Double monosomy is the aberrant chromosomal condition commonly found in the plant genome but so rare in the human genome.
Chromosomal aberrations or defects we can say, are classified broadly into aneuploidies. The monosomy is a kind of aneuploidy in which a single chromosome from a pair is missing.
Meaning, that instead of normal 46 chromosomes (23 pairs) we observe 45 chromosomes. For example, in monosomy X in females, known as Turner syndrome in which a whole single X chromosome is missing from a pair.
These types of chromosomal aberrations are classified into numerical chromosomal defects in which the number of chromosomes changes.
Monosomies are commonly found in the human genome, however, as we said, the double monosomy is very rare and hard to find. Here is the case and explanation of double monosomy.
What is double monosomy?
In the monosomy, a single chromosome is missing from the pair, meaning, 2n-1. Whilst in the double monosomy two different non-homologous chromosomes are missing from the genome, meaning, 2n-1-1, not 2n-2.
For example, Weber and Sparks, 1971; have reported a single case of double monosomy- 45, X0/ 46, XX -21.
What does it mean?
This was the case of a female- a female karyotype in which one X chromosome and one chromosome 21 wasn’t present. However, the case is of mosaic condition in which not all the cells have this condition. It is also evident that the original author has not explained the mosaic condition in depth.
The phenotypic effect of the condition is broad having symptoms of physical and mental retardation, speech problems and other mental and physical issues.
So technically speaking, no disorders are directly associated with the double monosomy, as only a single to a few notable cases are reported.
As per my opinion, phenotypic variation and severity of the condition depend on which type of double monosomy reported and with which pair it associated.
Let’s make it more clear, for example, symptoms of 45, X/46, XX -21 vary from 45, X0/46, XY -18. Here in case one, one X chromosome and one chromosome from the pair 21 is deleted.
In another case, one Y chromosome and one 18 number chromosome are deleted. Different genes are located on different chromosomes, so the effect of copy number variation also differs.
I searched on the internet and got only a single case of double monosomy in humans. The case is of double monosomy 45, X/ 46, XX -21. Phenotypically only mental retardation is reported predominantly.
I am giving you the link for the article, you can download it and read it:
- Weber F, M, Sparkes R, S, Muller H: Double monosomy mosaicism (45, X/45, XX, 21–) in a retarded child with multiple congenital malformations. Cytogenet Genome Res 1971;10:404-412. doi: 10.1159/000130161.