If a healthy woman in her forties, who does'nt have Niemann-Pick disease type A, but a carrier of all allele for Neimann-Pick disease, marries a healthy ..
man in his forties. And man's family history show no signs of Neimann-Pick disease type A. So what is the probablity that this couple will have
a) a child with Niemann-Pick disease type A?
B a child who carries the recessive allele for the Neimann-Pick disease type A?
c) a child who neither has nor carries the recessive allele for the Neimann-Pick disease type A?
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The answer to this lies in the type of genetic mutation that Niemann-Pick is. We need to ask ourselves two questions:
1) Is the trait sex-linked or autosomal?
2) Is the trait dominant or recessive?
Considering whether the trait is sex-linked or autosomal has profound implications on how the disease may be passed down. Most of the time, sex-linked disorders are mutations with one of the two X chromosomes in the mother. If the couple has a male, the only X chromosome the sone will receive is from the mother, giving the son a 50-50 chance of receiving the mutation. In general, these traits are recessive, and daughters generally do not receive them as a result (getting a second, functional X chromosome from the father). Hemophilia is the most famous example of a sex-linked trait where sons generally display the problems with clotting, but daughters simply become carriers.
In our case, Niemann-Pick is autosomal, which means, pretty much, "not sex-linked." This means that there is no difference between male and female because the mutation is on one of the 22 pairs of chromosomes that are not the sex chromosomes.
Now, we need to see whether the mutation is dominant or recessive. Dominance is a HUGE issue here because it will determine whether there is any possibility of the child receiving the condition whether or not the father has it. An example of a dominant mutation is Huntington's Chorea, where if a child only has one copy of the gene, that child will get Huntington's.
The opposite is the case for Niemann-Pick, which is a recessive condition. The child must have 2 copies of the defective gene before the disease takes hold.
So, knowing these facts about Niemann-Pick (that it is autosomal and recessive), we can analyze the possible situations for the children of this couple. Because the male has no ancestors with the problem, it's very unlikely he would be a carrier of the gene. We already know the female is a carrier.
You can make a Punnett Square to look at the possibilities for offspring, recognizing that the father would be of type SS and the mother would be of type Ss, where S is the dominant normal sphingomyelinase gene and s is the Niemann-Pick sphingomyelinase gene.
From the Punnett Square, you can glean that 50% of the children will be without the mutation completely, and that the other 50% will be "carriers" who have a copy of the mutated allele.
So, this gives us our answer:
A) 0% of the children will have Niemann-Pick
B) 50% will have the recessive, mutated allele
C) 50% will not carry the recessive allele.
I hope that helped! By the way, for us, it doesn't matter what kind of Niemann-Pick we have for the genetic analysis. They're all autosomal-recessive. It would affect a treatment decision, though!
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