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Exam 7: Linkage and Chromosome Mapping in Eukaryotes

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Question 21

True/False

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The cross GE/ge X ge/ge produces the following progeny: GE/ge 404,ge/ge 396,gE/ge 97,Ge/ge 103.From these data,one can conclude that the G and E loci assort independently.

A) True
B) False

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Question 22

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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% crossing over
B) linkage with 50% crossing over
C) linkage with approximately 33 map units between the two gene loci
D) independent assortment
E) 100% recombination

F) B) and D)
G) C) and D)

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Question 23

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Provide a brief definition for positive interference.

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: $\begin{array}{lc}\text { Phenotype } & \text { Number } \\\hline \text { light-straw } & 22 \\\text { wild-type } & 18 \\\text { light } & 990 \\\text { straw } & \underline{970} \\& \text { Total: } \underline{2000}\end{array}$ Compute the map distance between the light and straw loci.

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

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?

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?

Assume that a cross is made between AaBb and aabb plants and that the offspring fall into approximately equal numbers of the following groups: AaBb,Aabb,aaBb,aabb.These results are consistent with ________.

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?

What is the expected evolutionary significance of genetic recombination?

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

The cross GE/ge X ge/ge produces the following progeny: GE/ge 404,ge/ge 396,gE/ge 97,Ge/ge 103.From these data,one can conclude that the recombinant progeny are gE/ge and Ge/ge.

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?

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

The cross GE/ge X ge/ge produces the following progeny: GE/ge 404,ge/ge 396,gE/ge 97,Ge/ge 103.From these data,one can conclude that there are 20 map units between the G and E loci.

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

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

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

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.