Deck 3: The Chromosome Theory of Inheritance

Question 1

If the map distance between genes A and B is 10 map units and the map distance between genes B and C is 25 map units,what is the map distance between genes A and C?

A) 15 map units
B) 35 map units
C) either 15 map units or 35 map units,depending on the order of the genes
D) The map distance between A and C can not be predicted from these data.
E) 5 map units

Answer 1

C

Question 2

The R/r and S/s genes are linked and 10 map units apart.In the cross Rs/rS × rs/rs what fraction of the progeny will be RS/rs?

A) 5%
B) 10%
C) 25%
D) 40%
E) 45%

Answer 2

5%

Question 3

The pairwise map distances for four linked genes are as follows: A-B = 22 m.u. ,B-C = 7 m.u. ,C-D = 9 m.u. ,B-D = 2 m.u. ,A-D = 20 m.u. ,A-C = 29 m.u.What is the order of these four genes?

A) ABCD
B) ADBC
C) ABDC
D) BADC
E) CADB

Answer 3

B

Question 4

Which of the following processes can generate recombinant gametes?

A) Segregation of alleles in a heterozygote.
B) Crossing over between two linked heterozygous loci.
C) Independent assortment of two unlinked heterozygous loci.
D) Crossing over between two linked heterozygous loci and independent assortment of two unlinked heterozygous loci.
E) Segregation of alleles in a heterozygote,crossing over between two linked heterozygous loci and independent assortment of two unlinked heterozygous loci.

Question 5

The map of a chromosome interval is Q-10 m.u.-R-40 m.u.-S.Which interval would likely show the higher ratio of double to single chiasmata?

A) Q-R
B) R-S
C) The ratios would be the same in the two intervals.
D) Two chiasmata never occur in the same interval.

Question 6

Suppose a three-point testcross was conducted involving the genes X,Y,and Z.If the most abundant classes are XYz and xyZ and the rarest classes are xYZ and Xyz,which gene is in the middle?

A) X
B) Y
C) Z
D) cannot be determined
E) either Z or Y

Question 7

In an individual with genotype Lpq/lPQ the L gene is in the middle.What would be the genotypes of the gametes produced by a double crossover surrounding the L locus?

A) LPQ and lpq
B) LpQ and lPq
C) lpQ and LPq
D) Lpq and lPQ
E) cannot be determined

Question 8

In humans,the genes for red-green colour blindness (R = normal,r = colour-blind)and haemophilia A (H = normal,h = haemophilia)are both X-linked and only 3 map units apart.Suppose a woman has four sons,and two are colour-blind but have normal blood clotting and two have haemophilia but normal colour vision.What is the probable genotype of the woman?

A) HR/hr
B) Hr/hr
C) hr/hR
D) Hr/hR
E) HR/Hr

Question 9

Crossing over takes place in paired bivalents consisting of ______ chromatids,and involves _______ of the chromatids.

A) 2;2
B) 2;4
C) 4;2
D) 4;4
E) 8;4

Question 10

Recombination frequencies near 50 percent suggest that

A) two genes are on different chromosomes.
B) two genes lie very close together on the same chromosome.
C) two genes are on the same chromosome but lie very far apart.
D) two genes are on different chromosomes,or two genes are on the same chromosome but lie very far apart.
E) none of the choices are correct.

Question 11

The zipper-like connection between paired homologs in early prophase is known as a

A) spindle fiber.
B) synaptic junction.
C) synaptonemal complex.
D) chiasma.
E) none of the choices are correct.

Question 12

The map of a chromosome interval is A-10 m.u.-B-40 m.u.-C.From the cross Abc/aBC × abc/abc,how many double crossovers would be expected out of 1000 progeny?

A) 5
B) 10
C) 20
D) 40

Question 13

The measured distance between genes D and E in a two-point test cross is 50 map units.What does this mean in physical terms?

A) D and E are on different pairs of chromosomes.
B) D and E are linked and exactly 50 map units apart.
C) D and E are linked and at least 50 map units apart.
D) either D and E are on different pairs of chromosomes or D and E are linked and exactly 50 map units apart.
E) either D and E are on different pairs of chromosomes or D and E are linked and at least 50 map units apart.

