Deck 17: Large-Scale Chromosomal Changes

Question 1

A triploid with 5 chromosomes in each set (3n = 15)is discovered.What is the probability of a meiosis in which all univalents pass to the same pole?

A)1/5
B)1/10
C)1/15
D)1/16
E)1/25

Answer 1

D

Question 2

In the plant genus Triticum there are many different polyploid species,as well as diploid species.Crosses were made between three different species,and hybrids were obtained.The meiotic pairing was observed in each hybrid and is recorded in the following table.(A bivalent is two homologous chromosomes paired at meiosis,and a univalent is an unpaired chromosome at meiosis.) $\begin{array}{ll}\text { Species crossed to make hybrid } & \text { Pairing in hybrid } \\\text { 1. T. turdigum } \times \text { T. monococcum } & 7 \text { bivalents }+7 \text { univalents } \\\text { 2. T. aestivum } \times \text { T. monococcum } & 7 \text { bivalents }+14 \text { univalents } \\\text { 3. T. aestivum } \times \text { T. turdigum } & 14 \text { bivalents }+7 \text { univalents }\end{array}$

The first cross suggests a diploid and tetraploid individual were involved with one set of 7 chromosomes in common.Based on the other crosses,which species is the likely diploid individual?

A)T.turdigum
B)T.monococcum
C)T.aestivum
D)T.turdigum or T.monococcum

Answer 2

T.monococcum

Question 3

In corn R is a gene for red aleurone;its recessive allele r determines colorless aleurone.A cross is made between a diploid r/r female and a trisomic R/r/r male.If pollen grains with extra chromosomes are inviable,the ratio of red to colorless kernels expected in the progeny would be:

A)1 red : 5 colorless.
B)1 red : 2 colorless.
C)1 red : 1 colorless.
D)2 red : 1 colorless.
E)5 red : 1 colorless.

Answer 3

B

Question 4

A triploid of genotype a b/a B/a b is crossed to a triploid of genotype A b/a b/a b.If genes A (a)and B (b)are 20 map units apart,what proportion of the progeny will be phenotypically A B?

A)1/3
B)1/4
C)1/6
D)1/9
E)1/36

Question 5

In an animal bearing the heterozygous inversion ABCDE[*]FGHI / ABGF[*]EDCHI,in one meiocyte,a crossover occurred between the D and E loci and another crossover occurred between the F and G loci.The two crossovers involved the same two chromatids.What will be the proportion of abnormal meiotic products from that meiosis? (Note: [*] = centromere.)

A)0%
B)25%
C)50%
D)100%
E)It depends on the distance between D and E and between F and G.

Question 6

The red fox has 17 pairs of large,long chromosomes.The arctic fox has 26 pairs of smaller shorter chromosomes.What do you expect to be the chromosome number in somatic tissues of a red fox/arctic fox hybrid?

A)13
B)17
C)26
D)43

Question 7

In a tetraploid species,an individual with genotype B/b/b/b was crossed with an individual with genotype b/b/b/b.What is the expected genotypic ratio in the offspring?

A)1:1
B)1:2
C)1:4
D)1:8
E)1:16

Question 8

A wild-type chromosome can be represented as ABC[*]DEFGH,and from this a chromosomal aberration arises that can be represented ABC[*]DEGFH,where [*] represents the centromere.This aberration is known as a:

A)deletion.
B)duplication.
C)paracentric inversion.
D)pericentric inversion.
E)translocation.

Question 9

In the flowering plants (the angiosperms),the process of double fertilization occurs.This is where one sperm nucleus from the pollen grain fertilizes the egg,and the other fuses with the two polar nuclei in the female gametophyte to produce the triploid (3n)endosperm tissue.The aleurone layer (the outer layer of cells of the endosperm)can be colored or colorless.The allele C causes color (purple),and the allele c gives colorless aleurone.A plant is of genotype Cc and self-pollinates.What ratio of colored to colorless aleurone layered endosperms are expected in the progeny?

