A disease caused by gene X is an autosomal recessive disease but is modified by expression of a gene on another chromosome gene Y.If gene Y has at least one dominant allele,then the disease is aggressive and becomes evident at age 2.If gene Y has two recessive alleles,then the disease is mild and dormant until the age of 50.Two people who are heterozygous for both gene X and Y have a child.What are the chances the child will have the aggressive form of the disease?

Question

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When two heterozygous individuals for gene X and Y (both carriers of one dominant and one recessive allele) have a child, there are 16 possible combinations of alleles that the child could inherit. Out of these 16 combinations, only 3 will result in the aggressive form of the disease (YYXX, YYXx, YyXX), as in all these cases the child inherits at least one dominant allele for gene Y. Therefore, the chance that the child will have the aggressive form of the disease is 3 out of 16, or 3/16, which is choice C.

A Disease Caused by Gene X Is an Autosomal Recessive