1. You have discovered a mutant with altered eye development in Drosophila. The mutation causing this phenotype maps to chromosome II (i.e. not the X). You wonder whether genes that affect either eye development or function are clustered together, i.e. near your gene, and attempt to answer this question by trying to isolate additional mutations affecting the eyes in the region around your gene.

   1. How would you do such a screen? (Hint: use an appropriate chromosomal abnormality). 

   2. How would you know when to stop looking, i.e. when have you "saturated" the region? 

2. How can a series of overlapping deletions be used to define the map position of a gene?

3. In humans colorblindness and adrenoleukodystrophy are X-linked traits that map very close to one another. Colorblindness is relatively common: approximately 8% of males have it. Adrenoleukodystrophy (from which the affected male children die at a very early age) is much rarer: less than 1/10,000 male children show the trait.

1. What factors could contribute to give the very different amounts of males with colorblindness and males with adrenoleukodystrophy? 

2. In a study of boys with adrenoleukodystrophy it has been found that 64% of them are also colorblind.

   1. What percentage of boys with adrenoleukodystrophy would you expect to be colorblind?

   2. How do you explain the experimental results. (A consideration of the nature of the genetic defects would be appropriate.) 

 

4. a. How would you prove genetically that a mutation was a large deletion (i.e. deleting several known genes? 

2. How could you use such a deletion to find all the genes that can be mutated to a viable phenotype that map within the deleted area?

3. When would you stop doing the experiment in (b)?

 

5. Describe genetic (not molecular) experiments to map the extent of a free duplication formed from a small part of the X chromosome in C. elegans (the fragment does not affect sex determination; remember that hermaphrodites have two Xs, males have one).

6. Consider the results obtained by Beadle and Tatum.

   1. What would be the growth requirements of the following double mutants: arg-1arg-2, arg-1arg-3, and arg-2arg-3?

   2. Using your predictions in (a), state the epistatic relationships among the three genes. 

   3. How does epistasis relate to the biochemical pathway? 

7. Histone H4 (a major structural component of chromatin) is one of the most highly conserved proteins known. The amino acid sequence (102 amino acids) of this protein from cows and peas differs in only two places (from cows to peas: val 6 ile and lys 6 arg).

1. Why do you suspect that there are so few differences? 

2. Do you think that the gene for H4 (actually there are many copies) is an extremely "low spot" for mutations? 

3. What would you expect to see if you compared the DNA sequences for the H4 genes from cows and peas? 

4. Do you suspect that the changes that are seen would have major effects on H4 structure or function? Why? 

5. What types of mutations (i.e. what base changes) could result in the observed differences? 

8. Temperature-sensitive mutants can be very useful in the genetic dissection of biological systems. For example, with a temperature-sensitive allele, the time at which a gene is acting can be determined by shifting to restrictive temperature at var ious times in development. Two touch insensitive mutants of C. elegans were examined in this way. For both animals were grown at the permissive (15 degree C) and restrictive (25 degree C) temperatures and shifted to the opposite temperatures at various times after hatching (the animals grow at different rates at the two temperatures, but here I've listed times in 25 degree C. After the animals had become adults they were tested for touch sensitivity. For each set of data (a) graph % touch sensitive vs. time of shift (put both the up and down shifts on the same graph) and (b) determine when during development the genes are acting.

	time (hr):	0	8	16	24	32	40	48
mec-5 15 to 25		0	10	29	100	100	100	100
mec-5 25 to 15		100	100	61	0	0	0	0
mec-7 15 to 25		0	20	42	67	95	100	100
mec-7 25 to 15		100	82	65	38	7	0	0

3. Larval stage times at 25 degree Celsius are L1 (0-12 hr), L2 (12-20), L3 (20-27), L4 (27-36). After 36 hrs the animals are adults.

4. Explain how temperature-sensitive mutants can be used to determine when a gene must be expressed to determine the adult phenotype.

Answers