Deck 5: Genetic Linkage and Mapping in Eukaryotes

Question 1

For a given cross, the expected number of double recombinants is 20 and the observed number of double recombinants is 15. What is the interference calculation (I)?

A)0.25
B)1.33
C)20
D)0.75
E)15

Answer 1

A

Question 2

Which type of crossover event produces no recombinant chromosomes?

A)single-strand crossover
B)two-strand double crossover
C)three-strand double crossover
D)four-strand double crossover
E)None of the above; all crossover events produce recombinant chromosomes.

Answer 2

B

Question 3

Genes A and B are located 10cM from each other on a chromosome. Gene C is located 25cM from gene A and 15cM from gene B. What is the probability that the trihybrid ABC/abc will produce any kind of recombinant gamete?

A)76.5%
B)38.25%
C)23.5%
D)1.5%
E)0.75%

Answer 3

C

Question 4

What type of test would you use to determine whether observed data constitute evidence of genetic linkage or are simply a case of chance variation from expected values?

A)test cross
B)two-point test cross
C)three-point test cross
D)chi-square analysis
E)recombination frequency (r)calculation

Question 5

You perform a test cross of the dihybrid AaBb and score the phenotypes of 1000 progeny. Assuming independent assortment, how many of the progeny do you expect to display the dominant phenotype for both the A and B genes?

A)100
B)200
C)250
D)500
E)750

Question 6

Where does crossing over occur?

A)at the centromeres
B)at the telomeres
C)at nodules that occur randomly at "hotspots" along the synaptonemal complex
D)at nodules that occur in specific, evenly spaced locations along a chromosome
E)at both the centromeres and telomeres

Question 7

Genes A and B are located 10cM from each other on a chromosome. Gene C is located 25cM from gene A and 15cM from gene B. Assuming that I = 0, what is the probability that the trihybrid ABC/abc will produce an ABC gamete?

A)76.5%
B)38.25%
C)25%
D)1.5%
E).75%

Question 8

In a two-point test-cross analysis, a dihybrid F₁ fly is crossed to a __________.

A)pure-breeding mate with a dominant phenotype (AABB)
B)pure-breeding mate with a recessive phenotype (aabb)
C)heterozygote mate (AaBb)
D)mate that is dominant for one gene and recessive for the other (AAbb or aaBB)
E)second dihybrid F₁ fly (sibling cross)

Question 9

The syntenic genes A and Z are linked. A cross between two parents, AAZZ and aazz, produces F₁ progeny with the AaZz genotype. What are the possible arrangements of alleles on the F₁ progeny's chromosomes?

A)AZ/az
B)Az/aZ
C)Aa/Zz
D)AZ/Az or az/aZ
E)A/Z or a/z

Question 10

The alleles of linked genes tend to __________.

A)segregate together during gamete production
B)assort independently
C)be mutated more often than unlinked genes
D)experience a higher rate of crossing over
E)assort independently and show a higher rate of crossing over

Question 11

Assuming three genes are linked, how many recombinant genotypes would you expect and at what frequency?

A)two recombinant genotypes, more frequent than expected by independent assortment
B)four recombinant genotypes, more frequent than expected by independent assortment
C)six recombinant genotypes, more frequent than expected by independent assortment
D)four recombinant genotypes, less frequent than expected by independent assortment
E)six recombinant genotypes, less frequent than expected by independent assortment

Question 12

Genetic linkage leads to the production of a significantly greater than expected number of gametes containing chromosomes with __________.

A)allele combinations that are different from parental combinations
B)parental combinations of alleles
C)mutant alleles
D)dominant alleles
E)recessive alleles

Question 13

If you are given a recombination frequency of 34% between genes X and Y and 27% between X and Z, can you predict the order of the three genes?

A)Yes; the order is X-Z-Y.
B)Yes; the order is X-Y-Z.
C)Yes; the order is Z-X-Y.
D)No; based on this data alone, the order could be Z-Y-X or X-Y-Z.
E)No; based on this data alone, the order could be X-Z-Y or Z-X-Y.

Question 14

When does recombination occur?

A)before DNA replication
B)at the two-strand stage, when each member of a homologous chromosome pair is composed of two sister chromatids, and before DNA replication
C)at the four-strand stage, when each member of a homologous chromosome pair is composed of two sister chromatids, and before DNA replication
D)at the two-strand stage, when the sister chromatids have been separated, and after DNA replication
E)at the four-strand stage, when each member of a homologous chromosome pair is composed of two sister chromatids, and after DNA replication

Question 15

In fruit flies, red eyes (pr+_)are dominant to purple eyes (prpr)and normal wings (vg+_)are dominant to vestigial wings (vgvg). The genes are located on the same chromosome. A purebreeding red-eyed fly with vestigial wings was crossed with a pure-breeding purple-eyed fly with normal wings. All of the F₁ progeny had a WT phenotype. Which of the following represents the arrangement of alleles on the F₁'s chromosome?

