| Q: | A disease is studied in one family of patients and it is found that if fathers have the disease but the mothers do not, it is never passed down to sons. Which of the following holds true? |
| A | | the disease can only be X-linked dominant |
| B | | the disease can only be X-linked recessive |
| C | | the disease can be either X-linked dominant or X-linked recessive |
| D | | the disease is autosomal recessive |
The correct answer is C. The first thing we can tell about this genetic disease is that it is not Y-linked. Since sons obtain their only copy of their y chromosome from their fathers, there is no way for a father to have a y-linked disease and for the son to not contract it. X-linked for this disease makes sense, since the father doesn’t pass his x-chromsome to his son; the mother passes her x-chromosome to her son. The disease can be X-linked dominant because the mother is unaffected, meaning her x-chromosomes can’t have the disease, and the father doesn’t pass an x-chromosome to his son. The disease can also be x-linked recessive, and the father would still have it, but again, wouldn’t pass it on to his son.