A)1:1
B)2:1
C)3:1
D)10:1

Question 10

In humans,males have only one X chromosome and females have two,yet housekeeping X chromosome genes are expressed in roughly equal amounts in males and females.How can this be explained?

A)The X chromosome is hyperactivated in males.
B)The X chromosomes are hypoactivated in females.
C)One X chromosome in females is inactivated.
D)The Y chromosome in males actually contains similar genes as the X chromosome.

Question 11

A hybrid allotetraploid species (2n = 60)was backcrossed to one of the suspected parents (2n = 30).When the F₁ underwent meiosis,the prophase chromosome configuration was examined.If the guess about the suspected parent was correct,what would the chromosome configuration look like?

A)30 pairs
B)45 singles
C)60 singles
D)15 pairs and 15 singles
E)30 pairs and 15 singles

Question 12

A man is found to be karyotypically 47,XYY.The presence of an extra Y chromosome most likely results from:

A)nondisjunction in a maternal meiocyte at meiosis I.
B)nondisjunction in a maternal meiocyte at meiosis II.
C)nondisjunction in a paternal meiocyte at meiosis I.
D)nondisjunction in a paternal meiocyte at meiosis II.
E)unequal crossing over.

Question 13

In the following pedigree,the shaded symbols represent a rare X-linked disease of the blood.The propositus has 45 chromosomes and is sterile.Propose a single mechanism to account for both the sterility and the blood disease in this female.

A)The woman's father was a carrier for the disease on his X chromosome.
B)The woman is likely demonstrating a case of Turner syndrome.
C)The woman is likely demonstrating a case of Down syndrome.
D)The woman's mother's eggs likely experienced a nondisjunction event.

Question 14

A plant trisomic for a chromosome carrying the A gene has the genotype A/a/a.Which of the following represents a gamete this plant could NOT produce?

A)A/a
B)A
C)a
D)A/A

Question 15

In an animal bearing the heterozygous inversion ABCDE[*]FGHI / ABGF[*]EDCHI,in one meiocyte a crossover occurred between the D and E loci and another crossover occurred between the F and G loci.The two crossovers involved all four chromatids.What will be the proportion of abnormal meiotic products from that meiosis? (Note: [*] = centromere.)

A)0%
B)25%
C)50%
D)100%
E)It depends on the distance between D and E and between F and G.

Question 16

In a triploid of genotype B/b/b,what proportion of gametes will be B?

A)1/6
B)1/4
C)1/3
D)1/2
E)2/3

Question 17

The n + 1 female gametophytes (embryo sacs)produced by trisomic plants are usually more viable than the n + 1 male gametophytes (pollen grains).If 50% of the functional embryo sacs of a selfed trisomic plant are n + 1 but only 10% of the functional pollen grains are n + 1,what percentage of the offspring will be diploid?

A)0
B)5
C)45
D)90

Question 18

A particular hybrid species is defined as 3n = 12.It is discovered that at meiosis,pairing is always univalent + bivalent (never trivalent)in this species.How many bivalents will be present in prophase of meiosis I?

A)2
B)3
C)4
D)8
E)12

Question 19

Suppose that in Guinea pigs ABO blood types are determined like in humans,where I^A and I^B are expressed codominantly and i (blood type O)is recessive to both.During a set of routine crosses you notice something unusual: Repeated matings between a certain female with blood type AB and a certain male with blood type A produces F₁ Guinea pigs of all four blood types in the following ratios: 5A : 2B : 4AB : 1O.The genotypes of the parents are most likely:

A)I^A/I^B for the female and I^A/I^A for the male.
B)I^A/I^B for the female and I^A/i for the male.
C)I^A/I^B/i for the female and I^A/I^A for the male.
D)I^A/I^B/i for the female and I^A/i for the male.
E)None of the answer options are correct.

Question 20

In Neurospora,the genes his2 and leu2 are closely linked.From a cross,his2 × leu2,which is a rare octad of the following type,arose: his2, +
his 2. +
his 2. +
his 2, +
abort
abort
+, leu2
+, lou 2
This octad most likely arose by:

A)crossover between his2 and leu2,followed by first-division nondisjunction.
B)first-division nondisjunction.
C)post-meiotic mitotic nondisjunction.
D)rare triploidy.
E)second-division nondisjunction.

