deletion

In genetics, a deletion is the complete loss of a segment of a chromosome. This structural change can have severe consequences because it alters the normal ratio of genetic material within the genome. Even in diploid organisms, where two copies of each chromosome exist, deletions are typically lethal or cause serious health issues.

If a deletion occurs in a heterozygous state, one chromosome remains normal while the other is shortened. This results in genomic imbalance, which worsens with the size of the deletion. Furthermore, any recessive, harmful, or lethal alleles present on the normal counterpart of the deleted region will be expressed in the organism's phenotype. A human example is cri-du-chat syndrome, caused by a deletion on chromosome 5, leading to intellectual disability and other developmental problems. Techniques like fluorescence in situ hybridization (FISH) can detect even small genetic deletions involving just a few genes.12