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Question

A man with normal vision marries a red-green color blind woman. They have eight children in total(five boys and three girls). All of their sons are born with red-green color blindness, while None of their daughters have it.

1. Is the genetic disease recessive, dominant, co-dominant or incompletely dominant and why.

2. is the disease autosomal or sex-linked?

3. Why does the father always determine the sex of a child?

Answer 1

To help answer this question you should draw a pedigree and complete a punnet square.

From this information we can assume that the mom has two color-blind alleles because 100% of her sons are affected.

It is easier to answer question #2 first. It is clear that this condition is X-linked because all of the sons are affected and none of the daughters. Why does this matter? Look at the punnet square. Remember males are XY and females XX. Only a male can give a Y chromosome meaning their X chromosome comes from the mom. If the mom gives an affected X chromosome then the son will have a 100% chance of having the phenotype.

Now let's answer question #1. 100% of the sons are affected so let's assume that the mom is homozygous for the trait because they all got an affected X chromosome. None of her daughters present with the phenotype so even if they are carriers of the gene they do not have the condition. This makes the condition recessive.

Now #3. Remember our discussion about male and female genotypes? Since women are XX the only chromosome they can pass to gametes is an X. Males have XY meaning they can either pass an X or a Y to their gametes. Sex of a child depends on if the male X vs Y gametes fertilize the female X gamete.

Hope this helps!

Answer 2

To explain I have to answer out of order.

3. The father determines the sex of a child because they pass on a Y or an X chromosome. If they pass on the Y chromosome, then child is male, if they pass on the X gene then the child is female. The mother can only pass on an X chromosome.

2.The disease is sex-linked, we assume this since there is a consistent distinction between male and female children in inheriting the disease.

1.The gene is most likely to be recessive. The mother has two X chromosomes that can be inherited by the children, whereas the father has one X chromosome. If the gene was dominant, co-dominant, or incompletely dominant, then we would see some version of the red-green color blindness in the daughters. The father cannot pass on his dominant X chromosome to his sons, so the X chromosome from the mother controls the red-green color blindness. Assuming x to be recessive and X as dominant we can assume the genotype to be: Mother: xx, Father: XY, sons: xY, daughters: xX. Not having the dominant X chromosome leads to the red-green colorblind phenotype.