Deck 7: Linkage and Chromosome Mapping in Eukaryotes

Question 1

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?

Answer 1

48

Question 2

Under what circumstance might two loci be on the same chromosome but behave as if independently assorting in crosses?

Answer 2

If the genes are far apart, they may show independent assortment.

Question 3

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 _.

A) alternation of generations
B) incomplete dominance
C) hemizygosity
D) codominance
E) complete linkage

Answer 3

E

Question 4

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.

Question 5

What is the expected evolutionary significance of genetic recombination?

Question 6

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.

Question 7

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

Question 8

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 _.

A) complete linkage
B) hemizygosity
C) alternation of generations
D) independent assortment
E) incomplete dominance

Question 9

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:
a() Give the gene order and map units for these three loci.
b() What is the coefficient of coincidence?

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 ________.

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

Question 11

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

A) discontinuous inheritance
B) Mendelian inheritance
C) linkage
D) dominance and/or recessiveness
E) tetrad analysis

Question 12

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?

Question 13

The phenomenon in which one crossover decreases the likelihood of crossovers in nearby regions is called _.

A) chiasma
B) reciprocal genetic exchange
C) positive interference
D) negative interference
E) mitotic recombination

Question 14

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

Question 15

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?

Question 16

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.

Question 17

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?

Question 18

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

Question 19

Methods for determining the linkage group and genetic map in humans involve _.

A) DNA markers
B) zygotene and pachytene DNA synthesis
C) tetrad analysis and bromodeoxyuridine
D) twin spots and tetrad analysis
E) chiasmatype and classical analyses

Question 20

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 which arrangement of alleles?

Question 21

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.)

Question 22

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

Question 23

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

Question 24

(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?

Question 25

Provide a brief definition for positive interference.

Question 26

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?

Question 27

Three loci, mitochondrial malate dehydrogenase that forms a and b (MDHa, MDHb),
glucouronidase 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.

Question 28

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.

Question 29

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

Question 30

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?

Question 31

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?

Question 32

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

Question 33

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

Question 34

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.

Question 35

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?

Question 36

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.

Question 37

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

Question 38

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