Question 21

In yeast,the gene phe1 is on chromosome 3 and lys4 is on chromosome 7.The progeny (spores)from the cross +;+ × phe1;lys4 were:
$\begin{array}{ll}47 \% & +;+ \\44 \% & \text { phel; lys4 } \\4 \% & \text { phel; }+ \\5 \% & +; \text { lys } 4\end{array}$


This results suggests that:

A)a nondisjunction occurred.
B)one parent has a deletion.
C)one parent has an inversion.
D)one parent has a translocation.
E)unequal crossovers occurred.

Question 22

Which of the following chromosomal aberrations can result in unmasking recessive alleles resulting in their expression?

A)deletions
B)duplications
C)inversions
D)translocations

Question 23

The diagram below shows the chromosomes of a plant that is heterozygous at six loci as well as for a reciprocal translocation involving the chromosomes that carry these loci. This plant is crossed to a plant with normal chromosomes that is homozygous recessive at all loci,and the progeny was comprised of about 250 individuals,half of them phenotypically ABDEFG and half abdefg.This result suggests that:

A)multiple crossovers occurred.
B)no crossovers occurred.
C)segregation was always adjacent.
D)segregation was always alternate.
E)the six loci are unlinked.

Question 24

Assume you are studying an organism in which duplications or deficiencies in the gametes are not viable.An individual is heterozygous for a paracentric inversion.A four-chromatids double crossover,with one of the two crossovers within the inversion and the other outside of it,would lead to:

A)four viable gametes,two of them parental and two recombinant.
B)four viable gametes,two of them with the inversion and two without.
C)two viable gametes,both recombinant.
D)two viable gametes,both parental.
E)zero viable gametes.

Question 25

Assume you are studying an organism in which duplications or deficiencies in the gametes are not viable.An individual is heterozygous for a pericentric inversion.A four-chromatids double crossover,with one of the two crossovers within the inversion and the other outside of it,would lead to:

A)four viable gametes,two of them parental and two recombinant.
B)four viable gametes,two of them with the inversion and two without.
C)two viable gametes,both recombinant.
D)two viable gametes,both parental.
E)zero viable gametes.

Question 26

Suppose that in a certain diploid fish genes A and T are on chromosomes 4 and 7,respectively.A female fish is heterozygous at both loci and is also heterozygous for a reciprocal translocation between chromosomes 1 and 4.The translocation breakpoint on chromosome 4 is 10 map units away from the a allele (A is located on the normal chromosome 4). If this female is crossed to a male of genotype a/a;t/t and with normal chromosomes,what percentage of the progeny is expected to be phenotypically A;T and have only normal chromosomes?

A)0%
B)2)5%
C)22.5%
D)25%
E)45%

Question 27

An allotetraploid plant with 2n = 30 is backcrossed to one of its progenitor species that has an n = 8.A sterile progeny is produced.How many chromosomes will this sterile individual possess,and what are the chromosome numbers of the two progenitor species?

Question 28

The fungal cross ad,pan × +,+ (where ad and pan are two linked loci)gave only the following two types of tetrads: $$\begin{array} { l l l } & \text { Tetrad I } & \text { Tetrad 2 } \\\text { Spore 1 } & \text { ad,pan } & a d , \text { pan } \\\text { Spore 2 } & \text { aborted } & a d ,+ \\\text { Spore 3 } & \text { aborted } & +,\text { pan } \\\text { Spore 4 } & + _ { + } + & + ,+\end{array}$$
These results suggest:

A)a deletion in the wild-type parent.
B)a duplication in one or both parents.
C)an inversion in one of the parents.
D)a translocation in one of the parents,with adjacent segregation.
E)a translocation in one of the parents,with alternate segregation.

