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

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

Key Concepts

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.