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NBME 24 Answers

nbme24/Block 1/Question#40

A 26-year-old man and his 25-year-old wife come to ...

One in four will have 25% β-globin function and may require occasional transfusions

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 +11  upvote downvote
submitted by brethren_md(44),

Make a punnett square with a cross of B B+ and B B0; B+ represents 50% function while B0 represents 0% (null) function. So in this case, the husband would have a B B0 genotype while the wife has a B B+ genotype.

Cross of these two will result in the following genotypes; BB, BB0, BB+, B+B0 BB = 100% function, BB+ = 75% function, BB0 = 50% function, B+B0 = 25% function

So the answer will be 1 in 4 have a 25% function given the genotypes.

 +0  upvote downvote
submitted by medstruggle(6),

Whats the difference between “heterozygous null mutation in B globin gene” and “heterozygous mutation known to cause 50% decrease in B globin gene function of one allele”?

welpdedelp  I interpreted "null" as meaning full deletion while the other heterozygous mutations was only a 50% decrease in the function. One child would inherit 1 null mutation and 1 50% mutation, which would leave them with a 25% functional gene. +1