A)prvg/pr+vg+
B)pr+vg/prvg+
C)prpr+/vgvg+
D)prvg/prvg
E)pr+vg+/pr+vg+

Question 16

In a dihybrid cross exhibiting complete genetic linkage, what would you expect?

A)one parental allele combination occurring more frequently than another
B)two equally frequent gametes containing only parental allele combinations
C)only recombinant gametes
D)one parental allele combination occurring more frequently than another and no recombinant gametes
E)two equally frequent gametes containing only parental allele combinations and no recombinant gametes

Question 17

In fruit flies, red eyes (pr+_)are dominant to purple eyes (prpr)and normal wings (vg+_)are dominant to vestigial wings (vgvg). The genes are located on the same chromosome. A purebreeding red-eyed fly with vestigial wings was crossed with a pure-breeding purple-eyed fly with normal wings. All of the F₁ progeny had a WT phenotype. The recombination frequency between the two genes is 10%. If an F₁ individual was test crossed, what percentage of the progeny would you expect to have the WT phenotype?

A)5%
B)10%
C)50%
D)90%
E)95%

Question 18

Syntenic genes can assort independently when __________.

A)they are very close together on a chromosome
B)they are located on different chromosomes
C)crossing over occurs rarely between the genes
D)they are far apart on a chromosome and crossing over occurs frequently between the genes
E)they are far apart on a chromosome and crossing over occurs very rarely between the genes

Question 19

Incomplete genetic linkage of three genes in a trihybrid produces eight genetically different gamete genotypes. How many different gamete genotypes are produced in a four-gene cross with incomplete genetic linkage?

A)4
B)8
C)16
D)20
E)24

Question 20

For a given cross, the expected number of double recombinants is 20 and the observed number of double recombinants is 15. What is the coefficient of coincidence (c)?

A)0.25
B)1.33
C)20
D)0.75
E)15

Question 21

Recombination analysis allows for the mapping of genes because the frequency of crossing over is proportional to what distance?

Question 22

In a two-point test-cross analysis, what are the "two points" being tested?

Question 23

In most tests of genetic linkage, the number of double crossovers is less than the number expected due to what effect, which limits the number of crossovers that can occur in a short length of chromosome?

Question 24

After analysis of 100 pedigrees, the Z_max from the analysis of linkage between a disease gene D and the DNA marker P was found to be 3.5 at θ = 0.25. Which of the following is the best interpretation of these results?

A)No conclusion can be made regarding linkage of D and P.
B)The evidence supports linkage of D and P at 25 cM.
C)The evidence supports linkage of D and P at 2.5 cM.
D)The evidence supports linkage of D and P at 0.25 cM.
E)The evidence indicates that D and P are not linked.

Question 25

Incomplete genetic linkage is far more common than complete linkage. What is the term for gametes produced when recombination shuffles the alleles of linked genes?

Question 26

What organism exhibits complete genetic linkage, meaning there is no recombination between homologous chromosomes?

Question 27

The Z_max from the analysis of a disease gene N and the DNA marker H was 2.5 at θ = 0.1. Which of the following is the best interpretation of this result?

A)N and H are not linked.
B)N and H are separated by 10 cM.
C)N and H are separated by more than 10 cM.
D)N and H are separated by less than 10 cM.
E)No conclusion can be made regarding the linkage of N and H.

Question 28

Intragenic recombination can be detected __________.

A)based on recombination between homologs carrying different mutant alleles
B)by the recovery of 100% double-mutant chromosomes
C)by the recovery of both wild-type and double-mutant chromosomes
D)based on recombination between homologs with different mutant alleles and recovery of 100% double-mutant chromosomes
E)based on recombination between homologs with different mutant alleles and recovery of both wild-type and double-mutant chromosomes

Question 29

Which type of crossover event produces two parental and two recombinant chromosomes in gametes?

A)single-strand crossover
B)two-strand double crossover
C)three-strand double crossover
D)four-strand double crossover
E)None of the above; all crossover events produce only recombinant chromosomes.

Question 30

There are three genes located in the gene order A-B-C on a chromosome. Would you expect the recombination frequency to be higher between A and B or A and C?

Question 31

For a given haplotype, the frequencies of alleles for gene Y are Y = 0.65 and Y' = 0.35, and the frequencies at gene Z are Z = 0.25 and Z' = 0.75. What is the predicted frequency of the Y'Z' haplotype?

