Based on Table 17-1, how would you categorize the following genomes? (Letters H through J stand for four different chromosomes.)

Based on Figure 17-4, how many chromatids are in a trivalent?

Based on Figure 17-5, if colchicine is used on a plant in which 2n = 18, how many chromosomes would be in the abnormal product?

Basing your work on Figure 17-7, use colored pens to represent the chromosomes of the fertile amphidiploid.

If Emmer wheat (Figure 17-9) is crossed to another wild wheat CC (not shown), what would be the constitution of a sterile product of this cross? What amphidiploid could arise from the sterile product? Would the amphidiploid be fertile?

In Figure 17-12, what would be the constitution of an individual formed from the union of a monosomic from a first-division nondisjunction in a female and a disomic from a second-division nondisjunction in a male, assuming the gametes were functional?

In Figure 17-14, what would be the expected percentage of each type of segregation?

In Figure 17-19, is there any difference between the inversion products formed from breakage and those formed from crossing over?

Referring to Figure 17-19, draw a diagram showing the process whereby an inversion formed from crossing over could generate a normal sequence.

In Figure 17-21, would the recessive fa allele be expressed when paired with deletion 264-32? 265-11?

Look at Figure 17-22 and state which bands are missing in the cri du chat deletion.

In Figure 17-25, which species is most closely related to the ancestral yeast strain? Why are genes 3 and 13 referred to as duplicate?

Referring to Figure 17-26, draw the product if breaks occurred within genes A and B.

In Figure 17-26, the bottom panel shows that genes B and C are oriented in a different direction (note the promoters). Do you think this difference in orientation would affect their functionality?

In Figure 17-28, what would be the consequence of a crossover between the centromere and locus A?

Based on Figure 17-30, are normal genomes ever formed from the two types of segregation? Are normal genomes ever formed from an adjacent-1 segregation?

Referring to Figure 17-32, draw an inviable product from the same meiosis.

Based on Figure 17-35, write a sentence stating how translocation can lead to cancer. Can you think of another genetic cause of cancer?

Looking at Figure 17-36, why do you think the signal ratio is so much higher in the bottom panel?

Using Figure 17-37, calculate what percentage of conceptions are triploid. The same figure shows XO in the spontaneous-abortion category; however, we know that many XO individuals are viable. In which of the viable categories would XO be grouped?

In keeping with the style of Table 17-1, what would you call organisms that are MM N OO; MM NN OO; MMM NN PP?

A large plant arose in a natural population. Qualitatively, it looked just the same as the others, except much larger. Is it more likely to be an allopolyploid or an autopolyploid? How would you test that it was a polyploid and not just growing in rich soil?

Is a trisomic an aneuploid or a polyploid?

In a tetraploid B/B/b/b, how many quadrivalent possible pairings are there? Draw them (see Figure 17-5).

Someone tells you that cauliflower is an amphidiploid. Do you agree? Explain.

Why is Raphanobrassica fertile, whereas its progenitor wasn’t?

In the designation of wheat genomes, how many chromosomes are represented by the letter B?

How would you “re-create” hexaploid bread wheat from Triticum tauschii and Emmer?

How would you make a monoploid plantlet by starting with a diploid plant?

A disomic product of meiosis is obtained. What is its likely origin? What other genotypes would you expect among the products of that meiosis under your hypothesis?

Can a trisomic A/A/a ever produce a gamete of genotype a?

Which, if any, of the following sex-chromosome aneuploids in humans are fertile: XXX, XXY, XYY, XO?

Why are older expectant mothers routinely given amniocentesis or CVS?

In an inversion, is a 5′ DNA end ever joined to another 5′ end? Explain.

If you observed a dicentric bridge at meiosis, what rearrangement would you predict had taken place?

Why do acentric fragments get lost?

Diagram a translocation arising from repetitive DNA. Repeat for a deletion.

