Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

6. Chromosomal Variation

Chromosomal Rearrangements: Inversions

Genetics

6. Chromosomal Variation

Chromosomal Rearrangements: Inversions

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4:10 minutes

Problem 15c

Textbook Question

A normal chromosome and its homolog carrying a paracentric inversion are shown here. The dot (·) represents the centromere.
Normal ABC • DEFGHIJK
Inversion abc • djihgfe
Assume a crossover takes place in the region between A and B. Identify the gametes that are formed by this crossover event, and indicate which, if any, gametes are viable.

Verified step by step guidance

Understand the problem: A paracentric inversion is a chromosomal rearrangement where a segment of a chromosome is inverted but does not include the centromere. Here, the normal chromosome and its homolog with the inversion are given. A crossover occurs between genes A and B, and we need to determine the resulting gametes and their viability.

Analyze the crossover event: During meiosis, homologous chromosomes align and may undergo crossing over. In this case, the crossover occurs between the normal chromosome (ABC • DEFGHIJK) and the inverted chromosome (abc • djihgfe). This will result in recombinant chromatids with altered gene arrangements.

Determine the structure of recombinant chromatids: After the crossover, the recombinant chromatids will contain a mix of normal and inverted segments. This can lead to the formation of dicentric (two centromeres) or acentric (no centromere) chromatids, which are structurally abnormal.

Assess the viability of gametes: Gametes with dicentric chromatids are typically inviable because the dicentric bridge breaks during anaphase I of meiosis. Similarly, gametes with acentric chromatids are inviable because they lack a centromere and cannot be properly segregated. Only gametes with normal or inversion chromosomes (non-recombinant) are viable.

Summarize the outcome: The viable gametes will be those that retain the original normal chromosome (ABC • DEFGHIJK) or the inversion chromosome (abc • djihgfe). Recombinant gametes resulting from the crossover event are inviable due to structural abnormalities.

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Key Concepts

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

Chromosomal Inversion

A chromosomal inversion occurs when a segment of a chromosome is reversed end to end. In the case of a paracentric inversion, the inversion does not include the centromere. This structural alteration can affect gene expression and recombination during meiosis, particularly when crossovers occur within the inverted region, leading to potential formation of nonviable gametes.

Crossover and Recombination

Crossover is a genetic process that occurs during meiosis where homologous chromosomes exchange segments of genetic material. This recombination can create new allele combinations in gametes. The location of the crossover is crucial, as it can lead to viable or nonviable gametes depending on whether the crossover occurs within a normal or inverted chromosome segment.

Viability of Gametes

The viability of gametes refers to their ability to develop into a functional organism after fertilization. In the context of chromosomal inversions, certain gametes produced from crossovers may carry duplications or deletions of genetic material, rendering them nonviable. Understanding which gametes are viable requires analyzing the genetic content resulting from the crossover event.

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Related Practice

Multiple Choice

A person has a WT chromosome with the following segments. A B C • D E F G H. Which of the following shows how the chromosome would look after a pericentric inversion?

From the following list, identify the types of chromosome changes you expect to show phenotypic consequences.

Pericentric inversion

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Textbook Question

Inversions are said to 'suppress crossing over.' Is this terminology technically correct? If not, restate the description accurately.

565

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Textbook Question

Contrast the genetic composition of gametes derived from tetrads of inversion heterozygotes where crossing over occurs within a paracentric versus a pericentric inversion.

785

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Textbook Question

A normal chromosome and its homolog carrying a paracentric inversion are shown here. The dot (·) represents the centromere.

Normal ABC • DEFGHIJK

Inversion abc • djihgfe

Assume a crossover takes place in the region between F and G. Identify the gametes that are formed following this crossover, and indicate which, if any, gametes are viable.

560

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Textbook Question

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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Textbook Question

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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Textbook Question

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

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