Question

The Gumienny lab is trying to clone a gene that affects body size in the roundworm C. elegans. A mutation in this gene, called sma-12, makes small animals when homozygous. It was isolated in a screen that used the chemical mutagen ENU. sma-12 was mapped to Chromosome V (5). To more finely map the location to identify the causative mutation, they perform a two-factor mapping experiment with another Chromosome V-linked gene, unc-23, which has a known position on Chromosome V. This gene has an allele that, when homozygous, causes uncoordinated movement.

Use (-) for the mutants and (+) for the wild-type alleles.

3. We cross homozygous sma-12(-) mothers to homozygous unc-23(-) fathers.

What is the genotype of the mothers (write the genotype at both loci)?

What is the genotype of the fathers (write the genotype at both loci)?

4. What is the genotype of the F1 cross progeny?

e. You now take the F1 cross progeny males and cross them to true-breeding, homozygous sma-12 unc-23 mutant animals, and then you observe 1000 offspring. If the body size and movement traits were so closely linked that no recombination occurred, you would expect to count:

		phenotype
expected number	genotype	body size	movement
		small	uncoordinated
		small	normal
		normal	uncoordinated
		normal	normal

d. When 1000 of the F2 animals are scored, what we observe is:

	phenotype
observed number	body size	movement	Recombinant or parental?
6	small	uncoordinated
493	small	normal
487	normal	uncoordinated
4	normal	normal

Answer 1