Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement

문제 21

8장, 문제 21

The outcome of a single crossover between nonsister chromatids in the inversion loop of an inversion heterozygote varies depending on whether the inversion is of the paracentric or pericentric type. What differences are expected?

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Understand the key terms: A paracentric inversion occurs when the inversion does not include the centromere, while a pericentric inversion includes the centromere. Both types involve a segment of a chromosome being flipped 180 degrees, but their outcomes differ during meiosis due to the involvement of the centromere.

In a paracentric inversion, during meiosis, a single crossover within the inversion loop results in the formation of a dicentric chromatid (with two centromeres) and an acentric fragment (lacking a centromere). The dicentric chromatid often breaks during anaphase I, and the acentric fragment is lost because it cannot attach to the spindle apparatus.

In a pericentric inversion, a single crossover within the inversion loop results in chromatids with duplications and deletions of genetic material. This happens because the centromere is included in the inversion, and the crossover disrupts the normal alignment of homologous regions.

Compare the outcomes: In paracentric inversions, the primary issue is the mechanical instability caused by the dicentric and acentric chromatids, leading to chromosome breakage and loss. In pericentric inversions, the main problem is the production of unbalanced gametes due to duplications and deletions of genetic material.

Summarize the differences: Paracentric inversions lead to structural instability (dicentric and acentric chromatids), while pericentric inversions result in genetic imbalances (duplications and deletions). Both types of inversions reduce fertility due to the production of nonviable gametes.

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Crossover Events

Crossover events occur during meiosis when homologous chromosomes exchange genetic material. This process is crucial for genetic diversity, as it can result in new combinations of alleles. The outcome of a crossover can vary significantly based on the chromosomal structure and the specific regions involved, particularly in the context of inversions.

Inversion Types

Inversions are chromosomal rearrangements where a segment of a chromosome is reversed end to end. There are two main types: paracentric inversions, which do not include the centromere, and pericentric inversions, which do. The type of inversion affects the outcome of crossover events, particularly in how they can lead to viable or nonviable gametes.

Genetic Consequences of Inversions

The genetic consequences of inversions can lead to various outcomes during meiosis, including the production of gametes with duplications or deletions of genetic material. In paracentric inversions, crossover can result in acentric fragments that are lost, while pericentric inversions can produce gametes with altered gene dosage. Understanding these consequences is essential for predicting the viability of offspring.

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교과서 질문

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the F₁ and F₂ results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

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교과서 질문

A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.

Would you advise the woman that she will have to bring each pregnancy to term to determine whether the fetus is normal? If not, what else can you suggest?

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What can you predict about the probability of abnormality/normality of their future children?

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교과서 질문

Having correctly established the F₂ ratio in Problem 18, predict the F₂ ratio of a 'dihybrid' cross involving two independently assorting characteristics (e.g., P₁ = WWWWAAAA x wwwwaaaa).

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How would you explain the high incidence of past stillbirths?

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