A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right.

Using a drawing, demonstrate how these chromosomes would pair during meiosis. Be sure to label the different segments of the chromosomes.

Verified step by step guidance

1. Step 1: Identify the normal chromosomes and the rearranged chromosomes. The normal chromosome 2 has segments labeled A, B, C, D, and the normal chromosome 3 has segments E, F, G, H. The rearranged chromosomes show a translocation between chromosomes 2 and 3, where segments C and D from chromosome 2 are swapped with segments G and H from chromosome 3.
2. Step 2: Understand that during meiosis, homologous chromosomes pair by aligning their similar segments. Because of the translocation, the chromosomes cannot pair linearly as usual. Instead, they form a cross-shaped structure (a quadrivalent) to allow homologous regions to align properly.
3. Step 3: Draw the pairing structure by placing the normal chromosome 2 and the rearranged chromosome 2/3 opposite each other, and the normal chromosome 3 and the rearranged chromosome 2/3 opposite each other, forming a cross. Label each segment (A, B, C, D, E, F, G, H) on the respective chromosomes to show which segments align with each other.
4. Step 4: In the cross structure, ensure that segments A and B on the normal chromosome 2 align with A and B on the rearranged chromosome, segments C and D on the normal chromosome 2 align with C and D on the rearranged chromosome 2/3, and similarly for segments E, F, G, H on chromosome 3 and the rearranged chromosome 2/3.
5. Step 5: This cross-shaped pairing allows for proper synapsis during meiosis despite the rearrangement, and it is important to label the centromeres and segments clearly to visualize how the chromosomes align and segregate.

Key Concepts

Chromosomal Rearrangements

Chromosomal rearrangements involve structural changes in chromosomes, such as translocations, inversions, deletions, or duplications. In this case, a rearrangement between chromosomes 2 and 3 results in a hybrid chromosome containing segments from both. Understanding these rearrangements is crucial for predicting how chromosomes pair and segregate during meiosis.

Meiotic Pairing of Homologous Chromosomes

During meiosis, homologous chromosomes pair to ensure proper segregation. When a heterozygous rearrangement is present, chromosomes form complex pairing structures (e.g., loops or cross-shaped configurations) to align homologous segments. This pairing is essential to maintain genetic balance and avoid gametes with duplications or deletions.

Karyotype and Chromosome Segment Labeling

Karyotyping visually represents chromosomes and their segments, labeled here as A-H, to identify structural differences. Labeling helps track which chromosome parts correspond between normal and rearranged chromosomes, facilitating the understanding of pairing behavior and genetic consequences during meiosis.