Deck 14: Genetic Mapping in Eukaryotes

Question 1

Interference is a phenomenon in which

A)no double crossovers occur among nearby genes.
B)the number of observed double crossovers is greater than the number of expected double crossovers.
C)the number of observed double crossovers is less than the number of expected double crossovers.
D)the occurrence of one crossover interferes with the formation of another crossover nearby.
E)Both C and D

Answer 1

E

Question 2

One map unit is equal to a recombination frequency of

A)1%.
B)5%.
C)10%.
D)50%.
E)100%.

Answer 2

A

Question 3

How many phenotypic classes may be generated from a three-point testcross?

A)(3)² = 9
B)(2)³ = 8
C)(3)³ = 27
D)(2)² = 4
E)It cannot be predicted.

Answer 3

B

Question 4

Crossing-over is

A)the place on a homologous pair of chromosomes at which a physical exchange occurs.
B)an extremely rare event.
C)an event that only takes place during meiosis.
D)the reciprocal exchange of homologous regions of chromatids.
E)not useful in mapping genes.

Question 5

If genes are linked and an F₁ is testcrossed,

A)the genes cannot separate.
B)they can produce only one phenotype.
C)they only produce homozygotes.
D)they produce more recombinant phenotypes than parental phenotypes.
E)they produce more parental phenotypes than recombinant phenotypes.

Question 6

During a dihybrid cross involving two linked genes,18% of the resulting gametes showed a recombinant genotype.These two linked genes are ________ map units apart.

A)0.18
B)18
C)32
D)180
E)1.8

Question 7

If an AABB strain is mated with an aabb strain,an AB gamete produced by the resulting F₁ is

A)impossible.
B)parental.
C)recombinant.
D)the most common.
E)None of these

Question 8

If two genes are not linked,then the expected phenotypic ratio resulting from a testcross is

A)9:3:3:1.
B)1:2:1.
C)3:1.
D)1:1:1:1.
E)1:1.

Question 9

The closer together two genes are on a chromosome,

A)the more likely there will be a recombination event between them.
B)the less likely there will be a recombination event between them.
C)the greater the chance that a double crossover will occur between them.
D)the more likely they are to be epistatic.
E)the less likely they are to be good genetic markers.

Question 10

A two-point testcross is a cross between

A)a double heterozygote for two linked genes and a double recessive genotype.
B)a heterozygote for one of two linked genes and a double recessive genotype.
C)a double heterozygote for two linked genes and a homozygous dominant genotype.
D)a heterozygote for one gene and a homozygote for another.
E)individuals with two different genetic markers.

Question 11

T.H.Morgan and his colleagues found that among the offspring of genetic crosses,parental phenotypic classes were the most frequent,while recombinant classes occurred less frequently.This observation led Morgan to conclude that

A)all genes are linked.
B)alleles of linked genes assort independently.
C)alleles of some genes assort together.
D)genes on different chromosomes assort independently.
E)genes on different chromosomes do not assort independently.

Question 12

A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene. $\begin{array} { l r } + + + & 669 \\a b c & 653 \\+ + c & 121 \\a b + & 139 \\+ b + & 2280 \\a + c & 2215 \\+ b c & 3 \\a + + & 2\end{array}$

-Without performing any calculations,which of the following is the gene order?

A)abc
B)acb
C)cab
D)+++
E)None of these

Question 13

A chi-square analysis of linkage between genes k and f produces a P value of 0.99.What is the likelihood that these two genes are linked?

A)99%
B)10%
C)9.9%
D)1%
E)None of these

Question 14

When does crossing-over occur?

A)Prophase I of meiosis
B)Prophase II of meiosis
C)Interphase prior to meiosis
D)At any time during the second meiotic division
E)At any time throughout division

Question 15

In the offspring resulting from an F₁ testcross,what percentage of recombinant phenotypes is expected if the genes under study are independently assorting?

A)5%
B)33%
C)50%
D)90%
E)The percentage cannot be predicted.

Question 16

Genes that are linked

A)segregate to opposite poles during meiosis.
B)do not assort independently during meiosis.
C)segregate independently during meiosis.
D)are on nonhomologous chromosomes.
E)are always on the X chromosome.

Question 17

A DNA mutation that gives a distinguishable phenotype for a chromosome or gene is a

A)recessive allele.
B)lethal mutation.
C)parental type.
D)linkage group.
E)genetic marker.

Question 18

A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene. $\begin{array} { l r } + + + & 669 \\a b c & 653 \\+ + c & 121 \\a b + & 139 \\+ b + & 2280 \\a + c & 2215 \\+ b c & 3 \\a + + & 2\end{array}$

-Which gene pair is closer together; i.e.there are fewer map units between them?

A)a ? b
B)b ? c
C)a ? c
D)a ? b and b ? c are the same length
E)a ? c and b ? c are the same length

Question 19

In a species of Drosophila,genes q and r are found on the same chromosome 20 centimorgans apart.A cross was made between the following individuals: What proportion of the offspring would you expect to have the wild-type phenotype?

