McGraw-Hill Education SAT Subject Test Biology by Stephanie Zinn

McGraw-Hill Education SAT Subject Test Biology by Stephanie Zinn

Author:Stephanie Zinn
Language: eng
Format: epub
Publisher: McGraw-Hill Education
Published: 2019-12-19T16:00:00+00:00


Hemophilia is a disease that affects the ability of blood to clot properly. It is a recessively inherited disorder carried on the X chromosome. Figure 6-20 traces the occurrence of hemophilia in a family.

Figure 6-20

What is the likelihood that the individuals of the third generation marked with a question mark will have hemophilia? What is the likelihood that the female in the third generation will be a carrier for hemophilia?

Again, the first step in solving this problem is by listing the genotypes of each of the individuals in the pedigree that we know for certain. Because, in this pedigree, the individuals are not lettered, we’ll use the traditional numbering system of I-1, I-2, etc.

We know, for example, that individual II-3, the father of the children about whom we have asked, does not have hemophilia. Because he is a male, and hemophilia is carried on the X chromosome, we know that his one X chromosome has a dominant allele. His genotype can, therefore, be written as XAY. We also know that individual I-1, who has the hemophilia, has a recessive allele on his X chromosome; his genotype can be written as XaY.

To figure out the likelihood of the third-generation children having hemophilia, we need to know the genotype of their mother, individual II-2 (we have already figured out the genotype of their father). We know that the mother does not have hemophilia. But is she a carrier? Well, we know that her father had hemophilia, and that her father passed on an X chromosome to her (otherwise she would not be female). Therefore, we know that she is indeed a carrier of hemophilia. We can thus write her genotype as XAXa.

Now, we must turn to our original question, which asks the probability that each of her children will have hemophilia, and the probability that the female child will be a carrier. To answer this question, we can complete a Punnett square—it’s not hard to do because we already know the genotypes of the parents. We know that the father can pass on either an Xa allele or a Y allele; the mother can pass on either an XA allele or an Xa allele:

Our Punnett square in Figure 6-21 provides many of our answers for us. What is the likelihood that the individual III-1 will have hemophilia? Well, of the two female squares in our Punnett square, we see that neither has both recessive alleles. There is, thus, no chance that III-1 will have hemophilia. For individual III-2, we see that of the male genotypes in our cross have hemophilia. Thus, there is a 50% chance that this individual will be affected by hemophilia.



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