Using Table 3-1, answer the following questions about probability values:

Inspect Figure 3-8: which meiotic stage is responsible for generating Mendel’s second law?

In Figure 3-9,

Inspect Figure 3-10: what would be the outcome in the octad if on rare occasions a nucleus from the postmeiotic mitotic division of nucleus 2 slipped past a nucleus from the postmeiotic mitotic division of nucleus 3? How could you measure the frequency of such a rare event?

In Figure 3-11, if the input genotypes were a · B and A · b, what would be the genotypes colored blue?

In the progeny seen in Figure 3-13, what are the origins of the chromosomes colored dark blue, light blue, and very light blue?

In Figure 3-17, in which bar of the histogram would the genotype R₁/r₁ · R₂/R₂ · r₃/r₃ be found?

In examining Figure 3-19, what do you think is the main reason for the difference in size of yeast and human mtDNA?

In Figure 3-20, what color is used to denote cytoplasm containing wild-type mitochondria?

In Figure 3-21, what would be the leaf types of progeny of the apical (top) flower?

From the pedigree in Figure 3-25, what principle can you deduce about the inheritance of mitochondrial disease from affected fathers?

Assume independent assortment and start with a plant that is dihybrid A/a; B/b:

Normal mitosis takes place in a diploid cell of genotype A/a; B/b. Which of the following genotypes might represent possible daughter cells?

In a diploid organism of 2n = 10, assume that you can label all the centromeres derived from its female parent and all the centromeres derived from its male parent. When this organism produces gametes, how many male-and female-labeled centromere combinations are possible in the gametes?

It has been shown that when a thin beam of light is aimed at a nucleus, the amount of light absorbed is proportional to the cell’s DNA content. Using this method, the DNA in the nuclei of several different types of cells in a corn plant were compared. The following numbers represent the relative amounts of DNA in these different types of cells:

0.7, 1.4, 2.1, 2.8, and 4.2

Which cells could have been used for these measurements? (Note: In plants, the endosperm part of the seed is often triploid, 3n.)

Draw a haploid mitosis of the genotype a⁺; b.

In moss, the genes A and B are expressed only in the ga-metophyte. A sporophyte of genotype A/a; B/b is allowed to produce gametophytes.

When a cell of genotype A/a; B/b; C/c having all the genes on separate chromosome pairs divides mitotically, what are the genotypes of the daughter cells?

In the haploid yeast Saccharomyces cerevisiae, the two mating types are known as MATa and MATα. You cross a purple (ad^–) strain of mating type a and a white (ad⁺) strain of mating type α. If ad^– and ad⁺ are alleles of one gene, and a and α are alleles of an independently inherited gene on a separate chromosome pair, what progeny do you expect to obtain? In what proportions?

In mice, dwarfism is caused by an X-linked recessive allele, and pink coat is caused by an autosomal dominant allele (coats are normally brownish). If a dwarf female from a pure line is crossed with a pink male from a pure line, what will be the phenotypic ratios in the F₁ and F₂ in each sex? (Invent and define your own gene symbols.)

Suppose you discover two interesting rare cytological abnormalities in the karyotype of a human male. (A karyotype is the total visible chromosome complement.) There is an extra piece (satellite) on one of the chromosomes of pair 4, and there is an abnormal pattern of staining on one of the chromosomes of pair 7. With the assumption that all the gametes of this male are equally viable, what proportion of his children will have the same karyotype that he has?

Suppose that meiosis occurs in the transient diploid stage of the cycle of a haploid organism of chromosome number n. What is the probability that an individual haploid cell resulting from the meiotic division will have a complete parental set of centromeres (that is, a set all from one parent or all from the other parent)?

Pretend that the year is 1868. You are a skilled young lens maker working in Vienna. With your superior new lenses, you have just built a microscope that has better resolution than any others available. In your testing of this microscope, you have been observing the cells in the testes of grasshoppers and have been fascinated by the behavior of strange elongated structures that you have seen within the dividing cells. One day, in the library, you read a recent journal paper by G. Mendel on hypothetical “factors” that he claims explain the results of certain crosses in peas. In a flash of revelation, you are struck by the parallels between your grasshopper studies and Mendel’s pea studies, and you resolve to write him a letter. What do you write? (Problem 23 is based on an idea by Ernest Kroeker.)

From a presumed testcross A/a × a/a, in which A represents red and a represents white, use the χ² test to find out which of the following possible results would fit the expectations:

Look at the Punnett square in Figure 3-4.

Assuming independent assortment of all genes, develop formulas that show the number of phenotypic classes and the number of genotypic classes from selfing a plant heterozygous for n gene pairs.

