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Exam 5: Chromosome Mapping in Eukaryotes

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

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Compare the double-crossover c...

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Question 2

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Provide a brief definition for positive interference.

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A crossover in one region decreases the likelihood of crossovers in nearby regions.

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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.) a) AaBb (dominant alleles on same chromosome)female × ab/Y male b) AaBb (dominant alleles on homologs)female × ab/Y male c) aabb female × AB/Y male (no crossing over in male Drosophila)

What is the relationship between the degree of crossing over and the distance between two 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?

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

Linkage (viewed from results of typical crosses)always occurs when two loci are on 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?

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: $\begin{array}{lr}\text { Phenotypes } & \text { Number } \\ \text { spineless } & 321 \\ \text { wild } & 38 \\ \text { claret, spineless } & 130 \\ \text { claret } & 18 \\ \text { claret, hairless } & 309 \\ \text { hairless, claret, spineless } & 32 \\ \text { hairless } & 140 \\ \text { hairless, spineless } & 12\end{array}$ 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.

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

Diagram chromosomal events that will ultimately result in the segregation of alleles (A and a)during meiosis II rather than meiosis I.

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: $\begin{array}{lr}\text { smooth abdomen, straw body } & 820 \\ \text { Lobe } & 780 \\ \text { smooth abdomen, Lobe } & 42 \\ \text { straw body } & 58 \\ \text { smooth abdomen } & 148 \\ \text { Lobe, straw body } & 152\end{array}$ a) Give the gene order and map units between these three loci. b) What is the coefficient of coincidence?

The cross GE/ge × 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.

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 what arrangement of genes?

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

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

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?

Methods for determining the linkage group and genetic map in humans involve which of the following?

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 the following circumstance:

The coefficient of coincidence reflects the frequency of observed double crossovers compared to the frequency of expected double crossovers.What is the relationship between the coefficient of coincidence and interference?