Mode of Inheritance:
- Trisomy 18 is not due to a dominant or recessive sex-linked or autosomal inheritance.
- Instead, it occurs randomly during the formation of an egg and sperm.
- An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes.
- Nondisjunction is when homologous pairs don't split in Meiosis 1, or sister chromatids don't split in Meiosis 2.
- If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body's cells, causing Trisomy 18.
Genetic cause:
- Most cases of trisomy 18 result from having three copies of chromosome 18 in each cell in the body (instead of the usual two copies). This occurs randomly, as there is no mutation.
- Some people have an extra copy of chromosome 18 in only some of the body's cells. This is called mosaic trisomy 18. The development of individuals with this form of trisomy 18 may range from normal to severely affected.
- Very rarely, part of the long arm of chromosome 18 moves to another chromosome during the formation of reproductive cells. This mutation is a change within a chromosome, called a translocation. This is called partial trisomy 18. The physical signs of partial trisomy 18 are less severe.
Website Picture (No Author, Chromosome 18 - Conditions, 2009):
["The image above shows a balanced translocation involving 18p and chromosome 4. On the left side of the arrow, the translocation is balanced. If the balanced translocation becomes unbalanced, it may results in partial trisomy 18q. In this example, the extra chromosome material is attached to chromosome 4. This is illustrated on the right side of the arrow."]. Retrieved April 19, 2015
from http://www.chromosome18.org/WhatisaChromosome18Abnormality/TheConditions/OtherConditions/tabid/518/Default.aspx