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 coefficient of coincidence (c)?

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

Answer 1

D

Question 2

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

Answer 2

E

Question 3

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

Answer 3

E

Question 4

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.

Question 5

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 6

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 7

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 8

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 chance
B)four recombinant genotypes,more frequent than expected by chance
C)six recombinant genotypes,more frequent than expected by chance
D)four recombinant genotypes,less frequent than expected by chance
E)six recombinant genotypes,less frequent than expected by chance

Question 9

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 10

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

Question 11

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 12

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 13

The Huntington disease (HD)allele was discovered

A)by mapping it to blood enzyme genes,based on Lod scores of -2.
B)through pedigree analysis alone.
C)by linkage to a DNA genetic marker,D4S10 with a Lod score of 8.53.
D)through mapping to known homologs in other organisms such as mice.
E)through direct sequencing of mutations identified by SNPs.

Question 14

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 15

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 16

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 17

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 18

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 19

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 20

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

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

Question 21

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

Question 22

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

Question 23

Chromosomes that do not reshuffle the alleles of linked genes are identified as what?

Question 24

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

Question 25

Morgan realized that Bateson and Punnett had detected genetic linkage but 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 26

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

Question 27

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 28

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

Question 29

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 30

Does recombination frequency between linked genes increase as the distance between genes gets larger or smaller?

Question 31

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

Question 32

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 33

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

Question 34

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

Question 35

Refer to this map to answer the following 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

Bateson and Punnett saw early evidence of gene linkage.What terms did they use to describe genes that remained together frequently and the loss of parental alleles in favor of new,nonparental phenotypes in the F₂ progeny?

Question 37

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 38

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

Question 39

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

Question 40

Incomplete genetic linkage is far more common than complete linkage.Recombination between the homologs produces what type(s)of gametes?

Question 41

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

Question 42

The alleles of syntenic genes can be reshuffled by crossing over between homologous chromosomes to produce ________.

Question 43

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

Question 44

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

Question 45

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 46

What is the Lod score?

Question 47

Describe the three types of tetrads.

Question 48

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

Question 49

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

Question 50

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 51

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

Question 52

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

Question 53

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

Question 54

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

Question 55

The tendency of alleles of linked genes to segregate together during meiosis can be broken by ________.

Question 56

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