Question

I think there is a mistake in the second "check yourself" question on the Genetics/Biochemical Genetics page of the OCW Scholar course. The question is "The phenotypes of two haploid tryptophan auxotrophs (trp1 and trp2) are both found to be recessive. These two mutants are crossed and the resulting diploid CAN grow on minimal media. Which of the following statements is correct?". In my opinion the correct answer must be "The trp1 and trp2 mutations are in different genes", but the answer shown is "The trp1 and trp2 mutations are in the same gene". Can anybody help?

Answer 1

I've just reached that question and i am a little bit confused, to be honest. Actually i can't figure out an explanation for neither of the answers.
I'm just able to suppose that, if "The trp1 and trp2 mutations are in the same gene" would be right, these two mutations would "erase" their failures, i.e. together theses genes form a new gene which works.
This could explain why it must be in the same gene because only then it is possible that they interact and that they "fuse".
But this is only a hypothesis of my own and i have no idea that is based on reliable information or knowledge.

As nobody had replied to the question before i did, have you tried to post it in the biology class because there will be more guys able to support you (and me)?

Answer 2

I didn't know there was another biology forum, thanks for that! This is how I understand it: When the traits are recessive and the mutations are in different genes, they complement each other. The new diploid cell can grow, because when the mutations are in different genes, it now has one working and one defect gene from each of the haploid mutants. One working gene is enough for the recessive trait. If the mutation was in the same gene in both haploid mutant cells, the diploid cell still wouldn't have a working gene, and thus wouldn't grow, because both haploid mutant cells would contribute the defect gene to the new cell. This is why I think the answer they give is wrong.

Answer 3

I think you are right. Just ask sb in the Biology forum to assure it. Maybe you could write to MIT OCW and tell them the mistake...

Answer 4

they have to occur in the same gene.

Answer 5

@panlac01, could you please elaborate why you think they have to occur in the same gene? For the diploid cells to grow, the mutations have to complement each other, and for this to happen the mutations have to occur in different genes (see the help session video about complementation that is linked below the question: http://ocw.mit.edu/courses/biology/7-01sc-fundamentals-of-biology-fall-2011/genetics/biochemical-genetics/#?w=535).

Answer 6

@KristiGer

I'm not even sure if you check this anymore, but in case you do - I'm 99% sure that you are correct :) as you explained very well, this is the well-known process of "complementation". You could and maybe should, as someone else suggested, write to MIT OCW letting them know about this mistake. But who knows, maybe we're both wrong and there's some esoteric unknown explanation; however, having done a few of these problems my best guess is that you are correct.

Answer 7

Thanks @kma230! I also posted the question in the general biology forum, but didn't get any responses. I'll write to MIT OCW!

Answer 8

PS: I think there is another mistake in the very last answer to exam 3: the genotype for individual 4 should be (X+)Y, not (X-)Y.