one of whom exhibits mild mental disability. The granddaughter with disability has a son with severe mental disability. Fragile X DNA testing reveals the severely affected son to have greater than 700 triplet repeats near his fragile X locus on the X chromosome. Recall that normal fragile X alleles have 5-59 repeats upstream of the FMR-1 gene.
Based on this information, the most likely results of fragile X DNA testing for the normal man and his granddaughter with disability would be:
A. 67 repeats in the normal man, 45 and 18 repeats in his granddaughter with disability
B. 14 repeats in the normal man, 400 and 210 repeats in his granddaughter with disability
C. 62 repeats in the normal man, 237 and 42 repeats in his granddaughter with disability
D. 14 and 37 repeats in the normal man, 205 in his granddaughter with disability
E. 14 repeats in the normal man, 14 repeats in his granddaughter with disability
Answer with an explanation please! Much appreciated!
Based on this information, the most likely results of fragile X DNA testing for the normal man and his granddaughter with disability would be:
A. 67 repeats in the normal man, 45 and 18 repeats in his granddaughter with disability
B. 14 repeats in the normal man, 400 and 210 repeats in his granddaughter with disability
C. 62 repeats in the normal man, 237 and 42 repeats in his granddaughter with disability
D. 14 and 37 repeats in the normal man, 205 in his granddaughter with disability
E. 14 repeats in the normal man, 14 repeats in his granddaughter with disability
Answer with an explanation please! Much appreciated!
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So with this question the number of repeats is going to increase through generations to first have no effect (man and daughter) to having a little effect (granddaughters) to having a severe effect (her son).
We also know that the man is XY so he only has one X chromosome for the mutation. Because he only has one X chromosome to give, his daughter would inherit this X chromosome.
Now she has two daughters. One inherits a good X chromosome, one inherits a bad X chromosome (with extra repeats).
From this information, let's start eliminating choices.
A is incorrect because the granddaughter has a normal level of repeats in both chromosomes.
B is incorrect because both of the X chromosomes in the granddaughter are abnormal.
D is incorrect because the man has two x chromosomes.
E is incorrect because the girl only has one X chromosome.
Now C is the only answer left. So we know that this is right. 62 is a high number of repeats but probably wouldn't manifest in any detectable disability. But over two generations, these repeats increase to 205 which begins to cause a problem. Her other X chromosome (from the son in law) is within normal range. Then when the granddaughter has a son, the repeats increase to 700 and he has a huge problem.
When I started I said this mutation is linked back to the man. This is because he gave his X chromosome (I'll call it "x") to his daughter.
xY + XX -> Xx
man + wife -> daughter
The daughter then had two daughters:
Xx + XY -> XX + Xx
daughter + son in law -> granddaughter no disability + granddaughter with disability
This mutation had to have come from the father. The only other place it could come would be the son in law, but then both granddaughters would have to get it.
We also know that the man is XY so he only has one X chromosome for the mutation. Because he only has one X chromosome to give, his daughter would inherit this X chromosome.
Now she has two daughters. One inherits a good X chromosome, one inherits a bad X chromosome (with extra repeats).
From this information, let's start eliminating choices.
A is incorrect because the granddaughter has a normal level of repeats in both chromosomes.
B is incorrect because both of the X chromosomes in the granddaughter are abnormal.
D is incorrect because the man has two x chromosomes.
E is incorrect because the girl only has one X chromosome.
Now C is the only answer left. So we know that this is right. 62 is a high number of repeats but probably wouldn't manifest in any detectable disability. But over two generations, these repeats increase to 205 which begins to cause a problem. Her other X chromosome (from the son in law) is within normal range. Then when the granddaughter has a son, the repeats increase to 700 and he has a huge problem.
When I started I said this mutation is linked back to the man. This is because he gave his X chromosome (I'll call it "x") to his daughter.
xY + XX -> Xx
man + wife -> daughter
The daughter then had two daughters:
Xx + XY -> XX + Xx
daughter + son in law -> granddaughter no disability + granddaughter with disability
This mutation had to have come from the father. The only other place it could come would be the son in law, but then both granddaughters would have to get it.