This logic isn't right for 2 reasons. First, the parents may share genes. So if one parent has AB and the other BC then average odds of a child inheriting a parent's gene is 75% not 50%, because B is 100%. The second problem is that the same genes are spread throughout the population. So if you have 2 couples - (AB,CD) and (AC,BD) - then the chance of losing genes is based on the sum of the children. Extend this to a big population, and the chance of losing common genes drops dramatically.