A deletion is a type of chromosomal structural variation where a portion of the chromosome is missing or lost. This results in the loss of genetic material, which can have significant consequences depending on the size and location of the deletion. Deletions can affect a single gene, multiple genes, or large chromosomal segments, leading to various genetic disorders and phenotypic abnormalities.

Types of Deletions

Deletions can be classified based on their size, location, and genetic impact. But there are two main types of deletion which are classified based on which part of chromosome is deleted.  

1. Terminal Deletion:

   • Definition: A terminal deletion occurs at the "end of a chromosome". The deleted segment includes the terminal part of the chromosome, leading to the loss of the end segment.

   • Example: Cri-du-chat syndrome is caused by a terminal deletion on the short arm of chromosome 5 (5p-).

2. Interstitial Deletion:

   • Definition: An interstitial deletion occurs in the "middle of a chromosome", where a segment within the chromosome is deleted, leaving two separate segments joined together.

   • Example: Williams syndrome results from an interstitial deletion on chromosome 7, leading to the loss of several genes.

Consequences of Deletions

1. Loss of Gene Function: Deletions can result in the loss of essential genes, leading to various genetic disorders and developmental abnormalities. The severity of the impact depends on the size of the deletion and the genes involved.

2. Genetic Imbalance: Deletions can create an imbalance in the dosage of certain genes, leading to phenotypic changes and diseases. For instance, losing one copy of a gene (haploinsufficiency) can cause diseases if the remaining copy is not sufficient to perform the necessary function.

3. Meiotic Consequences: Deletions can lead to problems during meiosis, such as improper pairing and recombination of homologous chromosomes, resulting in unbalanced gametes. This can cause infertility, miscarriages, or congenital abnormalities in offspring.

4. Associated Disorders: Deletions are associated with various genetic disorders, including developmental and intellectual disabilities, congenital anomalies, and increased susceptibility to certain diseases like cancers.

Examples of Genetic Disorder Caused by Deletion

Cri-du-chat Syndrome: Chromosome: 5p,  Symptoms: Intellectual disability, distinctive facial features, and a high-pitched cat-like cry in infancy.

Summary: Deletions are a crucial aspect of chromosomal variations that lead to the loss of genetic material. They have profound implications for genetic health and can result in a variety of genetic disorders depending on their size and location. Understanding the types and consequences of deletions is essential for diagnosing and managing these conditions.