Question 1

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?

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Question 2

(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?

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Question 3

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

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Question 4

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

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Question 5

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 there are 20 map units between the G and E loci.

Accepted Answer

The fact that there are 404 GE/ge, 396 ge/ge, 97 gE/ge, and 103 Ge/ge progeny from the cross indicates that the G and E loci are closely linked, with some degree of interference. By summing the frequencies of the recombinant classes (gE/ge and Ge/ge), the total number of recombinants is found to be 200, which equals 1/5th of the total progeny (1000). Thus, the distance between the G and E loci is 20 map units (or centimorgans), which is the percentage of recombinant progeny. Therefore, the statement is true.

Question 6

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

Accepted Answer

Independent assortment occurs when genes are located on different chromosomes, which do not undergo homologous recombination during meiosis. Therefore, genes located on nonhomologous chromosomes will segregate independently during gamete formation.

Question 7

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

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Question 8

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.

Accepted Answer

The progeny ratios deviate significantly from the 1:1:1:1 ratio expected if the G and E loci were assorting independently, indicating linkage between the two loci.

Question 9

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

Accepted Answer

Linkage does not always occur when two loci are on the same chromosome because recombination can separate alleles of genes that are far apart on the same chromosome, making them assort independently during meiosis.

Question 10

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?

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Question 11

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.

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The answer of In the fruit fly,Drosophila melanogaster,a spineless (no...

Question 12

Three loci,mitochondrial malate dehydrogenase that forms a and b (MDHa,MDHb),glucuronidase that forms 1 and 2 (GUS1,GUS2),and a histone gene that forms + and -
(H⁺,ᴴ-),are located on chromosome #7 in humans.Assume that the MDH locus is at position 35,GUS at position 45,and H at position 75.A female whose mother was homozygous for MDHa,GUS2,and H⁺ and whose father was homozygous for MDHb,GUS1,and H- produces a sample of 1000 egg cells.Give the genotypes and expected numbers of the various types of cells she would produce.Assume no chromosomal interference.

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Question 13

Provide a brief definition for positive interference.

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Question 14

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?

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Question 15

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?

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Question 16

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

Accepted Answer

Positive interference describes a phenomenon in which a crossover event suppresses the likelihood of a proximal crossover event. As a result, positive interference leads to a reduction in the number of crossovers in nearby regions of a chromosome.

Question 17

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)

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Question 18

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

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Question 19

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?

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Question 20

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 recombinant progeny are gE/ge and Ge/ge.

Accepted Answer

The recombinant progeny are gE/ge and Ge/ge, as they have inherited a combination of alleles different from either of the parental combinations (GE/ge and ge/ge). The total number of recombinant progeny is 97 + 103 = 200, which is less than 50% of the total progeny (1000), confirming that the genes are linked and there is some degree of crossing over between them.

Quiz 5: Chromosome Mapping in Eukaryotes