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 Q#9 and Q#10  

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9- A woman who married her first cousin wants to know the risk of having a child with cystic fibrosis because her grandmother, who is also her husband's grandmother, died of cystic fibrosis. Which of the following is her risk?
A. 1/8
B. 1/16
C. 1/60
D. 1/120
E. 1/256

10- A physician makes the diagnosis of 47,XYY in a 16-year old boy. Which of the following options is most appropriate for the physician during the counseling session that follows the chromosome result?
A. Recommend karyotyping of the parents
B. Explain that the recurrence risk for such chromosomal aberrations is about 1%
C. Urge that the school receive a copy of the karyotype since these boys often have behavior problems
D. Recommend testosterone supplementation when the boy reaches puberty
E. Inform the parents that their child will be sterile

Explain why your answer is right. Thanks.
Will post answers later.


B. 1/2X1/2X1/2 X1/2=1/16

2/4 means 1/2 risk ( AA,Aa.Aa,aa) AA -Homozygous normal, aa diseased, aA CARRIER

Edited by reet on Feb 07, 2006 - 10:52 PM


Anser is B and B.
Can anyone please explain the first one???

What is the fraction of genes shared by siblings? parent-child? with aunt? cousin? grandparents?

for XYY, he has an extra Y; he will definately NOT need extra testoserone... its normal male, no mental retardation, and he will be fertile, it happens about 1% of the time.


i am still not confident on these questions,, but i just reframed my answer above..


Here's the answer that was given:

The grandmother has cystic fibrosis, so her children are obligate carriers. Each cousin therefore has a 1/2 chance of being a carrier. The woman's risk is 1/2 ¡Á 1/2 ¡Á 1/4 = 1/16 chance of having an affected
child Q10
The recurrence risk for aneuploidies caused by meiotic nondisjunction is about 1% in addition to the maternal age-related risk. It is not known why the risk for aneuploidy increases slightly after an affected child is born, but parental karyotypes are almost always normal. Surveys of penal institutions have revealed an increased incidence of 47,XYY individuals, but other conditions with mental disability are increased as well. As with other chromosomal syndromes, the phenotype of 47,XYY is variable and can be found coincidentally in normal males. It would therefore be inappropriate to label a child as abnormal in school unless there have been previous concerns about a medical disorder. Males with Klinefelter's syndrome (47,XXY), rather than those with 47,XYY syndrome, are often sterile and may require supplementation with male hormones.


the grandmother is affected one---> her father/or mother has 100% chance of heterozygote--> she has 50% chance of heterozygote.

Similarly, her husband's chance of heterozygote is 50%

So her child risk= 1/4*1/2*1/2= 1/16

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