Sickle cell anemia and albinism are both recessive traits in humans. The genes for these traits are located on different chromosomes. Imagine that a couple, already pregnant with twins, has just learned that they are both heterozygous for both of these traits. As the couple's genetic counselor, the couple asks you the following questions about how their carrier status will affect their offspring.

If the couple has fraternal twins, what is the probability that both children will be unaffected by both conditions?

Given that, both parents are heterozygous. Now, the chances for one child to inherit one defective gene from each parent = ½*1/2 = ¼.

Similarly, the probability of being affected by albinism = Probability of inheriting one defective gene from each parent = ½*½ = ¼

The probability of not being affected = 1 - the probability of being affected = 1-1/4 = ¾.

Now, the chances of not being affected by both albinism and sickle cell anemia = ¾*3/4 = 9/16.

The probability of neither twins be affected = 9/16*9/16 = 81/256.