or neutral, they can be harmful to the organism or they can change the organism in a way that makes them better able to survive and reproduce. Hardy-Weinberg equilibrium assumes that mutations don’t exist because they represent genetic change that is not due to the recombination of existing alleles.

Assumption 4: Random Mating Patterns

The Hardy-Weinberg principle assumes that every individual in a population has an equal chance of mating with every other individual, totally random mating. In a population at Hardy-Weinberg equilibrium, nobody gets to be picky. Random mating, also known as a panmictic population, almost never happens in real life.

Assumption 5: No Natural Selection

Natural selection is a pressure that favors one allele over another because the resulting trait, or phenotype, gives the individual an advantage. From a genetic standpoint, the only advantage that matters is one that helps the individual survive or reproduce more effectively. In a population at Hardy-Weinberg equilibrium, there are assumed to be no evolutionary pressures.

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