in drosophila melanogaster the gene for the recessive mutations brown eyes (bw)
ID: 215958 • Letter: I
Question
in drosophila melanogaster the gene for the recessive mutations brown eyes (bw) and humpy thorax (ht) are linked and 12 map units apart on chromosome 2. A paracentric inversion on chromosome 2 spanning the entire 12 map unit region that includes the two genes has been identified. Given a female that is heterozygous with cis linkage for the two genes a)With respect to the two genes, what would be the gametic genotypes that could be produced by a female homozygous for the inverted chromosome and in what proportions would they occur. b)With respect to the two genes, what would be the gametic genotypes that could be produced if the female carried one normal chromosome 2 and one paracentric inversion form and in what proportion would they occur?
Explanation / Answer
1. List the features (genetic or cytological) that identify and distinguish between the following: (a) deletions, (b) duplications, (c) inversions, (d) reciprocal translocations.
2. The two loci P and Bz are normally 36 m.u. apart on the same arm of a certain plant chromosome. A paracentric inversion spans about one-quarter of this region but does not include either of the loci. What approximate recombinant frequency would you predict for these loci in plants that are (a) heterozygous for the paracentric inversion? (b) Homozygous for the paracentric inversion?
(a)
27 percent;
(b)
36 percent.
3. Assume that the following loci in Drosophila are linked in the order a–b–c–d–e–f. A fly of genotype a?b?c?d?e?f/a?b?c?d?e?f is crossed to a wild-type fly. About half the progeny are fully wild type in phenotype, but the other half show the recessive phenotype corresponding to d and e. Propose an explanation for these results.
4. The normal sequence of markers on a certain Drosophila chromosome is 123·456789, where the dot represents the centromere. Some flies were isolated with chromosome aberrations that have the following structures: (a) 123·476589, (b) 123·46789, (c) 1654·32789, (d) 123·4566789. Name each type of chromosome mutation and draw diagrams to show how each would synapse with the normal chromosome.
5. A Neurospora heterokaryon is established between cells of the genotypes shown in the diagram below. Here leu, his, ad, nic, and met are all recessive alleles causing specific nutritional requirements for growth. A and a are the mating-type alleles (one parent must be A and the other a for a cross to occur). Usually a nucleus carrying A will not pair with a nucleus carrying a to form a heterokaryon, but the recessive mutant tol suppresses this incompatibility and permits heterokaryons to grow on vegetative medium. The recessive allele unprevents the fungus from growing at 37°C (un is a temperature-sensitive allele) and cannot be corrected nutritionally. The heterokaryon grows well on a minimal medium, as do most of the cells derived mitotically from it. However, some rare cells show the following traits:
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