Then I suppose the answer lies in whether the question writer expects me to consider frequencies. I doubt this is the case, but I’ll keep it in mind. More likely, however, given the question and its answer choices, is that the question writer wanted the test-taker to consider that HH and Hh are both possible genotypes for the “affected” person. Note that there is no option between 50% and 75%.
Given that question I would hold my nose and answer c) 50%. Then I would remind myself that it was probably some English major who wrote the question. :rolleyes:
If the test is well-written, you won’t encounter a question that ambiguous. Though I can’t guarantee that the MCAT is well-written… and I bet you’re dealing with a lot of less-well-written test prep material right now.
But you should be OK on this topic, you seem to have a good handle on the concept. Just be careful with your assumptions!
I’ll be sure to read the question carefully and entertain all the possibilities on the actual MCAT. Thank you, and thank you all for helping me with the original question. I agree - it’s ambiguous, and could definitely be clarified.
If the question didn’t specify, then I would assume that they’re testing whether the student has the judgment to know when to make a reasonable approximation and assume that the homozygous genotype is negligible. This is usually a good approximation. It would also be an approximation to say that the homozygous and heterozygous genotypes are equally common, but that would almost always be a terrible approximation.
It’s like in physics problems: They almost never explicitly tell you to ignore air resistance and the curvature of the Earth, but you should almost always ignore them anyway.
But, see, that’s the point I’m trying to make. You’re making this assumption that treating it as 50/50 is as close as you can get to leaving that factor out completely; that the 50/50 option is the best approximation to “we don’t know”. It’s not. There IS no best approximation to “we don’t know”, and 50/50 is just as wrong as 95/5, 80/20, or 51/49.
Huntington’s is not a simple mendelian trait. Here is what wiki
Huntington's disease - Wikipedia has to say (in part):
The point is that it is caused by a repeated segment of a particular codon and the severity as well as age of onset is determined by the number of times it is repeated. Moreover, the number of repeats tends to increase in each generation, resulting in earlier onset (usually). The probability of a double dose being negligible, you can assume that the probability of inheritance is essentially 50%. I can’t give a cite offhand, but I once read that the disease has a measurable effect long before symptoms appear.
Huntingtons is a Mendalian trait. It is a single gene disorder, and assortment is independent with equal expression whether maternally or paternally derived.
Yes the Huntingtons gene has a codon repeat. That does not change the frequency of assortment of the allele. It may affect the penetrance of the trait but that is a different matter.
I’m not going to claim that it’s appropriate here (or ever), but there is an intellectual tradition of assigning equal probabilities to different outcomes when nothing is known.
I would argue that the disease is Mendelian, but not simple Mendelian.
However, I will note that HTT Is listed in OMIM database (Online Mendelian Inheritance in Man:An Online Catalog of Human Genes and Genetic Disorders)
to OP:
I would pick 50%.
The question is bad, and I would report it after the test to the test-maker. There are procedures for that I believe.
But then the question becomes, what are the “outcomes” to which equal probabilities are being assigned. The example appears to define the outcomes that have equal probabilities as the two genotypes “GG” and “Gg”. Actually, however, these are three outcomes – GG, Gg, and gG. If we assume that the probability of being GG is equal to the combined probabilities of being either Gg or gG, then the corollary is that we are assuming that the frequency of the G allele in the population of “affected” individuals is 80%. That is:
For every 2 affected individuals who are heterozygous (Gg or gG), there are two affected individuals who are homozygous (GG and GG). So there are a total of six G alleles and two g alleles, or 80% G.
Given our state of ignorance, I think it would be much more reasonable to assume that the G and g alleles have equal probability in the overall population (not just the population of affected individuals). In this case, among affected individuals, we should expect twice as many heterozygotes (Gg or gG) as homozygotes (GG). So the “correct” answer would be 66.7%, and all the given response options are wrong.
And besides, this isn’t a situation where nothing is known. Even if we don’t know any of the statistics for Huntington’s Disease specifically, we do know from the statement of the question that it’s a disease resulting from a dominant gene. That tells us quite a lot: It’s very easy to select against a dominant trait, and thus all dominant genetic diseases are quite rare.
Wait. Gg and gG are different?
From the standpoint of probability, not genetics.
Well, they are separate outcomes. It’s like flipping a coin twice. There’s one way of getting two heads (HH), one way of getting two tails (TT), and two ways of getting a head and a tail (HT and TH). Similarly, there’s one way of being homozygous-normal (gg), one way of being homozygous-affected (GG), and two ways of being heterozygous (Gg and gG). So if the two alleles are equally prevalent in a population, the number of homozygotes of either type will be half the number of heterozygotes. This follows from the [Hardy-Weinberg principle](hardy-weinberg principle wiki).
I think if I got anything out of this discussion it is that the question is poorly written. I’ll have to lucubrate over the probability tangent later …