Question

Explain why a translocation cross occurs during metaphase of meiosis I when a cell contains a reciprocal translocation.

Short answer

In a cell with a reciprocal translocation, a translocation cross occurs during metaphase of meiosis I due to the exchange of chromosome parts between two non-homologous chromosomes. This leads to a quadrivalent structure for maintaining synapsis and crossing over. However, the quadrivalent alignment can cause abnormal segregation during anaphase, leading to genetic imbalances.

Step-by-step solution

Understanding Reciprocal Translocation

Reciprocal translocation involves two non-homologous chromosomes where parts of each chromosome are exchanged. This exchange can affect the position of genes and their regulation.

Recognize the Process of Metaphase I in Meiosis

Metaphase in meiosis I is the stage where the homologous chromosomes line up along the equatorial plate. Spindle fibers from both the poles attach to kinetochores of homologous chromosomes.

Identifying Translocation Cross

Chromosomes with reciprocal translocations do not align linearly during metaphase of meiosis I. Instead, they form a structure known as a quadrivalent or a translocation cross due to the swapping of the chromosome fragments. This is due to the need to maintain synapsis and crossing over between homologous segments during meiosis.

Effect of Translocation Cross

The alignment in the shape of a translocation cross may lead to abnormal segregation patterns during anaphase I. It can cause genetically unbalanced gametes leading to genetic disorders or miscarriages in organisms.