Question 29

A pure line of plants of genotype a/a;b/b;c/c;d/d;e/e (all recessive to wild type)was crossed to a wild type.One F₁ individual expressed the recessive alleles d and e.This individual arose most likely from:

A)a deletion in the quadruple homozygous recessive parent.
B)a deletion in the wild-type parent.
C)a gene mutation in the wild-type parent.
D)position effect variegation in the F₁.
E)a reversion in the quadruple homozygous recessive parent.

Question 30

Which of the following diagnostic features is indicative of an inversion having occurred?

A)formation of chromosomal "loop" structures in meiosis I
B)reduced fertility
C)pseudodominance
D)formation of chromosomal loops during meiosis and reduced fertility
E)reduced fertility and pseudodominance

Question 31

In the diagram below,each of the five lines indicates the extent of a Drosophila deletion.A through G represent adjacent chromosomal regions (for example,in line 3 regions D and E are deleted).
When a radioactive DNA fragment corresponding to a Drosophila gene of interest was incubated with chromosomes carrying each of the five deletions,it bound only to chromosomes carrying deletion 1 or deletion 5.Based on this information,the gene of interest is located in region:

A)A or B.
B)C.
C)C or D.
D)D.
E)E,F,or G.

Question 32

In a haploid fungus,genes a and b are located on chromosome 2 and 7,respectively.A cross is performed between a strain that is genotypically a;b and has normal chromosomes,and a strain that is genotypically wild type but has a reciprocal translocation between chromosomes 2 and 7.If crossing over never occurs in this fungus,what is the expected proportion of tetrads resulting from the cross? $$\begin{array} { l l l l l l } & \text { Tetrad I } & \text { Tetrad 2 } & \text { Tetrad 3 } & \text { Tetrad 4 } & \text { Tetrad 5 } \\\text { Spore 1 } & \text { aborted } & a ; b & A ; B & A ; B & A ; B \\\text { Spore 2 } & \text { aborted } & a ; b & A ; B & A ; B & \text { aborted } \\\text { Spore 3 } & \text { aborted } & \text { aborted } & \text { abarted } & a ; b & a ; b \\\text { Spore 4 } & \text { aborted } & \text { aborted } & \text { aborted } & a ; b & \text { aborted }\end{array}$$

A)All five tetrads will be present,in equal proportions.
B)Only tetrads 1 and 4 will be present,in a 1:1 ratio.
C)Only tetrads 2 and 3 will be present,in a 1:1 ratio.
D)Only tetrads 2,3,and 5 will be present,in a 1:1:2 ratio.
E)Only tetrad 5 will be present.

Question 33

In rabbits,genes T and R are located on the same chromosome,20 map units apart.A colleague sends you one of his pure-breeding t r/t r males,which you cross to a T R/T R individual.Each of the T R/ t r F₁ heterozygotes is then crossed back to your colleague's double homozygous recessive.Altogether,the backcrosses produce 50 rabbits,of which 24 are T R/t r,24 are t r/t r,1 is T r/t r and 1 is t R/t r. Which of the following explanations best justifies the results?

A)The colleague's rabbit is heterozygous for an inversion that spans at least 16 map units of the region between genes t and r.
B)The colleague's rabbit is heterozygous for an inversion that spans at least 18 map units of the region between genes t and r.
C)The colleague's rabbit is homozygous for an inversion that spans at least 16 map units of the region between genes t and r.
D)The colleague's rabbit is homozygous for an inversion that spans at least 18 map units of the region between genes t and r.
E)The colleague's rabbit carries an inversion that spans the whole region between genes t and r.

Question 34

In Zinnia plants (n = 16),the chromosomes of autotetraploids pair in twos.The locus for flower color (R = red,r = white)is close to the centromere on chromosome 5,and the locus for plant height (D = tall,d = dwarf)is also closely linked to the centromere on chromosome 11.Both dominant alleles show full dominance over any number of recessive alleles.
a)If you are observing meiosis in tetraploid Zinnias,what is the total number of bivalents (chromosome pairs)that will be visible during prophase I? Explain.
b)In a pollen cell from a tetraploid plant,how many chromosomes will be present?
c)If a tetraploid plant of genotype R/R/r/r;D/D/D/D is selfed,what proportion of progeny will be white flowered and tall?
d)If a tetraploid plant of genotype R/r/r/r;D/D/D/D is selfed,what proportion of progeny will be white flowered and tall?
e)If a tetraploid plant of genotype R/R/r/r;D/D/d/d is selfed,what proportion of progeny will be white flowered and dwarf? What proportion will be red flowered and tall?

