Dr. Smith notices a rare physical trait in one of his students that he suspects

Question

Dr. Smith notices a rare physical trait in one of his students that he suspects is genetic. To study this further, he finds more people with this trait by advertising online. To simplify the analysis, he limits his study to families with exactly three children (i.e., respondents each have two siblings). Assuming that the trait is autosomal, recessive and rare; and further assuming that his advertising has identified people with the trait at random, what fraction of the families would he expect to have 0/3 affected children, 1 affected child, 2 affected children and (all) 3 affected?

Explanation / Answer

Answer:

During rare autosomal inheritance, recessive allele fraction will be 1/4 or 0.25 and the dominant allele fraction will be 3/4 or 0.75.

The fraction of the families would he expect to have 0/3 affected children= 0.75 * 0.75 * 0.75= 0.42

The fraction of the families would he expect to have1 affected child= (0.25) + (0.75 * 0.75) =0.81

The fraction of the families would he expect to have 2 affected children= (0.25 * 0.25) + (0.75)=0.81

The fraction of the families would he expect to have (all) 3 affected = 0.25 * 0.25 * 0.25 = 0.02

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