Note: The first part of this problem was introduced in Chapter 2. The line of logic is extended here.

In the plant Arabidopsis thaliana, a geneticist is interested in the development of trichomes (small projections) on the leaves. A large screen turns up two mutant plants (A and B) that have no trichomes, and these mutants seem to be potentially useful in studying trichome development. (If they are determined by single-gene mutations, then finding the normal and abnormal function of these genes will be instructive.) Each plant was crossed with wild type; in both cases, the next generation (F₁) had normal trichomes. When F₁ plants were selfed, the resulting F₂’s were as follows:

F₂ from mutant A: 602 normal; 198 no trichomes

F₂ from mutant B: 267 normal; 93 no trichomes

In dogs, dark coat color is dominant over albino, and short hair is dominant over long hair. Assume that these effects are caused by two independently assorting genes. Seven crosses were done as shown below, in which D and A stand for the dark and albino phenotypes, respectively, and S and L stand for the short-hair and long-hair phenotypes.

Write the genotypes of the parents in each cross. Use the symbols C and c for the dark and albino coat-color alleles and the symbols H and h for the short-hair and long-hair alleles, respectively. Assume parents are homozygous unless there is evidence otherwise.

In tomatoes, one gene determines whether the plant has purple (P) or green (G) stems, and a separate, independent gene determines whether the leaves are “cut” (C) or “potato” (Po). Five matings of tomato-plant phenotypes give the following results:

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A mutant allele in mice causes a bent tail. Six pairs of mice were crossed. Their phenotypes and those of their progeny are given in the following table. N is normal phenotype; B is bent phenotype. Deduce the mode of inheritance of bent tail.

The normal eye color of Drosophila is red, but strains in which all flies have brown eyes are available. Similarly, wings are normally long, but there are strains with short wings. A female from a pure line with brown eyes and short wings is crossed with a male from a normal pure line. The F₁ consists of normal females and short-winged males. An F₂ is then produced by intercrossing the F₁. Both sexes of F₂ flies show phenotypes as follows:

Deduce the inheritance of these phenotypes; use clearly defined genetic symbols of your own invention. State the genotypes of all three generations and the genotypic proportions of the F₁ and F₂.

In a natural population of annual plants, a single plant is found that is sickly looking and has yellowish leaves. The plant is dug up and brought back to the laboratory. Photosynthesis rates are found to be very low. Pollen from a normal dark-green-leaved plant is used to fertilize emasculated flowers of the yellowish plant. A hundred seeds result, of which only 60 germinate. All the resulting plants are sickly yellow in appearance.

What is the basis for the green-and-white color variegation in the leaves of Mirabilis? If the following cross is made,

what progeny types can be predicted? What about the reciprocal cross?

In Neurospora, the mutant stp exhibits erratic stop-and-start growth. The mutant site is known to be in the mtDNA. If an stp strain is used as the female parent in a cross with a normal strain acting as the male, what type of progeny can be expected? What about the progeny from the reciprocal cross?

Two corn plants are studied. One is resistant (R) and the other is susceptible (S) to a certain pathogenic fungus. The following crosses are made, with the results shown:

What can you conclude about the location of the genetic determinants of R and S?

A presumed dihybrid in Drosophila, B/b; F/f, is test-crossed with b/b; f/f. (B = black body ; b = brown body; F = forked bristles; f = unforked bristles.) The results are

Use the χ² test to determine if these results fit the results expected from testcrossing the hypothesized dihybrid.

Are the following progeny numbers consistent with the results expected from selfing a plant presumed to be a dihybrid of two independently assorting genes, H/h; R/r? (H = hairy leaves; h = smooth leaves; R = round ovary; r = elongated ovary.) Explain your answer.

A dark female moth is crossed with a dark male. All the male progeny are dark, but half the female progeny are light and the rest are dark. Propose an explanation for this pattern of inheritance.

In Neurospora, a mutant strain called stopper (stp) arose spontaneously. Stopper showed erratic “stop and start” growth, compared with the uninterrupted growth of wild-type strains. In crosses, the following results were found:

In polygenic systems, how many phenotypic classes corresponding to number of polygene “doses” are expected in selfs

In the self of a polygenic trihybrid R₁/r₁; R₂/r₂; R₃/r₃, use the product and sum rules to calculate the proportion of progeny with just one polygene “dose.”

Reciprocal crosses and selfs were performed between the two moss species Funaria mediterranea and F. hygrometrica. The sporophytes and the leaves of the gametophytes are shown in the accompanying diagram.