Question 14

A dihybrid test cross is made between genes H and I.You wish to use the χ² test to test the hypothesis that the H and I genes are unlinked.How many degrees of freedom would there be in this test?

A) 1
B) 2
C) 3
D) 4
E) 0

Question 15

In Drosophila,singed bristles (sn)and cut wings (ct)are both caused by recessive,X-linked alleles.The wild type alleles (sn⁺ and ct⁺)are responsible for straight bristles and intact wings,respectively.A female homozygous for sn and ct⁺ is crossed to a sn⁺ct male.The F₁ flies are interbred.The F₂ males are distributed as follows: $$\begin{array} { | l | l | } \hline s n c t & 13 \\\hline s n c t ^ { + } & 36 \\\hline s n ^ { + } c t & 39 \\\hline s n ^ { + } c t ^ { + } & 12 \\\hline\end{array}$$
What is the map distance between sn and ct?

A) 12 m.u.
B) 13 m.u.
C) 25 m.u.
D) 50 m.u.
E) 75 m.u.

Question 16

In Drosophila,the genes b,c,and sp are linked and in the order given,the distances being b-c = 30 m.u.and c-sp = 20 m.u.These intervals exhibit 90% interference.How many double crossovers would be recovered in a three-point cross involving b,c,and sp out of 1000 progeny?

A) 3
B) 6
C) 54
D) 60
E) 600

Question 17

The T/t and S/s genes are linked and 10 map units apart.In the cross Ts/tS × ts/ts what fraction of the progeny will be Ts/ts?

A) 5%
B) 10%
C) 25%
D) 40%
E) 45%

Question 18

In Drosophila,singed bristles (sn)and carnation eyes (car)are both caused by recessive,X-linked alleles.The wild-type alleles (sn⁺ and car⁺)are responsible for straight bristles and red eyes,respectively.A sn car female is mated to a sn⁺ car⁺ male and the F₁ progeny are interbred.The F₂ males are distributed as follows.What is the value of ?² for a test of the hypothesis that the sn and car genes are unlinked? $$\begin{array} { | l | l | } \hline s n \text { car } ^ { + } & 55 \\\hline s n \text { car } ^ { + } & 45 \\\hline s n ^ { + } \mathrm { car } ^ { + } & 45 \\\hline \mathrm { sn } ^ { + } \mathrm { car } ^ { + } & 55 \\\hline\end{array}$$

A) 0.5
B) 1.0
C) 2.0
D) 0.4
E) 20

Question 19

Suppose the L and M genes are on the same chromosome but separated by 100 map units.What fraction of the progeny from the cross LM/lm × lm/lm would be Lm/lm?

A) 10%
B) 25%
C) 50%
D) 75%
E) 100%

Question 20

In Drosophila,the genes y (yellow body)and car (carnation eyes)are located at opposite ends of the X chromosome.In doubly heterozygous females (y⁺ car⁺/y car),a single chiasma is observed somewhere along the X chromosome in 90% of the examined oocytes.No X chromosomes with multiple chiasmata are observed.What percentage of the male progeny from such a female would be recombinant for y and car?

A) 5%
B) 10%
C) 45%
D) 55%
E) 90%

Question 21

Which of the following is usually true of a chi-square test performed when deciding whether two genes are linked?

A) the p-value is 2
B) degrees of freedom (df)= 2
C) the minimum sample size is 1000
D) the null hypothesis is that the loci are not linked
E) degrees of freedom (df)= 4

Question 22

Some of the larger human chromosomes typically contain multiple chiasmata during meiotic prophase.If you were to carefully study the distribution of these chiasmata,what would you find?

A) Chiasmata are randomly distributed along chromosomes.
B) All chromosome pairs have the same number of chiasmata.
C) A single chromosome pair always has the same number of chiasmata in every meiotic cell.
D) Chiasmata are spaced along a chromosome arm more regularly than would be expected by chance.
E) Chiasmata are spaced more irregularly along a chromosome arm than would be expected by chance.

