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BIOLOGY A married man and woman, both of whom are deaf, carry some recessive mutant alleles in three different "hearing genes": d1 is recessive to D1, d2 is recessive to D2, and d3 is recessive to D3. Homozygosity for a mutant allele at any one of these three genes causes deafness. In addition, homozygosity for any two of the three genes together in the same genome will cause prenatal lethality (and spontaneous abortion) with a penetrance of 25%. Furthermore, homozygosity for the mutant alleles of all three genes will cause prenatal lethality with a penetrance of 75%. If the genotypes of the mother and father are as indicated here, what is the likelihood that a live-born child will be deaf?
$$
\begin{array}{l}{\text { Mother: }\ D1\ d1,\ D2\ d2,\ d3\ d3} \\ {\text { Father: }\ d1\ d1,\ D2\ d2,\ D3\ d3}\end{array}
$$