The crosses are written with the female parent first.

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Assume that diploid plant A has a cytoplasm genetically different from that of plant B. To study nuclear-cytoplasmic relations, you wish to obtain a plant with the cytoplasm of plant A and the nuclear genome predominantly of plant B. How would you go about producing such a plant?

You are studying a plant with tissue comprising both green and white sectors. You wish to decide whether this phenomenon is due (1) to a chloroplast mutation of the type considered in this chapter or (2) to a dominant nuclear mutation that inhibits chlorophyll production and is present only in certain tissue layers of the plant as a mosaic. Outline the experimental approach that you would use to resolve this problem.

You have three jars containing marbles, as follows:

In tomatoes, red fruit is dominant over yellow, two-loculed fruit is dominant over many-loculed fruit, and tall vine is dominant over dwarf. A breeder has two pure lines: (1) red, two-loculed, dwarf and (2) yellow, many-loculed, tall. From these two lines, he wants to produce a new pure line for trade that is yellow, two-loculed, and tall. How exactly should he go about doing so? Show not only which crosses to make, but also how many progeny should be sampled in each case.

We have dealt mainly with only two genes, but the same principles hold for more than two genes. Consider the following cross:

A/a; B/b; C/c; D/d; E/e χ a/a; B/b; c/c; D/d; e/e

Assume independent assortment.

The accompanying pedigree shows the pattern of transmission of two rare human phenotypes: cataract and pituitary dwarfism. Family members with cataract are shown with a solid left half of the symbol; those with pituitary dwarfism are indicated by a solid right half.

A corn geneticist has three pure lines of genotypes a/a; B/B; C/C, A/A; b/b; C/C, and A/A; B/B; c/c. All the phenotypes determined by a, b, and c will increase the market value of the corn; so, naturally, he wants to combine them all in one pure line of genotype a/a; b/b; c/c.

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Assume independent assortment of the three gene pairs. (Note: Corn will self or cross-pollinate easily.)

In humans, color vision depends on genes encoding three pigments. The R (red pigment) and G (green pigment) genes are close together on the X chromosome, whereas the B (blue pigment) gene is autosomal. A recessive mutation in any one of these genes can cause color blindness. Suppose that a color-blind man married a woman with normal color vision. The four sons from this marriage were color-blind, and the five daughters were normal. Specify the most likely genotypes of both parents and their children, explaining your reasoning. (A pedigree drawing will probably be helpful.) (Problem 50 is by Rosemary Redfield.)

Consider the accompanying pedigree for a rare human muscle disease.

The plant Haplopappus gracilis has a 2n of 4. A diploid cell culture was established and, at premitotic S phase, a radioactive nucleotide was added and was incorporated into newly synthesized DNA. The cells were then removed from the radioactivity, washed, and allowed to proceed through mitosis. Radioactive chromosomes or chro-matids can be detected by placing photographic emulsion on the cells; radioactive chromosomes or chromatids appeared covered with spots of silver from the emulsion. (The chromosomes “take their own photograph.”) Draw the chromosomes at prophase and telophase of the first and second mitotic divisions after the radioactive treatment. If they are radioactive, show it in your diagram. If there are several possibilities, show them, too.

In the species of Problem 52, you can introduce radioactivity by injection into the anthers at the S phase before meiosis. Draw the four products of meiosis with their chromosomes, and show which are radioactive.

The plant Haplopappus gracilis is diploid and 2n = 4. There are one long pair and one short pair of chromosomes. The diagrams below (numbered 1 through 12) represent anaphases (“pulling apart” stages) of individual cells in meiosis or mitosis in a plant that is genetically a dihybrid (A/a; B/b ) for genes on different chromosomes. The lines represent chromosomes or chromatids, and the points of the V’s represent centromeres. In each case, indicate if the diagram represents a cell in meiosis I, meiosis II, or mitosis. If a diagram shows an impossible situation, say so.

The pedigree below shows the recurrence of a rare neurological disease (large black symbols) and spontaneous fetal abortion (small black symbols) in one family. (A slash means that the individual is deceased.) Provide an explanation for this pedigree in regard to the cytoplasmic segregation of defective mitochondria.

A man is brachydactylous (very short fingers; rare autosomal dominant), and his wife is not. Both can taste the chemical phenylthiocarbamide (autosomal dominant; common allele), but their mothers could not.

One form of male sterility in corn is maternally transmitted. Plants of a male-sterile line crossed with normal pollen give male-sterile plants. In addition, some lines of corn are known to carry a dominant nuclear restorer allele (Rf) that restores pollen fertility in male-sterile lines.