A translocation is a type of chromosomal structural rearrangement where segments of chromosomes are exchanged between non-homologous chromosomes. This can lead to significant changes in the genome, potentially disrupting gene function and regulation, and is often associated with various genetic disorders and cancers.

Types of Translocations

Translocations can be classified into several types based on their nature and the chromosomal segments involved:

1. Reciprocal Translocation: Reciprocal translocations occur when segments from two non-homologous chromosomes are exchanged (Balanced Translocation). This involves a mutual exchange of material without any net loss or gain of genetic material. A classic example is  "The Philadelphia chromosome" in chronic myeloid leukemia (CML) is a result of a reciprocal translocation between chromosomes 9 and 22. This means that genetic material excahnge is balanced, the rearrangement can disrupt gene function or create fusion genes that contribute to disease (see in above figure-A).

2. Non-Reciprocal Translocation: A non-reciprocal translocation, also known as an unbalanced translocation, involves the transfer of a segment of one chromosome to another non-homologous chromosome without any corresponding exchange. This results in an unequal distribution of genetic material, leading to potential genetic imbalances and disorders. Genetic Imbalance: This can lead to a gain or loss of genetic material, causing genetic disorders due to the altered dosage of genes. Various Phenotypic Consequences are due to unbalanced translocation such as developmental abnormalities, miscarriages, or congenital defects depending on the genes involved.

3. Robertsonian Translocation: Robertsonian translocations involve the fusion of two acrocentric chromosomes (chromosomes with the centromere near one end), typically resulting in a chromosome with two long arms and the loss of the short arms (See in above figure-B). Example: The most common Robertsonian translocation is between chromosomes 13 and 14, which can increase the risk of trisomy 21 (Down syndrome). Consequences: This can lead to an unbalanced distribution of genetic material in gametes, leading to conditions such as Down syndrome or Patau syndrome (trisomy 13).

Mechanisms of Translocation Formation

1. Chromosomal Breakage and Repair:

   • Translocations can occur when two chromosomes break and the fragments are erroneously rejoined, leading to an exchange of segments between non-homologous chromosomes.

2. Non-Homologous End Joining (NHEJ):

   • This is a repair mechanism that can incorrectly join broken chromosome ends from different chromosomes, leading to translocations.

3. Recombination Errors:

   • During meiosis, improper alignment and recombination between non-homologous chromosomes can result in translocations.

Examples of Genetic Disorders and Cancers Associated with Translocations

1. Chronic Myeloid Leukemia (CML): Chromosomes: 9 and 22 (BCR-ABL fusion gene), Symptoms: Excessive proliferation of white blood cells due to the formation of a fusion gene that promotes cancer growth.