Deck 6: Extensions of Mendelian Genetic Principles

Question 1

The closer two genes are together on a chromosome,

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

Answer 1

A

Question 2

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

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

Answer 2

D

Question 3

Genes that are located on the same chromosome are

A) linked.
B) chromologs.
C) homologous.
D) homozygous.
E) coupled.

Answer 3

A

Question 4

If alleles of different genes always maintain their parental association rather than assorting independently into offspring, this is because

A) they are really alleles of a single gene.
B) there was a meiotic error.
C) they are on the same chromosome but far apart.
D) they are very close together on the same chromosome.
E) Mendel's experiments were flawed.

Question 5

Crossing over is

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

Question 6

T. H. Morgan and his group 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) genes on different chromosomes do not assort independently.
B) alleles of linked genes assort independently.
C) genes on different chromosomes assort independently.
D) all genes are linked.
E) alleles of some genes assort together.

Question 7

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

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

Question 8

Genes that are linked

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

Question 9

The chromosome in the previous question is now said to be

A) polymorphic.
B) autosomal.
C) recombinant.
D) homologous.
E) aneuploid.

Question 10

In F2s resulting from a testcross, what percent of recombinant phenotypes is expected if the genes under study are independently assorting?

A) 2
B) 35
C) 90
D) 50
E) There is no predictable percentage.

Question 11

When does crossing over occur?

A) Anytime during the second meiotic division
B) Anytime at random during division
C) Prophase II of meiosis
D) Prophase I of meiosis
E) Interphase prior to meiosis

Question 12

One genetic map unit is equal to a recombination frequency of

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

Question 13

A chromosome contains alleles for blue eyes and blond hair. After meiosis the same chromosome contains alleles for blue eyes and brown hair. This occurred because of

A) crossing over.
B) interference.
C) independent assortment.
D) segregation.
E) nondisjunction.

Question 14

The accuracy of map distances based on recombination frequencies

A) decreases as the chance of multiple crossovers increases.
B) is greatest when genes are at a distance of 7 mu or less.
C) decreases as the distance between genes increases.
D) increases as the distance between genes increases.
E) All but D.

Question 15

For a two-point testcross, if the percentage of recombinants is 50 percent, the two genes may be

A) on different chromosomes.
B) not linked.
C) distant from each other on the same chromosome.
D) both A and B.
E) all of the above.

Question 16

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

Question 17

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

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

Question 18

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

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

Question 19

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

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

Question 20

A two-point testcross is a cross between

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

Question 21

A chromosomal arrangement in which each homolog carries the wild-type allele of one gene and the mutant allele of the other gene is called coupling.

Question 22

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 23

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

Question 24

How could you determine whether two genes, A and C, showing 50 percent recombination in genetic crosses, are on different chromosomes or far apart on the same chromosome?

Question 25

What are the forms of crossovers that may occur between two genes that are far apart on the same chromosome?

Question 26

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

Question 27

One would correct for the effects of multiple crossovers in calculating map distances by applying a linear mapping function.

Question 28

Describe experimental evidence that proves that crossing over occurs at the 4-chromatid stage of meiosis rather than prior to meiosis.

Question 29

A triple-point testcross of a plant is used to determine the order of linked genes g, t, and l. The wild-type parental phenotypes are green, tall, and leafy, represented by the triple heterozygote g+t+l+/gtl. The mutant plants are red, dwarf, and sparse, represented by the triple recessive genotype gtl/gtl. The least frequent phenotypes in the progeny of the testcross are red, tall, and leafy, and green, dwarf, and sparse. What is the order of the three genes?

Question 30

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 31

Alfred Sturtevant discovered that recombination frequencies could be used as a quantitative measure of the genetic distance between two genes on a genetic map.

Question 32

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

Question 33

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

Question 34

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

Question 35

The map distance between the two outside genes in a linkage group of three genes can be calculated by adding the map distances between the two adjacent pairs of the three genes.

Question 36

Explain how Creighton and McClintock showed, using evidence from corn, that genetic recombination is associated with the physical exchange of parts between homologous chromosomes.

Question 37

Why can a male fruit fly be used as a testcross individual with a female heterozygote to test for recombinant phenotypes in X-linked genes?

Question 38

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

Question 39

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

Question 40

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

Question 41

Why is the probability of a double crossover in two different regions of a chromosome NOT equal to the product of the probabilities of the two events occurring separately?