Question 23

Individuals heterozygous for the RB⁺ and RB^- alleles can develop tumours as a result of

A) a mitotic crossover that leads to homozygosity for both RB⁺ and RB^-.
B) a somatic mutation in the RB⁺ allele that leads to homozygosity for RB^-.
C) a somatic mutation in the RB^- allele that leads to homozygosity for RB⁺.
D) the fact that RB^- is dominant to RB⁺.
E) either a mitotic crossover that leads to homozygosity for both RB^- or a somatic mutation in the RB⁺ allele that leads to homozygosity for RB^-.

Question 24

Suppose an individual is heterozygous for a pair of alleles (e.g. ,A/a).Under what conditions would a crossover in a somatic cell of this individual lead to a clone of cells homozygous for a? (Pick the most precise answer. )

A) The crossover would have to occur between the A locus and the centromere and involve two homologous (non-sister)chromatids.
B) The crossover would have to occur between the A locus and the end of the chromosome and involve two homologous (non-sister)chromatids.
C) The crossover would have to occur on the same chromosome arm as the A locus and involve two homologous (non-sister)chromatids.
D) The crossover would have to occur on the same chromosome as the A locus and involve two homologous (non-sister)chromatids.
E) The crossover would have to occur between the A locus and the centromere and involve two sister chromatids (not homologous)chromatids.

Question 25

Abnormal chromosomes used by Stern and others to study crossovers are called

A) molecular markers
B) chiasmata
C) physical markers
D) genetic markers
E) non-disjunction

Question 26

By definition,a tester is always

A) syntenic
B) recessive
C) X-linked
D) linked
E) heterozygous

Question 27

Suppose an individual is heterozygous for two linked pairs of alleles on the same chromosome arm,Ab/aB such that the A locus is closer to the centromere than the B locus.Under what conditions would a crossover in a somatic cell generate a twin spot,i.e. ,two adjacent clones of cells,one clone homozygous for a and the other clone homozygous for b?

A) The crossover would have to occur between the A locus and the centromere and involve two homologous (non-sister)chromatids.
B) The crossover would have to occur between the A locus and the B locus and involve two homologous (non-sister)chromatids.
C) The crossover would have to occur between the B locus and the end of the chromosome locus and involve two homologous (non-sister)chromatids.
D) A double crossover would have to occur,with one crossover between the A locus and the centromere and a second crossover between the A and B loci and both crossovers would have to involve two homologous (non-sister)chromatids.
E) No crossover in a somatic cell could generate a twin spot.

Question 28

Recombination due to crossing over occurs only rarely in mitosis.

Question 29

Syntenic genes are

A) always recombinant
B) always unlinked
C) assort independently
D) always linked
E) located on the same chromosome

Question 30

Which of the following was useful in the identification of the gene that causes cystic fibrosis (CF)?

A) observation of independent assortment of PON and CF
B) sequencing of the complete human genome
C) identification of another trait that is often inherited with CF
D) comparison of karyotypes of affected and unaffected individuals
E) chorionic villus sampling

Question 31

Two genes are considered linked when F₂ progeny more commonly show the recombinant genotype.

Question 32

In fruit flies,the recessive pr and cn mutations cause brown and bright-red eyes,respectively (wild-type flies have brick-red eyes).The double mutant pr cn combination has orange eyes.A female who has wild-type eyes is crossed to an orange-eyed male.Their progeny have the following distribution of eye colours: $$\begin{array} { | l | r | } \hline \text { wild-type } & 8 \\\hline \text { brown } & 241 \\\hline \text { bright-red } & 239 \\\hline \text { orange } & 12 \\\hline & 500 \\\hline\end{array}$$
The mother of these progeny resulted from a cross between two flies from true breeding lines.What are the genotypes of these two lines?

A) pr pr cn⁺ cn⁺ and pr⁺ pr⁺ cn cn
B) pr⁺ pr⁺ cn⁺ cn⁺ and pr pr cn cn
C) pr⁺ pr cn⁺ cn and pr pr cn cn
D) pr⁺ pr cn cn and pr pr cn⁺ cn
E) Either pr pr cn⁺ cn⁺ and pr⁺ pr⁺ cn cn or pr⁺ pr⁺ cn⁺ cn⁺ and pr pr cn cn could be true.

Question 33

What happens physically during the process of crossing over?

