When two mutant alleles of a gene are brought into the same cell, the merodiploid might acquire a wild-type phenotype by intragenic complementation or by recombination. How could you distinguish between these two possibilities?

ANSWER:

• Repeat the test in a recA host. The RecA protein is required for homologous recombination, but not for complementation.
• You can also get a good hint based upon the frequency that cells acquire the wild-type phenotype. Complementation should occur for 100% of the cells that acquire both copies of the gene, but recombination is relatively rare (typically less than 0.2% of the cells that acquire both copies will yield recombinants).

If the donor copy of a gene had a nonsense mutation in a particular base pair and the recipient had a base substitution mutation located one base pair away, could the gene be repaired by recombination?

ANSWER: Recombination occurs by cutting and rejoining at the phosphate bond between two base pairs. Thus reciprocal recombination between the two adjacent pairs would yield a wild-type recombinant at both base pairs and the corresponding double-mutant recombinant. [Get a pencil and a piece of paper and try drawing it out!]

Sometimes interference between closely spaced recombination events (cross-overs) results in disagreement between the gene order predicted from two-factor and three-factor crosses for very closely linked mutations. Suggest two different in vivo recombination experiments that would allow you to figure out the gene order.

ANSWER:

• Deletion mapping! Remember that 2-factor and 3-factor crosses depend upon frequencies but deletion mapping depends upon a yes or no answer.
• Since the problem is that the two markers are too close together to accurately map, if you didn't have a deletion map you could test linkage of each marker to one or more other genes which are more distant.

Please send comments, suggestions, or questions to smaloy@sciences.sdsu.edu
Last modified October 18, 2004