From a large stock of Neurospora rearrangements available from the fungal genetics stock center, what type would you choose to synthesize a strain that had a duplication of the right arm of chromosome 3 and a deletion for the tip of chromosome 4?

You observe a very large pairing loop at meiosis. Is it more likely to be from a heterozygous inversion or heterozygous deletion? Explain.

A new recessive mutant allele doesn’t show pseudodominance with any of the deletions that span Drosophila chromosome 2. What might be the explanation?

Compare and contrast the origins of Turner syndrome, Williams syndrome, cri du chat syndrome, and Down syndrome. (Why are they called syndromes?)

List the diagnostic features (genetic or cytological) that are used to identify these chromosomal alterations:

The normal sequence of nine genes on a certain Drosophila chromosome is 123 · 456789, where the dot represents the centromere. Some fruit flies were found to have aberrant chromosomes with the following structures:

Name each type of chromosomal rearrangement, and draw diagrams to show how each would synapse with the normal chromosome.

The two loci P and Bz are normally 36 m.u. apart on the same arm of a certain plant chromosome. A paracentric inversion spans about one-fourth of this region but does not include either of the loci. What approximate recombinant frequency between P and Bz would you predict in plants that are

As stated in Solved Problem 2, recessive mutation in certain mice called waltzers causes them to execute bizarre steps. W. H. Gates crossed waltzers with pure-breeding normal mice and found, among several hundred normal progeny, a single waltzing female mouse. This mouse was mated with a waltzing male, and her offspring were waltzers. When mated with a homozygous normal male, all her progeny were normal. Some of these normal males and females were intercrossed, and, unexpectedly, none of their progeny were waltzers. T. S. Painter examined the chromosomes of some of Gates’s waltzing mice that showed a breeding behavior similar to that of the original, unusual waltzing female. He found that these mice had the normal number of 40 chromosomes. In the unusual waltzers, however, one member of a chromosome pair was abnormally short. Interpret these observations as completely as possible, both genetically and cytologically.

A salivary-gland chromosome of Drosophila has six bands as shown in the following illustration. Below the chromosome are shown the extent of five deletions (Del 1 to Del 5):

Recessive alleles a, b, c, d, e, and f are known to be in the region, but their order is unknown. When the deletions are combined with each allele, the following results are obtained:

In this table, a minus sign means that the deletion uncovers the recessive allele (the recessive phenotype is observed), and a plus sign means that the corresponding wild-type allele is still present. Match each salivary band with a gene.

A fruit fly was found to be heterozygous for a paracentric inversion. However, obtaining flies that were homozygous for the inversion was impossible even after many attempts. What is the most likely explanation for this inability to produce a homozygous inversion?

Orangutans are an endangered species in their natural environment (the islands of Borneo and Sumatra), and so a captive-breeding program has been established using orangutans currently held in zoos throughout the world. One component of this program is research into orangutan cytogenetics. This research has shown that all orangutans from Borneo carry one form of chromosome 2, as shown in the accompanying diagram, and all orangutans from Sumatra carry the other form. Before this cytogenetic difference became known, some matings were carried out between animals from different islands, and 14 hybrid progeny are now being raised in captivity.

In corn, the genes for tassel length (alleles T and t) and rust resistance (alleles R and r) are known to be on separate chromosomes. In the course of making routine crosses, a breeder noticed that one T/t;R/r plant gave unusual results in a testcross with the double-recessive pollen parent t/t;r/r. The results were

A yellow body in Drosophila is caused by a mutant allele y of a gene located at the tip of the X chromosome (the wild-type allele causes a gray body). In a radiation experiment, a wild-type male was irradiated with X rays and then crossed with a yellow-bodied female. Most of the male progeny were yellow, as expected, but the scanning of thousands of flies revealed two gray-bodied (phenotypically wild-type) males. These gray-bodied males were crossed with yellow-bodied females, with the following results:

In corn, the allele Pr stands for green stems, pr for purple stems. A corn plant of genotype pr/pr that has standard chromosomes is crossed with a Pr/Pr plant that is homozygous for a reciprocal translocation between chromosomes 2 and 5. The F₁ is semisterile and phenotypically Pr. A backcross with the parent with standard chromosomes gives 764 semisterile Pr, 145 semisterile pr, 186 normal Pr, and 727 normal pr. What is the map distance between the Pr locus and the translocation point?

