What is the probability that exactly two of the two offspring will be affected in a family with dominant disease with reduced penetrance? 

Genetics
A dominantly inherited genetic disease is identified over several generations of a large family. However, about half the families have dominant disease with complete penetrance, whereby if a parent is affected there is a 50% probability that any one offspring will be affected. Similarly, about half the families have dominant disease with reduced penetrance, whereby if a parent is affected there is a 25% probability that any one offspring will be affected.
Suppose in a particular family one parent and two of the two offspring are affected.