Deck 7: Linkage and Chromosome Mapping in Eukaryotes

Question 1

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

Answer 1

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

Question 2

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 F? females were then mated to homozygous double mutant males,and 1000 offspring were produced.Give the phenotypes and frequencies expected.

Answer 2

scute = 425; ruby = 425; wild-type = 75; scute-ruby = 75

Question 3

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 F? Lobe females were then backcrossed with homozygous smooth abdomen,straw-body males,and the following phenotypes were observed:
smooth abdomen,straw body 820
Lobe 780
smooth abdomen,Lobe 42
straw body 58
smooth abdomen 148
Lobe,straw body 152
(a)Give the gene order and map units for these three loci.
(b)What is the coefficient of coincidence?

Answer 3

(a)Lobe is in the middle.
smooth abdomen---5---Lobe-----------15-------------straw body
(b)zero

Question 4

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

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

Question 5

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

Question 6

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

Question 7

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

Question 8

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

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

Question 9

What is the expected evolutionary significance of genetic recombination?

Question 10

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

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 F? female progeny were mated to fully homozygous (mutant)males,and the following progeny (1000 total)were observed:
Phenotypes Number Observed
spineless 321
wild-type 38
claret,spineless 130
claret 18
claret,hairless 309
hairless,claret,spineless 32
hairless 140
hairless,spineless 12
(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 F? 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 12

Phenotypically wild-type F? 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:
Phenotype Number
light-straw 22
wild-type 18
light 990
straw 970
Total: 2000
Compute the map distance between the light and straw loci.

Question 13

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

Question 14

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

Question 15

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?

Question 16

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

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

Question 17

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 genes?

Question 18

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 F? 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 19

In the early 1900s,two scientists noted that 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 20

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 F? 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 21

(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 22

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 23

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.

Question 24

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 25

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?,??)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 26

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.

Question 27

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

Question 28

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

Question 29

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

Question 30

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

Question 31

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

Question 32

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 33

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 34

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

Question 35

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

Question 36

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 37

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.

Question 38

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

Question 39

Provide a brief definition for positive interference.