A) Two homologous chromatids break and rejoin at random sites within each chromatid.
B) The genetic information on one chromatid is replaced by copying genetic information from a homologous chromatid without there being any physical exchange between the chromosomes.
C) Two homologous chromatids break and rejoin at precisely the same site along the chromosome so that there is no loss or gain of material on either product.
D) A segment of one chromatid is cut out and then pasted into the other chromatid.
E) It is not known what occurs during crossing over.

Question 34

In fruit flies,the recessive pr and cn mutations cause brown and bright-red eyes,respectively (wild-type flies have brick-red eyes).The double mutant pr cn combination has orange eyes.A female who has wild-type eyes is crossed to an orange-eyed male.Their progeny have the following distribution of eye colours: $$\begin{array} { | l | r | } \hline \text { wild-type } & 8 \\\hline \text { brown } & 241 \\\hline \text { bright-red } & 239 \\\hline \text { orange } & 12 \\\hline & 500 \\\hline\end{array}$$
What is the map distance between the pr and cn genes?

A) 20 m.u.
B) 2 m.u.
C) 4 m.u.
D) 46 m.u.
E) 8 m.u.

Question 35

Sturtevant's detailed mapping studies of the X chromosome of Drosophila established what genetic principle?

A) That genes are arranged in a linear order on the chromosomes.
B) That genes are carried on chromosomes.
C) That sex determination is controlled by the X and Y chromosomes.
D) That segregation of an allelic gene pair is accompanied by disjunction of homologous chromosomes.
E) That different pairs of chromosomes assort independently.

Question 36

Consider a pair of homologous chromosomes heterozygous for three genes (e.g.ABC/abc)during prophase I of meiosis.Let the sister chromatids of one homolog be numbered 1 and 2;and the sister chromatids of the other homolog be numbered 3 and 4.A crossover that would result in genetic recombination (e.g. ,Abc or aBC)could involve which chromatids?

A) 1 & 2 or 3 & 4
B) 1 & 3 or 2 & 4
C) 1 & 4 or 2 & 3
D) 1 & 3 or 1 & 4 or 2 & 3 or 2 & 4
E) any two of the four chromatids

Question 37

In fruit flies,the recessive pr and cn mutations cause brown and bright-red eyes,respectively (wild-type flies have brick-red eyes).The double mutant pr cn combination has orange eyes.A female who has wild-type eyes is crossed to an orange-eyed male.Their progeny have the following distribution of eye colours: $$\begin{array} { | l | r | } \hline \text { wild-type } & 8 \\\hline \text { brown } & 241 \\\hline \text { bright-red } & 239 \\\hline \text { orange } & 12 \\\hline & 500 \\\hline\end{array}$$
What is the genotype of the mother of these progeny?

A) pr cn/pr⁺ cn⁺
B) pr⁺ cn/pr⁺ cn
C) pr⁺ cn/pr cn⁺
D) pr cn⁺/pr cn⁺
E) pr cn/pr cn

Question 38

Which of the following best describes a linkage map that contains a high density of genetic markers?

A) recombinant
B) saturated
C) three-point
D) random
E) sporadic

Question 39

Observation of which of the following led to the discovery of mitotic recombination?

A) twin spots
B) chromosome knobs
C) linkage mapping
D) interference
E) chiasmata

Question 40

In fruit flies,the recessive pr and cn mutations cause brown and bright-red eyes,respectively (wild-type flies have brick-red eyes).The double mutant pr cn combination has orange eyes.A female who has wild-type eyes is crossed to an orange-eyed male.Their progeny have the following distribution of eye colours: $$\begin{array} { | l | r | } \hline \text { wild-type } & 8 \\\hline \text { brown } & 241 \\\hline \text { bright-red } & 239 \\\hline \text { orange } & 12 \\\hline & 500 \\\hline\end{array}$$
Which classes are the parental types?

A) wild-type and orange
B) brown and bright-red
C) wild-type and brown
D) bright-red and orange
E) There is no way to determine this.

Question 41

C.Stern studied recombination between two homologous X chromosomes in Drosophila in which one chromosome had two cytologically visible abnormalities at opposite ends.What did he find?

Question 42

What is a testcross?

Question 43

Why does the formula for map distance between the two outside genes in a three-point testcross count the double crossovers twice whereas the formula for the distance between an outside gene and the middle gene counts the double crossovers only once?

