Apply the concept of sex linkage to explain why color blindness is more prevalent in men than in women.

Question

B) Mary is concerned that she may be a carrier for hemophilia, a sex-linked condition located on the X chromosome. Mary is married to John, who doesn’t have hemophilia. Assuming Mary is a carrier, what are the genotype ratios expected for Mary and John’s kids (specify for boys and for girls)?
C) Mary and John have 2 boys, none of them has hemophilia. Can we use this fact as proof that Mary does not carry the allele for hemophilia? Explain your argument.

Answer 1

Answer:
A) Red-green colorblindness is an X-linked trait. What this means is that the trait is associated with the sex of the individual. Because the trait is associated with the sex of an individual, women carrying the recessive allele for color blindness will more likely give it to their sons because women have two X chromosomes that can potentially pass on their recessive allele for colorblindness, while a man only has one X chromosome that gives him a 50% chance of inheriting their mother's recessive colorblindness allele. For women, it would be more difficult to get the recessive X-linked allele because they possess two X chromosomes and both chromosomes would have to possess the recessive colorblindness allele for the woman to have red-green colorblindness.B) The genotype ratios for Mary and John's kids are 1:1:1:1, 25% chance of a female isn't a carrier of hemophilia (nor has it), 25% chance of a female who is a carrier of hemophilia, 25% chance of a male without hemophilia, and 25% of a male with hemophilia.C) The chances of having a male and him not having hemophilia are 25% so the chances of this occurring are 1 in 16 occurrences which is 6.25%. It can happen, just by chance even if Mary is a carrier. Every time they have a baby boy, the baby has 50% chance of having hemophilia and 50% chance of not having it. Just like when you toss a coin you have 50% of getting tails and 50% chance of getting heads. However, when you toss the coin once and get tails, it doesn't mean that the next time you toss it you will get heads, you may get tails again, because with every toss you have the same chance of getting either one. Thus, although the chance of having two boys and both of them being healthy is only 6.25%, that doesn't exclude the possibility of having another boy with hemophilia. Just like tossing a coin twice and getting heads both times doesn't mean the coin doesn't have a tail side.