A)0.10
B)0.20
C)0.40
D)0.80
E)None of these

Question 20

Statistical analysis of linkage between any two genes has 1 degree of freedom since there are only two classes of data,parental and recombinant.

Question 21

Is it possible to correct for the effects of multiple crossovers in calculating map distances by applying a linear mapping function.

Question 22

Is it possible for crossing-over to occur between two linked genes,yet for all of the resulting offspring to have parental genotypes?

Question 23

Two genes that are located far from each other on the same chromosome will show a higher frequency of recombination than two genes that are close together on the chromosome.

Question 24

Two genes on a Drosophila chromosome are 70 mu apart.How do you think this number was calculated,and what percentage of recombination would you expect to see between them experimentally?

Question 25

Given three genes in order x y z,it is more accurate to measure the distance between x and z directly than to add up the shorter distances between x and y and y and z.

Question 26

What is the order of these three genes on the chromosome?

Question 27

A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene. $\begin{array} { l r } + + + & 669 \\a b c & 653 \\+ + c & 121 \\a b + & 139 \\+ b + & 2280 \\a + c & 2215 \\+ b c & 3 \\a + + & 2\end{array}$

-The cross represented by a^+b+c+/abc × abc/abc is

A)a two-point testcross.
B)a three-point testcross.
C)a parental cross.
D)a haploid cross.
E)None of these

Question 28

Genes may show 50% recombination either when the genes are on different chromosomes or when the genes are far apart on the same chromosome.

Question 29

An F₁ plant with the genotype DdGg is mated to a ddgg plant.The resulting seeds are planted and the following phenotypes produced in these numbers:
DG 47
Dg 396
dG 412
dg 55
a.What were the genotypes of the parental strains that produced the F₁?
b.What is the distance between the genes?

Question 30

If in calculating the distance between two genes using data from a testcrossed dihybrid you arrive at a genetic distance of 70 map units,there is likely to be something wrong with your calculations,the experimental data,or with the experiment itself.

Question 31

A red,round,true-breeding tomato is bred with a yellow,oval,true-breeding tomato,and the F₁s are testcrossed to a homozygous-recessive tester.This results in the offspring below.Calculate the chi-squared value to test the likelihood of linkage.
yellow oval 11
red round 14
yellow round 7
red oval 8

Question 32

A female fruit fly with the recessive mutant phenotype of white eyes and miniature wings is mated with a male possessing the wild-type phenotype of red eyes and normal wings.Among the F₁s,all the females are wild-type while all the males exhibit the mutant phenotype.The F₂s resulting from a testcross exhibit predominantly (63%)parental phenotypes.How do you explain these results?

Question 33

Explain how Curt Stern showed,using evidence from Drosophila,that genetic recombination is associated with the physical exchange of parts between homologous chromosomes.

Question 34

In a particular breed of flowering plants,two genes on the same chromosome determine flower color and leaf color.For flower color,blue (B)is dominant to pink (b),and for leaf color,white (W)is dominant to green (w).A true-breeding plant with blue flowers and white leaves is crossed with a plant that has pink flowers and green leaves.The offspring are observed and the following phenotypes are tallied.
blue flowers and white leaves: 370 plants
blue flowers and green leaves: 14 plants
pink flowers and white leaves: 12 plants
pink flowers and green leaves: 363 plants
What is the frequency of recombination between these two genes?

Question 35

A coefficient of coincidence of zero is the same as an interference value of zero.

Question 36

Of the three pairs of genes,which ones are closest to each other and which ones are farthest apart?

Question 37

A linkage map illustrates the exact physical locations of genes on a chromosome.

Question 38

Testcrosses may be used to determine if two genes are linked.

Question 39

Parental strains are always homozygous dominant for all genes or homozygous recessive for all genes.

Question 40

Here is a genetic map: A 18 mu B 10 mu C .
AAbbCC is crossed to aaBBcc to produce an F₁ generation.
If you produce 1000 offspring from testcrossing the F₁,what genotypes do you expect,and in what numbers? Assume that there is no interference.

Question 41

What does the chi-square test tell us about the linkage of two genes?

Question 42

Why does the recombination frequency often lead to an underestimation of the true map distance between linked genes?

Question 43

It is not necessary to use a male from a tester strain to genetically map genes on the X chromosome.How can this be done and why does it work?

Question 44

What is the difference in the likelihood of linkage between two genes with a lod score of 2 and two genes with a lod score of 3?

Question 45

The genotypes and numbers of progeny resulting from a testcross are as follows:
t^{+g+ l+} 72
tgl 69
t^+gl 21
tg^+l+ 18
t^+gl+ 9
^tg+l 8
t^+g+l 2
tgl⁺ 1
Total 200
Calculate the genetic distance between genes g and l.