Distinguish among Klinefelter, Down, and Turner syndromes. Which syndromes are found in both sexes?

Show how you could make an allotetraploid between two related diploid plant species, both of which are 2n = 28.

In Drosophila, trisomics and monosomics for the tiny chromosome 4 are viable, but nullisomics and tetraso-mics are not. The b locus is on this chromosome. Deduce the phenotypic proportions in the progeny of the following crosses of trisomics.

A woman with Turner syndrome is found to be colorblind (an X-linked recessive phenotype). Both her mother and her father have normal vision.

Suppose you have a line of mice that has cytologically distinct forms of chromosome 4. The tip of the chromosome can have a knob (called 4^K) or a satellite (4^S) or neither (4). Here are sketches of the three types:

You cross a 4^K/4^S female with a 4/4 male and find that most of the progeny are 4^K/4 or 4^S/4, as expected.

However, you occasionally find some rare types as follows (all other chromosomes are normal):

Explain the rare types that you have found. Give, as precisely as possible, the stages at which they originate, and state whether they originate in the male parent, the female parent, or the zygote. (Give brief reasons.)

A cross is made in tomatoes between a female plant that is trisomic for chromosome 6 and a normal diploid male plant that is homozygous for the recessive allele for potato leaf (p/p). A trisomic F₁ plant is backcrossed to the potato-leaved male.

A tomato geneticist attempts to assign five recessive mutations to specific chromosomes by using trisomics. She crosses each homozygous mutant (2n) with each of three trisomics, in which chromosomes 1, 7, and 10 take part. From these crosses, the geneticist selects trisomic progeny (which are less vigorous) and backcrosses them to the appropriate homozygous recessive. The diploid progeny from these crosses are examined. Her results, in which the ratios are wild type: mutant, are as follows:

Which of the mutations can the geneticist assign to which chromosomes? (Explain your answer fully.)

A petunia is heterozygous for the following autosomal homologs:

Two groups of geneticists, in California and in Chile, begin work to develop a linkage map of the medfly. They both independently find that the loci for body color (B = black, b = gray) and eye shape (R = round, r = star) are linked 28 m.u. apart. They send strains to each other and perform crosses; a summary of all their findings is shown here:

An aberrant corn plant gives the following RF values when testcrossed:

(The locus order is centromere-d–f–b–x–y–p.) The aberrant plant is a healthy plant, but it produces far fewer normal ovules and pollen than does the control plant.

The following corn loci are on one arm of chromosome 9 in the order indicated (the distances between them are shown in map units):

C  gives colored aleurone; c, white aleurone.

Bz  gives green leaves; bz, bronze leaves.

Wx  gives starchy seeds; wx, waxy seeds.

Sh  gives smooth seeds; sh, shrunken seeds.

D  gives tall plants; d, dwarf.

A plant from a standard stock that is homozygous for all five recessive alleles is crossed with a wild-type plant from Mexico that is homozygous for all five dominant alleles. The F₁ plants express all the dominant alleles and, when backcrossed to the recessive parent, give the following progeny phenotypes:

Propose a hypothesis to explain these results. Include

Chromosomally normal corn plants have a p locus on chromosome 1 and an s locus on chromosome 5.

P gives dark green leaves; p, pale green leaves.

S gives large ears; s, shrunken ears.

An original plant of genotype P/p ; S/s has the expected phenotype (dark green, large ears) but gives unexpected results in crosses as follows:

Explain these results, showing the full genotypes of the original plant, the tester, and the F₁ plants. How would you test your hypothesis?

