What a translocation is

Eukaryotic cells have evolved telomeres, which tend to prevent translocations from occurring. They allow cells to distinguish where a chromosome ends and prevent the attachment of chromosomal DNA to the natural ends of a chromosome (197).
Mechanism #1 of getting translocations: If cells are exposed to things that cause chromosomes to break, the broken ends lack telomeres, become reactive, and bind to another chromosome that is also reactive. DNA repair enzymes will usually recognize these reactive ends and join them back together.
- If multiple chromosomes are broke, the reactive ends can be joined incorrectly to produce abnormalities.
Mechanism #2 of getting translocations: Abnormal crossovers. A reciprocal translocation can be produced when 2 nonhomologous chromosomes cross over. This is rare (198).
Reciprocal translocations are also called balanced translocations. They usually occur with no phenotypic consequence because genetic material isn't lost (198).
- Their offspring, however, is at risk of getting an unbalanced translocation. In these translocations, significant portions of genetic material are duplicated and/or deleted.
  - These ones have phenotypic consequences. They can even cause death.
  - They can occur because some of the chromosomes do not segregate evenly.
A Robertsonian translocation arises from breaks near the centromeres of 2 nonhomologous acrocentric chromosomes (199).
- Most common type in humans.
- Only involves the acrocentric chromosomes 13, 14, 15 21, and 22.

Reciprocal translocations and segregation of chromosomes

Whether or not people with balanced translocations produce gametes with unbalanced chromosomes depends on segregation in meiosis I (199).
A translocation cross can occur when the homologous chromosomes attempt to synapse with each other, but form an unusual structure because of the translocations (199).
For translocated chromosomes, the expected segregation pattern is governed by the centromeres (199).
Each haploid should receive one centromere located on chromosomes 1 and 2. This can occur 1 of 2 ways.
- Alternate segregation:
  - Following meiosis II, 4 haploid cells are produced. 2 are normal, 2 have reciprocal (balanced) translocations.
  - One daughter cell receives 2 normal chromosomes, and the other cell gets to translocated chromosomes.
  - Alternate segregation is a process in which the chromosomes diagonal to each other within the translocation cross sort into the same cell.
- Adjacent-1 segregation:
  - Adjacent chromosomes segregate into the same cell
  - After anaphase of meiosis I, each daughter receives 1 normal and 1 translocated.
  - After meiosis II is completed, 4 haploid cells are produced. All of them are genetically imbalanced because part of 1 chromosome has been deleted and another has been duplicated.
  - If these abnormal gametes unite with a normal gamete, the zygote is expected to be abnormal genetically and probably phenotypically.
Adjacent-2 segregation can occur. In this case, centromeres do no not segregate as they should. 1 daughter receives both copies of the centromere on chromosome 1; in the other, both copies are on chromosome 2 (200).
- 4 abnormal haploid cells that are unbalanced are made.
- This is very rare.
In many cases, the haploids from adjacent-1 segregation are not viable, lowering fertility. This condition is called semisterility (200).

	Created by Olivia McRitchie over 8 years ago

NCLEX Biology (Genetics) Note on Ch 8: Variation in Chromosome Structure and Number, created by Olivia McRitchie on 04/04/2018.