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Ch. 2 - Mitosis and Meiosis
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 2 - Mitosis and MeiosisProblem 31b
Chapter 2, Problem 31b

A species of cereal rye (Secale cereale) has a chromosome number of 14, while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of 28. Sterile hybrids can be produced by crossing Secale with Elymus.
Given that none of the chromosomes pair at meiosis I in the sterile hybrid (Hang and Franckowlak, 1984), speculate on the anaphase I separation patterns of these chromosomes.

Verified step by step guidance
1
Understand the chromosome numbers: Secale cereale has 14 chromosomes (2n = 14), and Elymus canadensis has 28 chromosomes (2n = 28). The sterile hybrid will have a total of 14 + 28 = 42 chromosomes.
Recognize that the sterile hybrid is sterile because the chromosomes from the two species are not homologous and cannot pair during meiosis I. This means there will be no synapsis or crossing over.
During anaphase I of meiosis, homologous chromosomes typically separate to opposite poles. However, in this hybrid, since no homologous pairs are formed, the chromosomes will segregate randomly and independently.
Speculate on the separation pattern: Each chromosome will move to one of the two poles independently of the others. This random segregation will result in an unequal distribution of chromosomes in the daughter cells, leading to non-viable gametes.
Conclude that the lack of homologous pairing and random segregation of chromosomes during anaphase I is a key reason for the sterility of the hybrid, as it prevents the formation of balanced gametes with the correct chromosome number.

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Key Concepts

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

Chromosome Number and Meiosis

Chromosome number refers to the total number of chromosomes in a cell. In meiosis, homologous chromosomes pair up and segregate into daughter cells. In the case of the sterile hybrid between Secale cereale (14 chromosomes) and Elymus canadensis (28 chromosomes), the mismatch in chromosome numbers leads to difficulties in pairing during meiosis, resulting in no homologous chromosomes aligning at meiosis I.
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Hybrid Sterility

Hybrid sterility occurs when the offspring of two different species are unable to reproduce. In this case, the hybrid between Secale and Elymus is sterile because the differing chromosome numbers prevent proper pairing and segregation during meiosis, leading to gametes that lack the necessary genetic material for successful reproduction.
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Anaphase I and Chromosome Separation

Anaphase I is the stage of meiosis where homologous chromosomes are pulled apart to opposite poles of the cell. In the sterile hybrid, since no chromosomes pair during meiosis I, the separation pattern would involve random movement of unpaired chromosomes, leading to an unequal distribution and potentially resulting in gametes with varying and incomplete sets of chromosomes.
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Related Practice
Textbook Question

Consider a diploid cell that contains three pairs of chromosomes designated AA, BB, and CC. Each pair contains a maternal and a paternal member (e.g., Am and Ap). Using these designations, demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paired as a result of replication and/or synapsis.

Assume that during meiosis I none of the C chromosomes disjoin at metaphase, but they separate into dyads (instead of monads) during meiosis II. How would this change the alignments that you constructed during the anaphase stages in meiosis I and II? Draw them.

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Textbook Question

Consider a diploid cell that contains three pairs of chromosomes designated AA, BB, and CC. Each pair contains a maternal and a paternal member (e.g., Am and Ap). Using these designations, demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paired as a result of replication and/or synapsis.

Assume that each gamete resulting from Problem 29 fuses, in fertilization, with a normal haploid gamete. What combinations will result? What percentage of zygotes will be diploid, containing one paternal and one maternal member of each chromosome pair?

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Textbook Question

A species of cereal rye (Secale cereale) has a chromosome number of 14, while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of 28. Sterile hybrids can be produced by crossing Secale with Elymus.

What would be the expected chromosome number in the somatic cells of the hybrids?

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Textbook Question

An interesting procedure has been applied for assessing the chromosomal balance of potential secondary oocytes for use in human in vitro fertilization. Using fluorescence in situ hybridization (FISH), Kuliev and Verlinsky (2004) were able to identify individual chromosomes in first polar bodies and thereby infer the chromosomal makeup of 'sister' oocytes. Assume that when examining a first polar body you saw that it had one copy (dyad) of each chromosome but two dyads of chromosome 21. What would you expect to be the chromosomal 21 complement in the secondary oocyte? What consequences are likely in the resulting zygote, if the secondary oocyte was fertilized?

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Textbook Question

Assume that you were examining a first polar body and noted that it had one copy (dyad) of each chromosome except chromosome 21. Chromosome 21 was completely absent. What would you expect to be the chromosome 21 complement (only with respect to chromosome 21) in the secondary oocyte? What consequences are likely in the resulting zygote if the secondary oocyte was fertilized?

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Textbook Question

Kuliev and Verlinsky (2004) state that there was a relatively high number of separation errors at meiosis I. In these cases the centromere underwent a premature division, occurring at meiosis I rather than meiosis II. Regarding chromosome 21, what would you expect to be the chromosome 21 complement in the secondary oocyte in which you saw a single chromatid (monad) for chromosome 21 in the first polar body? If this secondary oocyte was involved in fertilization, what would be the expected consequences?

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