GENETIC DISEASES RESEARCH PROJECT

When a genetic mutation occurs in a population, a new characteristic may be displayed. If the mutation causes health problems, a person with the mutation is considered to have a genetic disease. This activity is intended to give you an opportunity to do research to determine the allele frequencies of a genetic disease. You will write about the effects of a genetic disease, search out the number of people with the disease (not an easy task), and use this information to determine the frequency of alleles, as well as the number of genotypes/phenotypes in the population. You will need at least a week to gather information.

You will recall in the Hardy-Weinberg principle that , where P represents the frequency (probability) of a dominant allele, and q represents the frequency of the recessive allele. Also recall that when this binomial is expanded we obtain . represents the frequency of a person who has inherited two dominant alleles (pure dominant homozygote) and will display the dominant trait (the trait will be visible to an observer). represents the frequency of the person with the dominant trait, who has inherited one dominant and one recessive allele (heterozygote), and so is a carrier of the recessive trait. represents the frequency of a person who has inherited two recessive alleles (pure recessive homozygote) and will display the recessive trait.

Shown below are examples of calculations needed to determine allele frequency.

* Given: Disease A is the recessive trait and affects people.

Solution: Since the disease is recessive, it affects only people who have inherited two recessive alleles. So and =0.1. From we can determine that . Now that we know both P and q, we can determine the number of carriers. The carriers are people who have a recessive allele in their genetic make-up yet display the dominant trait. That is, they have one recessive and one dominant allele.

The frequency of the:

carrier is ,

pure dominant is , and

the recessive trait is .

*Note: The sum of the three values above is one.

* Given: Disease B a dominant trait affecting individuals.

Solution: A dominant disease affects those individuals who have inherited one or two dominant alleles. In the expansion of , the dominant frequency is =. The frequency of individuals not affected is so and . We know that and so conclude . There are no carriers of this disease. Those with one dominant and one recessive allele aren't considered carriers because their genetic make-up contains the dominant allele and so they will have the disease.

* Given: Disease C a recessive trait that is sex linked (X chromosome/recessive) and affects male births.

Solution: A female has two X chromosomes, whereas a male has one X and one Y chromosome. An allele that is linked to the X chromosome will be contained on only the X chromosome (there will not be a corresponding allele on the Y chromosome). When a recessive genetic disease is linked to the X chromosome a male can only receive one of these recessive alleles. In order for a female to inherit this recessive trait she would need two recessive alleles, but it is different for the male. He needs only to inherit one recessive allele. That is because he has no equivalent gene on the Y chromosome to dominate the recessive X chromosome allele. Therefore, every male that is affected has the recessive X chromosome allele, and so we conclude that , and .

* Given: Disease D a recessive trait with the frequency of carriers being .

Solution: We know that and since , we can substitute for P and obtain . The quadratic formula calculates so or . Since the trait is recessive we conclude that and . Let's take this problem farther and use the frequency values to determine what the genetic make-up would be for a population of 1,000,000 people. The number of people with a particular genetic make-up is found by multiplying the frequency values times the total population. So the number of people who are:

pure dominant is

carriers is

pure recessive is

*The sum of the values above is not equal to the population because of rounding error.