Two purebred pea plants are crossed. One strain has dominant round seeds; the other has recessive wrinkled seeds. (a) What phenotypes would be seen in the F₁ generation plants, and in what proportions? (b) What phenotypes would be seen in the F₂ generation plants, and in what proportions? (c) If an F₁ generation plant is crossed with a plant producing wrinkled seeds, what phenotypes are seen in the progeny, and in what proportions?

Two pea plants with round seeds are crossed. In the F₁ generation, all the plants have round seeds. What can you say about the genotype of the parental plants?

The F₁ plants from the cross in Problem 2 are next crossed at random. There are 129 plants in the F₂ generation. The majority, 121 plants, produce round seeds. However, there are 8 plants that produce wrinkled seeds. From this information, what were the genotypes of the original parental plants?

Purebred white-eyed male fruit flies are crossed with wild-type red-eyed females. If the progeny are crossed with each other repeatedly, which generation will be the first to contain white-eyed female flies?

Purebred wild-type male flies are crossed with purebred white-eyed female flies. If the progeny are crossed with each other repeatedly, which generation will be the first to contain white-eyed male flies?

A new species of fruit fly is found on an uncharted island. The flies are brightly colored, with blue and green bodies. After studying these insects for a year or two, researchers find one male with an all-black body. When this male is crossed with wild-type females, all of the male progeny in the F₁ generation are black, and all of the female progeny have the blue and green coloring. This same pattern (all black males and colored females) is repeated in the F₂, F₃, and F₄ generations. Explain these observations.

Two purebred flowering plants are crossed. One has red flowers and small leaves (RRll) and the other has white flowers and large leaves (rrLL). Using a Punnett square analysis, and assuming that the genes are unlinked, predict the type and frequency of phenotypes in the F₂ generation.

If the F₁ plants in Problem 7 had genes for red flower color that exhibited incomplete dominance, the heterozygous Rr flower color would be pink. In that instance, what percentage of the F₁ plants in Problem 7 would have pink flowers? What percentage of the F₂ generation would have pink flowers?

Two purebred fruit flies are crossed. The male has white eyes and vestigial wings. The female has red eyes and normal wings. All F₁ flies have red eyes and vestigial wings. Using a Punnett square analysis, predict the percentage of F₂ generation males that will have red eyes and normal wings. Assume that the wing trait is not sex-linked.

A new and exotic species of fly is found, with green eyes (G) and striped wings (S). A mutant fly of the same species is found that has orange eyes (g) and clear (unstriped) wings (s). The mutant is cultured for many generations to obtain a purebred strain with the double-mutant phenotype. A ggss female fly is mated with a wild-type GGSS male. The F₁ progeny all have green eyes and striped wings, as expected. An F₁ male is mated with an F₁ female. Among the F₂ progeny of this cross, only two kinds of flies are observed: 75% with green eyes and striped wings, and 25% with orange eyes and clear wings. Some expected F₂ progeny (such as flies with green eyes and clear wings) are absent. Explain this result.

Both meiosis and mitosis are initiated with a complete replication of the cell’s chromosomes in S phase. The replication of each chromosome produces a pair of sister chromatids. During the cell division immediately following replication, how are the chromosomes in the sister chromatid pairs distributed to daughter cells in mitosis and meiosis?

Two purebred plants, with genotypes AABBCCDDEEFF and aabbccddeeff, are crossed. In the F₁ generation, all individuals are heterozygous for all traits. Geneticists probe the linkage of these various genes by doing a series of crosses, examining two traits in each cross. When all the crosses are done and the data are tabulated, the researchers find that in the F₁ plants, meiosis produces gametes that contain the following combinations of alleles at the indicated frequencies (which correspond to crossover frequencies):

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With these data, determine how the genes are distributed along the chromosomes. Draw a map, using the crossover frequencies as distances.

On one chromosome there are three linked genes designated M, N, and O. If crossing over occurs between M and O 5% of the time, and between N and O 8% of the time, what are the possible arrangements of the genes on the chromosome?

In the central dogma developed by Francis Crick and others, three kinds of RNA play important roles: rRNAs, tRNAs, and mRNAs. Describe two features that are characteristic of each type of RNA.

In the classic Hershey-Chase experiment (see the How We Know section for this chapter), the T2 phage was labeled with either ³⁵S or ³²P before using it to infect a bacterial host. In this experiment, would it have been possible for these researchers to label one batch of T2 phage with both ³⁵S and ³²P and still get a definitive result? Why or why not? What would the results of the experiment be?

In the bacterium Escherichia coli, the amino acid tryptophan is synthesized in a multistep pathway, beginning with an organic precursor called chorismate:

The numbers denote steps in the pathway, each catalyzed by an enzyme. There are five bacterial genes that encode polypeptides associated with these enzymes, called trpA, trpB, trpC, and so on. A researcher isolates a series of mutations that eliminate the capacity of the cells to synthesize tryptophan, each mutation affecting one of the five genes. Each mutant cell is tested for its ability to grow on a medium containing either tryptophan, chorismate, or intermediate X or Y. The following results are obtained (+ means growth; − means absence of growth; WT means wild type):

Which genes encode the enzymes involved in steps 1, 2, and 3 of the tryptophan biosynthetic pathway? Given this information, what can you say about the structure of these enzymes?

In mitosis and meiosis, all cellular chromosomes are replicated before cell division. In a diploid cell, there are two copies of each autosomal chromosome. For a typical diploid eukaryotic cell and a hypothetical chromosome A, the two copies of the chromosome can be labeled A1 and A2. During replication, each chromosome (including A1 and A2) is converted into two new chromosome copies that are transiently held together. The immediate tethered products of a replicated chromosome are called sister chromatids. The two sets of sister chromatids resulting from replication of chromosomes A1 and A2 can be labeled A1/A1* and A2/A2*. Thus, in all, four chromosomes are held in two pairs of sister chromatids. For both mitosis and meiosis, replication is eventually followed by a cell division (the first of two cell divisions in the case of meiosis). In that cell division, how are the four chromosomes segregated into the daughter cells in mitosis? How are the four chromosomes segregated into daughter cells in the first cell division of meiosis?

Food for thought: As described in the chapter, substantial segments of the genome of many organisms are transcribed into RNA that does not encode protein and does not correspond to either rRNA or tRNA. Without studying several additional chapters in this book, you may not be familiar with this RNA. However, considering some potential functions for this RNA is a useful prelude to your continued study of molecular biology. Based on the discussion of the RNA world hypothesis in Chapter 1 and other knowledge you may have about RNA, suggest one or two potential functions for this non-protein-coding RNA.