A male rat that is phenotypically normal shows reproductive anomalies when compared with normal male rats, as shown in the table in Problem 64. Propose a genetic explanation of these unusual results, and indicate how your idea could be tested.

A tomato geneticist working on Fr, a dominant mutant allele that causes rapid fruit ripening, decides to find out which chromosome contains this gene by using a set of lines of which each is trisomic for one chromosome. To do so, she crosses a homozygous diploid mutant with each of the wild-type trisomic lines.

(Problem 66 is from Tamara Western.)

The Neurospora un-3 locus is near the centromere on chromosome 1, and crossovers between un-3 and the centromere are very rare. The ad-3 locus is on the other side of the centromere of the same chromosome, and crossovers occur between ad-3 and the centromere in about 20 percent of meioses (no multiple crossovers occur).

Explain these results, and state how you could test your idea. (Note: In Neurospora, ascospores with extra chromosomal material survive and are the normal black color, whereas ascospores lacking any chromosome region are white and inviable.)

Two mutations in Neurospora, ad-3 and pan-2, are located on chromosomes 1 and 6, respectively. An unusual ad-3 line arises in the laboratory, giving the results shown in the table below. Explain all three results with the aid of clearly labeled diagrams. (Note: In Neurospora, ascospores with extra chromosomal material survive and are the normal black color, whereas ascospores lacking any chromosome region are white and inviable.)

662

Deduce the phenotypic proportions in the progeny of the following crosses of autotetraploids in which the a⁺/a locus is very close to the centromere. (Assume that the four homologous chromosomes of any one type pair randomly two by two and that only one copy of the a⁺ allele is necessary for the wild-type phenotype.)

The New World cotton species Gossypium hirsutum has a 2n chromosome number of 52. The Old World species G. thurberi and G. herbaceum each have a 2n number of 26. When these species are crossed, the resulting hybrids show the following chromosome pairing arrangements at meiosis:

Interpret these observations phylogenetically, using diagrams. Clearly indicate the relationships between the species. How would you prove that your interpretation is correct?

There are six main species in the Brassica genus: B. carinata, B. campestris, B. nigra, B. oleracea, B. juncea, and B. napus. You can deduce the interrelationships among these six species from the following table:

Several kinds of sexual mosaicism are well documented in humans. Suggest how each of the following examples may have arisen by nondisjunction at mitosis:

In Drosophila, a cross (cross 1) was made between two mutant flies, one homozygous for the recessive mutation bent wing (b) and the other homozygous for the recessive mutation eyeless (e). The mutations e and b are alleles of two different genes that are known to be very closely linked on the tiny autosomal chromosome 4. All the progeny had a wild-type phenotype. One of the female progeny was crossed with a male of genotype b e/b e; we will call this cross 2. Most of the progeny of cross 2 were of the expected types, but there was also one rare female of wild-type phenotype.

Which of the explanations in part b is compatible with this result? Explain the genotypes and phenotypes of the progeny of cross 3 and their proportions.

In the fungus Ascobolus (similar to Neurospora), ascospores are normally black. The mutation f, producing fawn-colored ascospores, is in a gene just to the right of the centromere on chromosome 6, whereas mutation b, producing beige ascospores, is in a gene just to the left of the same centromere. In a cross of fawn and beige parents (+f × b+), most octads showed four fawn and four beige ascospores, but three rare exceptional octads were found, as shown in the accompanying illustration. In the sketch, black is the wild-type phenotype, a vertical line is fawn, a horizontal line is beige, and an empty circle represents an aborted (dead) ascospore.

The life cycle of the haploid fungus Ascobolus is similar to that of Neurospora. A mutational treatment produced two mutant strains, 1 and 2, both of which when crossed with wild type gave unordered tetrads, all of the following type (fawn is a light brown color; normally, crosses produce all black ascospores):