SAT Subject Biology Practice Question 321: Answer and Explanation

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Question: 321

11. Hemophilia is a disorder in which blood fails to clot. John, a male hemophiliac, marries Jane, a normal woman, and together they have four children, two boys (Mark and Mike) and two girls (Molly and Mary). None of the children display the symptoms of hemophilia. Mark, Mike, Molly, and Mary all marry normal individuals and have children. None of Mark's or Mike's children, male or female, display symptoms of hemophilia, but the sons of Molly and Mary all display symptoms of hemophilia while the daughters of Molly and Mary do not.

Which of the following best explains the reason that Mark, Mike, Molly, and Mary do not display symptoms of hemophilia, even though their father, John, is a hemophiliac?

A. Hemophilia is an X-linked disorder, and John can pass on only his Y chromosome.
B. Hemophilia is an X-linked disorder and even though Molly and Mary received a hemophiliac X chromosome from John, Jane gave them a normal X chromosome.
C. Hemophilia is a Y-linked disorder and therefore cannot be displayed in females.
D. Hemophilia is a Y-linked disorder and Mark and Mike must have received an X chromosome from John.
E. Hemophilia is an X-linked disorder, and even though Mark and Mike received a hemophiliac X chromosome from John, Jane gave them a normal X chromosome.

Correct Answer: B

Explanation:

B As soon as you see "Hemophilia," you should be thinking "X-linked disorder." Then use the technique for avoiding the temptation trap, which is particularly dangerous here, because the passage and the questions are confusing, and it's very tempting to just guess blindly. Resist! Take the paragraph apart piece by piece, sentence by sentence. Write out genotypes as you read through, and construct Punnett squares to help you see probability. Out of this family, the only members that express this condition are males. This is a tip-off for X-linked disorders, which are more common in males because they have only a single X chromosome. (In any case, you should remember the two most common X-linked disorders: hemophilia and color blindness.) John's genotype is XhY. He passed his Y chromosome to Mark and Mike; they also received a normal X from Jane, thus they do not have hemophilia, nor can they pass it on to their kids. Molly and Mary received Xh from John but also received a normal X from Jane, thus they are carriers of hemophilia but do not display its symptoms.

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