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Exam 7: Linkage and Chromosome Mapping in Eukaryotes

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Question 1

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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?

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Question 2

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Assume that a cross is made between AaBb and aabb plants and that the offspring occur in the following numbers: 106 AaBb, 48 Aabb, 52 aaBb, 94 aabb. These results are consistent with ________.

A) sex -linked inheritance with 30 percent crossing over
B) linkage with 50 percent crossing over
C) 100 percent recombination
D) linkage with approximately 33 map units between the two gene loci
E) independent assortment

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Question 3

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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?

Describe a convenient method for determining gene order from three -point cross results.

In the early 1900s, two scientists noted there were many more genes than chromosome pairs, thus setting the stage for the suggestion that some gene loci might be linked during meiotic processes. Who were these two scientists?

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

Phenotypically wild -type F1 female Drosophila, whose mothers had light eyes (lt) and fathers had straw (stw) bristles, produced the following offspring when crossed to homozygous light -straw males: Compute the map distance between the light and straw loci.

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

What is the expected evolutionary significance of genetic recombination?

When two genes fail to assort independently, the term normally applied is

If two gene loci are on nonhomologous chromosomes, genes at these loci are expected to assort independently.

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

Sister chromatid exchanges increase in frequency in the presence of X rays, certain viruses, ultraviolet light, and certain chemical mutagens. In what autosomal recessive disorder is there known to be an increase in sister chromatid exchanges?

Assume that a cross is made between AaBb and aabb plants and that all the offspring are either AaBb or aabb. These results are consistent with _.

Provide a brief definition for positive interference.

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?

Given that loci A and B in Drosophila are sex -linked and 20 map units apart, what phenotypic frequencies would you expect in male and female offspring resulting from the following crosses? (Assume that A and B are dominant to a and b, respectively.)

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: 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 for 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.

In Drosophila, assume that the gene for scute bristles (s) is located at map position 0.0 and that the gene for ruby eyes (r) is at position 15.0. Both genes are located on the X chromosome and are recessive to their wild -type alleles. A cross is made between scute -bristled females and ruby -eyed males. Phenotypically wild -type F1 females were then mated to homozygous double mutant males, and 1000 offspring were produced. Give the phenotypes and frequencies expected.

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?