Question 35

Below are a list of chromosomal aberrations (a-d)and a list of potential diagnostic features (1-7).On the blank line following each aberration,write the number(s)of all the diagnostic features that may apply to that aberration.Note that each diagnostic feature may be used more than once.
List of chromosomal aberrations:
a)deletion _____
b)duplication _____
c)inversion _____
d)translocation _____
List of potential diagnostic features:
1)formation of chromosomal "cruciform" structures in meiosis I
2)formation of chromosomal "loop" structures in meiosis I
3)Recombinant frequencies between genes on the same chromosome are (much)lower than expected.
4)reduced fertility (other than semisterility)
5)semisterility
6)pseudodominance
7)pseudolinkage (two genes on separate chromosomes do not segregate independently).

Question 36

A reciprocal translocation happened between chromosomes 1 and 2 in a wild species of strawberries.Below is a diagram of (unreplicated)chromosomes 1 and 2 for the translocated strain of strawberries.N1 and N2 indicate the normal chromosomes 1 and 2,respectively;T1 and T2 are their translocated counterparts. Which chromosomes must segregate from one another at anaphase I to form viable meiotic products?

A)N1 and N2 separate from T1 and T2.
B)N1 separates from N2,T1,and T2.
C)N1 and T1 separate from N2 and T2.
D)N2 and T1 separate from N1 and T2.
E)T1 separates from T2,N1,and N2.

Question 37

Assume you are studying an organism in which duplications or deficiencies in the gametes are not viable.An individual is heterozygous for a pericentric inversion.A four-chromatids double crossover,with both crossovers occurring within the inversion,would lead to:

A)four viable gametes,two of them parental and two recombinant.
B)four viable gametes,two of them with the inversion and two without.
C)two viable gametes,both recombinant.
D)two viable gametes,both parental.
E)zero viable gametes.

Question 38

A meiocyte of an organism heterozygous for a reciprocal translocation goes through meiosis and results in four viable meiotic products.The most likely explanation is that:

A)there was adjacent segregation.
B)there was alternate segregation.
C)a suppressor mutation occurred.
D)the translocation breakpoints were very close to the centromeres.
E)the translocation reverted.

Question 39

The two loci Y (y)and R (r)are normally 20 map units apart.A particular dihybrid individual is heterozygous for an inversion that spans 98% of the region between Y and R,but does not include Y or R.If this dihybrid is testcrossed to an individual with normal chromosomes,the recombinant frequency in progeny will be:

A)more than 20%.
B)20%.
C)less than 20%,but more than 10%.
D)10%.
E)less than 10%.

Question 40

Assume that x is a new mutant allele in corn.An x/x plant is crossed with a triple-10 individual (trisomic for chromosome 10)carrying only normal dominant alleles at the x locus.Trisomic progeny for chromosome 10 are recovered and crossed back to x/x.
a)What ratio of dominant to recessive phenotypes is expected if the x locus is not in chromosome 10?
b)What ratio of dominant to recessive phenotypes is expected if the x locus is in chromosome 10?

Question 41

You discover a Drosophila male that is heterozygous for a reciprocal translocation between the second and third chromosomes,each break having occurred near the centromere (which for these chromosomes is near the center).
a)Draw a diagram showing how these chromosomes would synapse at meiosis.
b)You find that this fly has the recessive genes bw (brown eyes)and eb (ebony body)on the second and third chromosomes,respectively,and wild-type alleles on the translocated ones.The fly is crossed with a female having normal chromosomes and homozygous for bw and eb.What phenotypic classes and in what ratio would be expected in the progeny of this cross? (Hint: Zygotes with an extra chromosome arm or deficient for one do not survive.Also,remember that there is no crossing over in Drosophila males.)

