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473 is right. There are two alpha globulin genes on chromsome 16. Each alpha globulin gene contains two alleles giving a total of four alleles. Alpha thal trait is defined by loss of two alleles. If both alleles are lost from the same gene, this is a cis-configuration. If one allele is lost from each gene, this is a trans-configuration. The patient in question with alpha thal trait has inherited 2 mutated alleles that were lost from the same gene(cis configuration). This is NOT a case in which there was a mutated allele lost from each gene(trans-configuration). Since the patient is Asian, you can assume that the cis-configuration mutation is likely present as it is most commonly associated with this group; this can also be suggested by the fact that the patient in question has a +ve family history of hydrops fetalis. So this is a case where the patient has two mutated alleles in one alpha globulin gene and the other alpha globulin gene is totally normal (aa/--). If this patient has a child with a genotypically normal female(aa/aa), the chance of the offspring having alpha thal trait (aa/--) is 50% while the chance of being genotypically normal (aa/aa) is 50% as well.
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