A)0.16
B)0.26
C)0.74
D)0.84
E)1.1

Question 32

What is observed when syntenic genes are close enough to one another that they are unable to assort independently?

Question 33

Which type of crossover event produces all four recombinant chromosomes?

A)single-strand crossover
B)two-strand double crossover
C)three-strand double crossover
D)four-strand double crossover
E)None of the above; all crossover events produce a combination of parental and recombinant chromosomes.

Question 34

What unit of physical distance between genes on a chromosome provides a convenient way to relate the recombination frequencies for linked genes with their positions and order along a chromosome?

Question 35

Refer to this map to answer the questions: a)What is the distance between gene E and gene F?
b)Assuming I = 0, what is the probability of no crossovers between gene D and gene E?
c)Assuming I = 0, what is the probability of no crossovers between gene E and gene F?
d)Considering both gene pairs, what is the proportion of nonrecombinant gametes?
e)What is the predicted frequency of each parental gamete (DEF/def )?
f)What is the recombination frequency between gene D and gene E?
g)What is the recombination frequency between gene E and gene F?
h)What is the frequency of two single recombinant gametes between genes D and E?
i)What is the frequency of two single recombinant gametes between genes E and F?
j)What is the frequency of each of the double-recombinant gametes, DeF and dEf?
k)Fill in the blanks below to prove that the sum of the frequencies of the eight predicted gamete genotypes is equal to 1.0:
_______ + _______ + _______ + _______ + _______ + _______ + _______ + _______ = 1
DEF def Def dEF DEf deF DeF dEf

Question 36

When constructing a genetic map of Zea mays, Creighton and McClintock used genetic markers as well as structural differences in the homologous copies of chromosome 9 that can be seen under the microscope. What are these structural differences called?

Question 37

What type of test would you use to determine the relative order of more than two genes?

Question 38

Two genes, A and X, exhibit incomplete linkage. The frequency of each parental gamete (AX and ax)is 45%. What is the approximate frequency of the Ax gamete?

Question 39

Morgan realized that Bateson and Punnett had detected genetic linkage but they couldn't explain their results because they performed the wrong cross. What type of cross did Morgan use for the analysis of genetic linkage of autosomal genes in Drosophila?

Question 40

What is the term for chromosomes that do not reshuffle the alleles of linked genes?

Question 41

If there is a maximum Lod score of 4.2 at θ = 0.31, what can you say about the linkage and the distance between two genes?

Question 42

The products of meiosis yield the following results:
PD = 1200 TT = 750 NPD = 50
What is the recombination frequency?

Question 43

Ascospores in yeast are not arranged in a particular order. Thus, the four haploid ascospores contained within a yeast ascus are known as what?

Question 44

The specific array of alleles making up a set of linked genes on a single chromosome is called a ________.

Question 45

A chromosome with a different combination of alleles than parental that is created by crossing over between homologous chromosomes is termed ________.

Question 46

Alleles of linked genes usually segregate together during meiosis. When they don't, it is because ________ has occurred between them.

Question 47

Describe why allelic phase is a major obstacle in mapping human genes.

Question 48

What would a Lod score of 3.2 tell you about genetic linkage? What information does the θ value tell you?

Question 49

How is Newton Morton's statistical method helpful for calculating the overall probability of genetic linkage when allelic phase is unknown?

Question 50

What is the relationship between linked genes and syntenic genes? Are syntenic genes always linked?Are linked genes always syntenic? Describe what is meant by each term.

Question 51

5% recombination is equal to ________ map unit(s)(m.u.)or centimorgan(s)(cM)of distance between linked genes.

Question 52

Genetic linkage can be spotted by comparing the observed frequencies of ________ with the frequencies expected (assuming independent assortment).

Question 53

Recombination frequency between linked genes increases as the distance between genes gets larger or gets smaller?

Question 54

If too little time has passed for crossing over to randomize haplotypes or if natural selection favors certain haplotypes, what would you expect to see?

Question 55

The mapping of the human genome in the mid-1980s was integral for identifying new human genetic markers because of the identification of what types of polymorphic DNA markers?

Question 56

Describe the three types of tetrads that can be formed when a diploid yeast sporulates to form haploid gametes.

Question 57

What ratios of these three tetrads would you expect to see if two genes are linked? Why is each type of tetrad more or less common? What ratios would you expect to see if the two genes are unlinked? Why?

Question 58

Recombination frequency differs between males and females. Which sex has a higher rate of recombination, heterogametic (males)or homogametic (females)?

Question 59

What is the Lod score?

Question 60

What is the highest possible frequency of recombination between linked genes that can be generated by any type of crossover event?

Question 61

Crossing over occurs frequently during meiosis. Can crossing over occur during mitosis? Explain why mitotic crossing over may or may not be possible.