Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 10 - Eukaryotic Chromosome Abnormalities and Molecular Organization

Problem 22a

Chapter 10, Problem 22a

Two experimental varieties of strawberry are produced by crossing a hexaploid line that contains 48 chromosomes and a tetraploid line that contains 32 chromosomes. Experimental variety 1 contains 40 chromosomes, and experimental variety 2 contains 56 chromosomes.
Do you expect both experimental lines to be fertile? Why or why not?

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Determine the ploidy levels of the parent lines. The hexaploid line has 48 chromosomes, so the base chromosome number (x) is 48 ÷ 6 = 8. Similarly, the tetraploid line has 32 chromosomes, so its base chromosome number is 32 ÷ 4 = 8. Both lines share the same base chromosome number (x = 8), which is important for compatibility during hybridization.

Analyze the chromosome number of experimental variety 1 (40 chromosomes). Since 40 is not a multiple of the base chromosome number (8), this suggests that the chromosomes may not pair evenly during meiosis, leading to potential fertility issues.

Analyze the chromosome number of experimental variety 2 (56 chromosomes). Since 56 is a multiple of the base chromosome number (8), this suggests that the chromosomes can pair evenly during meiosis, which is more likely to result in fertility.

Explain the relationship between chromosome pairing and fertility. Fertility in polyploid organisms depends on the ability of chromosomes to pair evenly during meiosis. Uneven chromosome numbers (e.g., 40 in variety 1) can lead to unbalanced gametes and reduced fertility, while even chromosome numbers (e.g., 56 in variety 2) are more likely to produce balanced gametes and maintain fertility.

Conclude by stating that experimental variety 1 is unlikely to be fertile due to its uneven chromosome number, while experimental variety 2 is more likely to be fertile because its chromosome number allows for proper pairing during meiosis.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ploidy Levels

Ploidy refers to the number of sets of chromosomes in a cell. In this case, the hexaploid line has six sets (48 chromosomes) and the tetraploid line has four sets (32 chromosomes). Understanding ploidy is crucial because it affects the fertility and viability of the offspring produced from these crosses, as mismatched ploidy levels can lead to complications in meiosis.

Chromosome Number and Fertility

The number of chromosomes in an organism can significantly influence its fertility. In this scenario, experimental variety 1 has 40 chromosomes, while variety 2 has 56. Fertility is often compromised when the chromosome numbers of the parents differ significantly, as it can lead to unbalanced gametes and difficulties during meiosis, potentially resulting in sterile offspring.

Meiosis and Gamete Formation

Meiosis is the process by which gametes (sperm and eggs) are produced, involving two rounds of cell division that reduce the chromosome number by half. For successful reproduction, gametes must have compatible chromosome numbers. In this case, the differing chromosome counts in the experimental varieties may hinder proper pairing and segregation during meiosis, affecting the fertility of the resulting hybrids.

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Textbook Question

In humans that are XX/XO mosaics, the phenotype is highly variable, ranging from females who have classic Turner syndrome symptoms to females who are essentially normal. Likewise, XY/XO mosaics have phenotypes that range from Turner syndrome females to essentially normal males. How can the wide range of phenotypes be explained for these sex-chromosome mosaics?

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Textbook Question

A plant breeder would like to develop a seedless variety of cucumber from two existing lines. Line A is a tetraploid line, and line B is a diploid line. Describe the breeding strategy that will produce a seedless line, and support your strategy by describing the results of crosses.

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Textbook Question

In Drosophila, seven partial deletions (1 to 7) shown as gaps in the following diagram have been mapped on a chromosome. This region of the chromosome contains genes that express seven recessive mutant phenotypes, identified in the following table as a through g. A researcher wants to determine the location and order of genes on the chromosome, so he sets up a series of crosses in which flies homozygous for a mutant allele are crossed with flies homozygous for a partial deletion. The progeny are scored to determine whether they have the mutant phenotype ('m' in the table) or the wild-type phenotype ('+' in the table). Use the partial deletion map and the table of progeny phenotypes to determine the order of genes on the chromosome.

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Textbook Question

Two experimental varieties of strawberry are produced by crossing a hexaploid line that contains 48 chromosomes and a tetraploid line that contains 32 chromosomes. Experimental variety 1 contains 40 chromosomes, and experimental variety 2 contains 56 chromosomes.

How many chromosomes from the hexaploid line are contributed to experimental variety 1? To experimental variety 2?

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Textbook Question

How many chromosomes from the tetraploid lines are contributed to experimental variety 1? To experimental variety 2?

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Textbook Question

In the tomato, Solanum esculentum, tall (D−)(D−) is dominant to dwarf (dd) plant height, smooth fruit (P−) is dominant to peach fruit (pp), and round fruit shape (O−) is dominant to oblate fruit shape (oo). These three genes are linked on chromosome 1 of tomato in the order dwarf–peach–oblate. There are 12 map units between dwarf and peach and 17 map units between peach and oblate. A trihybrid plant (DPO/dpo) is test-crossed to a plant that is homozygous recessive at the three loci (dpo/dpo). The accompanying table shows the progeny plants. Identify the mechanism responsible for the resulting data that do not agree with the established genetic map.

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