Question

A man and woman have a son who is color blind, a recessive sex-linked trait carried on the X-chromosome, but neither parent is color blind. The woman is now pregnant with their second child. Which of the following statements is true about the second child?

Answer 1

A daughter will be color blind because the father is a carrier of the recessive allele.

A son will be color blind because the mother is a carrier of the recessive allele.

A daughter will not be color blind because the father is not a carrier of the recessive allele.

A son will be color blind because the father's mother must have been a carrier for recessive allele.

Answer 2

I can still help you with this, if you are still online @ me and we can go through it together. @becca9898

Answer 3

@becca9898 We reproduce in a sexual way (fyi, there is sex without reproduction). Because we reproduce in a sexual process we produce gametes (sperm and egg) through meiosis. Meaning the sperm and egg have half the chromosomes of non-gamete cells.

Humans have 23 pairs, so 46 total chromosomes. They are identical in shape and have the exact same genes, except for the sex determining chromosomes, X and Y. Remember genes can have different phenotypes. Meaning a gene for black hair on one chromsome could code for blonder hair on the other chromosomes. We call these slightly different coding alleles.

Anyway, we get half out chromosomes from the mother and half from the father. If the father gives an Y to the sperm that fertilizes the egg, we will have an XY (male) offspring. If the father gives an X, we will have a XX (female). So, we have 50% sperm with an X and 50% of sperm with an Y.

For females 100% of the eggs have an X, but remember females have two X chromosomes. So really we have 50% of X chromosome #1 and 50% X chromosome #2.

So, because males only have one X they are much more likely to have colorblindness. If neither parent has colorblindness then the male cannot have the faulty gene on his X, and it must be on only one of the female's X chromosome. This means only the males can have colorblindness, and since only 50% will be male and only 50% of the X chromosomes from the female are affected we have 50% of the males as colorblind.

This means that the total offspring with color blindness should be 25% of all offspring. We lose 50% because the are female, and another 50% of the males that are left.