Imagine that you are a member of a committee assigned to evaluate a report on neurofibromatosis, a disorder inherited through a dominant allele. The report concludes that, because 75% of the offspring of parents who are heterozygous for neurofibromatosis will have the disorder, eventually 75% of the population will have neurofibromatosis. Do you agree of disagree with the report’s conclusion? Explain your reasoning.
I’m having difficulty answering this particular lab question in my AP bio class. On one hand, you can agree that since it is a dominant allele it will persist throughout the population and that 75% of heterozygous mate offspring will be affected. On another, you can also argue that just because 3/4 of offspring from heterozygous parents will be affected, it won’t necessarily affect the whole population to the point where 75% are affected with neurofibromatosis. Can anyone help?
If there were some selective advantage to having neurofibromatosis, then I could see the frequency of the allele increasing. But the opposite should happen if there is a disadvantage to carrying the allele. The actual prevalence of Type I NF is less than 1 in 200,000 individuals (that would be < 0.000005, and that would include all heterozygotes as well as homozygous dominant individuals—most of us must be homozygous recessives for NF1 it seems) so either the allele has only just arisen (not likely, but not impossible, either) or it is selected against and there will be no increase—never mind a rise to 75%—in the frequency of NF within the population (at least, not due to genetics; perhaps there can be an increase due to improvements in diagnosis and screening, but probably not an increase due to genetic transmission.)