Deletions

A deletion is pretty much as the name implies. When discussing deletions at the chromosome level we are talking about losses of large segments of DNA, on the order of tens of thousands of bases or more. These can be observed on stained chromosomes under a light microscope.

Deletions are often formed by crossing over between direct repeats on the same chromosome as illustrated in this figure from the journal Nature. However, other mechanisms can generate deletions.

The effect of a deletion depends on what genes or other genetic elements happen to be part of the region that is deleted. Therefore, there is a lot of variation between deletions in the effects that they have. Obviously, the larger the deletion, the more chance that an important gene is deleted. Individuals who are heterozygous for a normal chromosome and a deletion chromosome will express only those genes from the normal copy in the deletion region and can thus express recessive phenotypes for these if they carry a recessive allele. Individuals who are homozygous for the deletion chromosome will not carry any copy of genes located in the region. This can, of course, have significant phenotypic effects depending on the gene.

There are many human syndromes that are associated with chromosome deletions.