Question 44

When setting up crosses to determine map distances,why do geneticists prefer to cross the hybrid individuals to individuals homozygous for the recessive alleles in the cross?

Question 45

Map distances between each pair of three linked genes L,M,and N are measured by two-point test crosses and determined to be: L-M = 20 cM,M-N = 25 cM,and L-N = 38 cM.Explain why the measured distance between the two farthest apart genes is not equal to the sum of the two shorter distances.

Question 46

In Drosophila,the genes y,f,and v are all X-linked.y f v females are crossed to wild-type males and the F₁ females are test-crossed.The F₂ are distributed as follows.
Calculate the coefficient of coincidence.

Question 47

How does Mendel's law of independent assortment relate to the concept of linkage?

Question 48

Chiasmata are structures of cross over between non-sister chromatids of homologous chromosomes.

Question 49

What are chiasmata?

Question 50

Genetic mapping distance calculated for a chromosome is directly related to physical distance along that chromosome.

Question 51

In Drosophila,the genes y,f,and v are all X-linked.y f v females are crossed to wild-type males and the F₁ females are test-crossed.The F₂ are distributed as follows.

Determine the linkage arrangement of these three genes and calculate the map distances.

Question 52

In Drosophila,the genes y,f,and v are all X-linked.y f v females are crossed to wild-type males and the F₁ females are test-crossed.The F₂ are distributed as follows.
State what a coefficient of coincidence of 0.5 would mean.

Question 53

When mapping three genes,why do geneticists prefer to do one three-point cross rather than three two-point crosses? Give two reasons.

Question 54

What use is genetic mapping? Give one example where it has been of value to society.

Question 55

When considering the chi-square test,a p value of 0.05 is often considered significant but it is actually an arbitrary assignment of significance.

Question 56

Chromosomal interference is not uniform and may significantly vary within a region of a single chromosome.

Question 57

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows:
What is the genotype of the females that gave rise to these progeny?

Question 58

Three-point crosses are more tedious and less accurate than two-point crosses.

Question 59

Chiasmata can be seen through a light microscope and are sites of recombination.

Question 60

Genes chained together by linkage relationships are known collectively as a linkage group.

Question 61

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows:
Using the two-point cross method,what are the distances between these three genes?

Question 62

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows:
Using the three-point cross method,determine which gene is in the middle (which has to be flipped).

Question 63

In peas,tall (T)is dominant to short (t),red flowers (R)is dominant to white flowers (r),and wide leaves (W)is dominant to narrow leaves (w).A tall,red,wide-leaved plant is crossed to a short,white,narrow-leaved plant and the progeny are as follows:
What is the genotype of the tall,red,wide-leaved parent?

Question 64

In peas,tall (T)is dominant to short (t),red flowers (R)is dominant to white flowers (r),and wide leaves (W)is dominant to narrow leaves (w).A tall,red,wide-leaved plant is crossed to a short,white,narrow-leaved plant and the progeny are as follows:
Give the linkage arrangements.

Question 65

A dihybrid test cross is made between the genes C and D with the following results.Calculate ?² to test the hypothesis that the C and D genes are unlinked.

Question 66

Suppose the map for a particular human chromosome interval is: a-1-b-1-c-1-d-1-e-1-f,where the numbers indicate map distances.
In a man heterozygous for all six genes,what fraction of his sperm would be recombinant in the a-f interval?

Question 67

Suppose the map for a particular human chromosome interval is: a-1-b-1-c-1-d-1-e-1-f,where the numbers indicate map distances.
In a man of the genotype ABCDEF/abcdef,what event would give rise to a sperm with the genotype abcdeF?

Question 68

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows:
Calculate the coefficient of coincidence and comment on its meaning.

Question 69

Females heterozygous for the recessive second chromosome mutations pn,px,and sp are mated to a male homozygous for all three mutations.The offspring are as follows:
Which method provides more confidence as to the correct order of the genes?

Question 70

A female mouse from a true-breeding wild-type strain was crossed to a male mouse with apricot eyes (ap)and grey body (gy).The F₁ mice were wild-type for both traits.When the F₁ were interbred,the F₂ were distributed as follows (200 total):
Explain these results.