If individual #1 in the following pedigree <image 1> is a heterozygote for a rare, AR disease, what is the probability that individual #7 will be affected by the disease? Assume that #2 and the spouses of #3 and #4 are not carriers.
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There is a 1/2 chance that an offspring of any mating Aa x AA, will be a carrier (Aa). So, there is a 1/2 chance that #3 will be Aa, and likewise for #4.  If #3 is a carrier, there is again a 1/2 chance that #5 will be a carrier, and likewise for #6.  If #5 and #6 are both Aa, then there is a 1/4 chance that this monohybrid cross will result in #7 having the genotype aa and, therefore, affected by the disease.  Thus, the joint probability is 1/2 x 1/2 x 1/2 x 1/2 x 1/4 = 1/64