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Exam 17: Large-Scale Chromosomal Changes

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Question 21

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

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

E) A) and D)
F) All of the above

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Question 22

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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?

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The first is an allotetraploid and the c...

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Question 23

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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:

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:

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?

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 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?

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?

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?

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:

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.

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?

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.

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:

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?

Two strains of Australian wombat were crossed.Meiotic metaphase configurations of the F₁ hybrids are diagrammed below. In both strains of wombats,n = 4.The black dots are the centromeres,a and b are genetic markers located on chromosome 2,c is located on chromosome 3,and d is located on chromosome 4. a)How do the chromosomes of the two parental strains differ? Use terminology that most accurately describes each chromosomal rearrangement. Chromosome 1: Chromosome 2: Chromosome 3: Chromosome 4: b)In the two parental strains,the map distance between the a and b loci is not the same.Explain why. c)Assume that during meiosis in the F₁ above,a crossover always occurs between the a locus and the centromere of chromosome 2. (i)What proportion of gametes produced by the F₁ hybrid will be viable? (Note: You must consider the behavior of all 4 chromosomes in answering this question.) (ii)What will be the genotypes of the viable gametes at the a and b loci? (iii)What will be the genotypes of the viable gametes at the c and d loci?

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

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?