Quiz 8: Variation in Chromosome Structure and Number

The two main types of chromosomal aberrations are the structural and the numerical aberrations. While translocations, deletions and duplications are the examples of the structural chromosomal aberrations, the nullisomies, monosomies, and the trisomies are the examples of numerical chromosomal aberrations. While the numerical chromosomal aberrations cause a change in the total amount of genetic material, the aberrations such as the reciprocal translocations do not cause a change in the total amount of the genetic material. The structural chromosomal aberrations include deletions, duplications, inversions, and translocations. The deletions and duplications cause a change in the total amount of genetic material while the inversions and translocations do not cause a change in the total amount of genetic material.

Several disorders of humans are well studied and their causes ascertained. One primary cause of several disorders is genetic, with several syndromes and cancers having a genetic etiology. Cancers are linked to chromosomal disorders such as deletions and duplications. Comparative genomic hybridization (CGH) is a high throughput technique that helps in comparing cancer samples and normal samples towards detecting chromosomal deletions and duplications in the cancer samples. In the CGH technique, two fluorescent labels (probes), the green and red DNA strands are prepared using DNA of the cancer cells and the normal cells as template strands respectively. These probes are made to hybridize on to the denatured DNA strands of the cancer and normal samples. Further to the hybridization, a comparison of green and red fluorescence intensity obtained will give an indication of the chromosomal content in the cancer samples in relation to the normal samples. An absence of either deletions or duplications in the cancer samples in comparison to the normal samples will give a red to green fluorescence ratio of 1. A ratio of less than 1 indicates the presence of deletions in the cancer samples and a ratio of more than 1 indicates the presence of duplications in the cancer samples.

The chromosome with a reciprocal translocation and an inversion along with two normal chromosomes are illustrated below: The pairing of such chromosomes during metaphase of meosis I is illustrated below: