Exam 5: Chromosome Mapping in Eukaryotes

Provide a brief definition for positive interference.

Who was the student in Morgan's laboratory who first realized that the sequence of genes could be determined by Morgan's proposal that two genes located relatively close to each other are less likely to form chiasma between them than if the two genes were relatively far apart?

Two lines of work indicated that crossing over actually involves breakage and reunion of chromatid material. What organisms were involved, and who did the work?

Positive interference occurs when a crossover in one region of a chromosome reduces crossovers in nearby regions.

Assume that two genes are 80 map units apart on chromosome II of Drosophila and that a cross is made between a doubly heterozygous female and a homozygous recessive male. What percent recombination would be expected in the offspring of this type of cross?

In the fruit fly, Drosophila melanogaster, a spineless (no wing bristles) female fly is mated to a male that is claret (dark eyes) and hairless (no thoracic bristles). Phenotypically wild-type F₁ female progeny were mated to fully homozygous (mutant) males, and the following progeny (1000 total) were observed: Phenotypes Number Obsreved spineless 321 wild 38 claret, spineless 130 claret 18 claret, hairless 309 hairless, claret, spineless 32 hairless 140 hairless, spineless 12 a) Which gene is in the middle? b) With respect to the three genes mentioned in the problem, what are the genotypes of the homozygous parents used in making the phenotypically wild-type F₁ heterozygote? c) What are the map distances between the three genes? A correct formula with the values "plugged in" for each distance will be sufficient. d) What is the coefficient of coincidence? A correct formula with the values "plugged in" will be sufficient.

If interference is complete, what would be the frequency of double crossovers?

Mendel predicted that some genes will be carried in the same chromosome.

Assume that the genes for tan body and bare wings are 15 map units apart on chromosome II in Drosophila. Assume also that a tan-bodied, bare-winged female was mated to a wild-type male and that the resulting F₁ phenotypically wild-type females were mated to tan-bodied, bare-winged males. Of 1000 offspring, what would be the expected phenotypes, and in what numbers would they be expected?

At what stage of the meiotic cell cycle and during what chromosomal configuration does crossing over occur?

What are two commonly used DNA landmarks for mapping human genes?

Assume that there are 12 map units between two loci in the mouse and that you are able to microscopically observe meiotic chromosomes in this organism. If you examined 200 primary oocytes, in how many would you expect to see a chiasma between the two loci mentioned above?

Restriction fragment length polymorphisms (RFLPs), microsatellites, and single-nucleotide polymorphisms (SNPs) were used by Morgan and Sturtevant to discover linkage in Drosophila.

What advantage does BrdU (bromodeoxyuridine) have in the study of chromosome structure and recombination?

Assume that investigators crossed a strain of flies carrying the dominant eye mutation Lobe on the second chromosome with a strain homozygous for the second chromosome recessive mutations smooth abdomen and straw body. The F₁ Lobe females were then backcrossed with homozygous smooth abdomen, straw body males, and the following phenotypes were observed: smooth abdomen, straw body 820 Lobe 780 smooth abdomen, Lobe 42 straw body 58 smooth abdomen 148 Lobe, straw body 152 a) Give the gene order and map units between these three loci. b) What is the coefficient of coincidence?

Exchanges occur between homologous chromosomes, but never between sister chromatids.

The genes for mahogany eyes and ebony body are approximately 25 map units apart on chromosome III in Drosophila. Assume that a mahogany-eyed female was mated to an ebony-bodied male and that the resulting F1 phenotypically wild-type females were mated to mahogany, ebony males. Of 1000 offspring, what would be the expected phenotypes, and in what numbers would they be expected?

What term is applied when two genes fail to assort independently, that is, they tend to segregate together during gamete formation?

(a) In a three-point mapping experiment, what three general classes of offspring are expected (assuming crossovers occur)? (b) How many different genotypic classes are expected?

Linkage (viewed from results of typical crosses) always occurs when two loci are on the same chromosome.