Sickle cell anemia and albinism are both recessive traits in humans. Imagine that a couple, already pregnant with fraternal twin
s, has just learned that they (the couple) are both heterozygous for both of these traits. As the couple's genetic counselor, the couple asks you what is the probability of having both of the twins be albino and have sickle-cell anemia.
By doing a test cross of albinism and sickle cell anemia you'll see that the probability that one child is unaffected by either conditions will be 9/16. Both children would be 9/16 * 9/16.
Since fraternal twins are developed from two separate fertilization events the probability chance will be independent.
