How to Calculate the X-linked Carrier Frequency
A gene is said to be sex-linked when the disease or trait caused by it is inherited through either the X or Y sex chromosome. X-linked disorders more commonly affect males because males only need to inherit one affected X-chromosome. Females must receive 2 copies of the gene, one from each parent, to have the disease. A female with one only one affected X chromosome is a carrier for the disease and typically shows no symptoms. You can calculate the frequency of these disorders in a given population using the Hardy-Weinberg equation.In the Hardy-Weinberg equation, p^2 + 2pq + q^2 = 1 P is the frequency of the dominant allele. In the case of X-linked disorders, p is the frequency of the normal X chromosome and q is the frequency of the X chromosome containing the gene for the disorder. The 2pq portion of the equation describes a carrier for an X-linked disorder. All the frequencies added together will equal 100 percent or p+q=1.
Things You'll Need
- Calculator
Instructions
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1
Decide if you have been given the value for p or q to substitute in your equation. In order to find the frequency of the carriers, you must first be given an allele frequency, and then use the Hardy-Weinberg equation to solve for the piece of information you are looking for.
For example, if you know that 20 percent of the males in a population have Hemophilia, then p + .20 =1 and 1-.20=p, so p =.80. You can use this information to calculate the frequencies for females, and of carriers.
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2
Substitute the values calculated in Step 1 into the Hardy-Weinberg equation. For example, 2 pq is the frequency of carrier females in the population, p^2 is the frequency of females having 2 normal X chromosomes that are neither a carrier nor affected by the disorder and q^2 is the frequency of females with Hemophilia.
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3
Solve the equation to find the frequency females that will be carriers. In the example problem, 2pq= 2* .20 *.80= .32 or 32 percent of the females will be carriers.
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