Is the genetic disease recessive, dominant, codominant, or incompletely dominant, and why?

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.

Answer 1

To understand this answer, first we must consider the genetics of males vs females. Males have XY chromosomes, whiles females have XX. Sperm cells will also contain either the X or Y from the male parent, while everyone gets 1 of the X chromosomes from the mother. This gene is also X linked, meaning it's located on the X chromosome.

The mother is colorblind. For now we don't know if the disease is dominant or recessive. If it is dominant, and the mother has 2 copies of the colorblind gene, then all the kids would be colorblind. Try your own punnet squares for both XcX and XcXc vs XY. If the mother is XcX (only has 1 copy), then we would expect half of the boys and half of the girls to be colorblind. We don't see this in the above situation.

Now lets consider if colorblindness is recessive. Look at the previous punnet squares, but now think of it in terms of being recessive. The mother must have the genotype XcXc, as she is colorblind. Look back on that punnett square of XcXc vs XY but now view it as a recessive gene. All males would be colorblind, as males only have 1 X, if they have the effected gene, there is no "normal" X to offset it. And no females would be colorblind as they only have 1 copy. This matches the description given above.

We can conclude that this is a recessive gene. Try out some punnet squares with the XcX and XcY and other variations to see how this gene is passed on to males and females differently.

Answer 2

Hi! Since this question brings attention to the biological sex of the offspring, we can assume it's a sex-linked trait. In this problem, the mom is the one suffering from color blindness, which means this trait must be carried in the X chromosome. The dad is not a sufferer, which means he is X^wtY (wt = wild type). Now, in order to answer the question, we can start by figuring out if the mom is heterozygous for the color blindness trait (X^wtX^colorblind, which would tell us the trait is dominant) or homozygous (X^colorblindX^colorblind). Let's assume each one may be true and think about the traits the offspring would exhibit for each scenario:

1. If the mom was heterozygous, then at least half of the male offspring would inherit the X^wt chromosome from her (this can be figured out by using a Punnett square). As X^wtY, this group of male offspring would not exhibit color blindness since none of the chromosomes are carrying the color blindness allele. However, the problem tells us that all of the male offspring are color blind, so it is not very likely that the mom is heterozygous after all.
2. If the mom was homozygous (X^colorblindX^colorblind), then all of the male offspring would inherit one X^colorblind chromosome. As X^colorblindY, they would all exhibit colorblindness. This checks out with what the problem stated! Now, let's think about the female offspring. If the mom is homozygous, then all of the female offspring would inherit one X^colorblind chromosome from mom and one X^wt from dad. In other words they would all be heterozygous for the colorblind trait (X^wtX^colorblind). If color blindness was a dominant trait, then they would all be color blind. However the problem states that none of the female offspring have the disease, so we can conclude that color blindness is a recessive trait. It will only be expressed if the organism is carrying both disease-associated alleles.

Incomplete dominance refers to a mixture of both alleles (for example, a flower with the red color allele and white color allele would be pink) and co-dominance is when both alleles are concurrently expressed (like blood types in humans!). None of these scenarios are mentioned in the problem.

I hope this helps!

Answer 3

Hi Kathryn!

So first, we need to recall the definitions for codominance and incomplete dominance. Codominance is a mixture of two dominant traits, like a black and white chicken giving birth to a speckled chicken. Incomplete dominance however is a blend of two traits, such as a red and white flower producing a pink flower. Since the children in this question either are colorblind or are not, neither of these inheritance patterns are correct.

When dealing with color blindness, it is important to remember that the gene for this trait is found on the X chromosome (sex chromosome). With that in mind, we know that males receive an X and a Y for their sex chromosomes, while females receive two X chromosomes (one from each parent).

The father donates a Y chromosome to every son and each son receives a colorblind X from their mother, since we know they all have the condition. The daughters on the other hand will receive the healthy X from their father and another X from their mother. Though we do not know the mothers genotype, we can assume that she has both colorblind X's because she passed one on to all 5 of her male children. This means that she would also pass a colorblind X chromosome to her daughters as well. Since the daughters were not affected by having only one colorblind gene, we know that this must be a recessive trait.

I hope this helps! Feel free to ask if you are in need of further explanation!