Question 42

In the fungus Neurospora,a standard lab strain carrying the auxotrophic mutation m was crossed to a prototrophic strain isolate from a burned sugar cane field.There were three types of asci as shown below,where the symbol "ab" means an aborted ascospore whose m locus constitution could not be tested.
$$\begin{array} { l l l } \text { Type I } & \text { Type 2 } & \text { Type 3 } \\m & a b & m \\m & a b & m \\m & a b & a b \\m & a b & a b \\+ & a b & a b \\+ & a b & a b \\+ & a b & + \\+ & a b & +\end{array}$$
Explain the origin of each ascus type.

Question 43

A pure-breeding flowering plant with a long stem and small leaves was crossed to a pure-breeding individual of the same species that has a short stem and large leaves.The F₁ is comprised entirely of individuals with long stems and large leaves.When these F₁ individuals are testcrossed,they only produced about half as many individuals as expected.The phenotypic ratios were: 462 short stems and large leaves,450 long stems and small leaves,52 long stems and large leaves,and 50 short stems and small leaves.
These 1022 plants were further crossed to a tester,and many of them displayed semi-sterility,in particular:
of the 462 plants with short stems and large leaves,362 were semisterile.
of the 450 plants with long stems and small leaves,90 were semisterile.
of the 52 plants with long stems and large leaves,40 were semisterile.
of the 50 plants with short stems and small leaves,10 were semisterile.
Provide a plausible,comprehensive genetic explanation for these results.

Question 44

In Drosophila,the phenotype bar-eye is due to a tandem duplication of several bands on the X chromosome.Occasionally (1 out of 1600),all true-breeding stocks of bar-eyed flies yield an even more extreme phenotype called ultrabar,which is in fact due to a triplication of this chromosomal segment.The hypothesis used to explain ultrabar's origin is that a slight mispairing of chromosomes can occur whenever there is a duplication.If crossing-over occurs (in females),then recombinants would form as follows:
What other evidence,not involving cytological observation,would you seek to substantiate this hypothesis? Be as exact as you can.There are at least two possible answers.

Question 45

In Drosophila,h determines hairy body (h⁺ = smooth)and se determines sepia eyes (se⁺ = red).The loci are approximately 40 m.u.apart on an autosome.A cross was made: se h⁺/se h⁺ X se⁺h/se⁺h,and the F₁ were all wild type.F₁ females were testcrossed to se h/se h males and the offspring were:
39 sepia eyes.
42 hairy body. a)What progeny and in what proportions were expected?
b)Provide an explanation of the observed results if they differ from expectations.

Question 46

You make meiotic chromosome preparations of two species of tetraploid plants.In one,there is normal pairing of chromosomes and tetrads (quadivalents)are observed,while in the other,there is abnormal pairing in some preparations and trivalents and univalents are seen.How would you account for the differences between these two species?

Question 47

In a lab strain of Drosophila,cinnabar (cn)and brown (bw)are recessive eye color mutations known to be 41 map units apart on chromosome 2.When similar mutant alleles were induced in a strain from nature,the same linkage of cn and bw was observed.However,when a wild-type strain from nature was crossed with a cn bw/cn bw lab strain to create the genotype + +/cn bw,and females of this type were testcrossed to cn bw/cn bw males from the lab strain,the following phenotypic proportions were obtained in the progeny:
$$\begin{array} { l l l } + & + & 25,200 \\\text { cn } & b w & 21,009 \\\text { cn } & + & 11 \\+ & b w & 36\end{array}$$
a)What is unexpected about these results?
b)What is the most likely explanation? (Summarize your model with a chromosome diagram with labelled genes.)
c)On your diagram show the precise origin of the cn + and the + bw classes.

Question 48

a)Diagram the pairing structure of the inversion heterozygote abcdefg/abfedcg.Assume the centromere is to the left of a.Is this a paracentric or pericentric inversion?
b)Diagram the early anaphase configuration of the bivalent in part (a)if a chiasma occurred between d and e.