Key Difference – Monosomy vs Trisomy
Chromosomal nondisjunction causes abnormal chromosome numbers in daughter cells. It can occur during cell division in mitosis and meiosis. As a result of nondisjunction in meiosis, aneuploid individuals are developed after the fertilization. Aneuploidy is a mutation in which chromosomal number is abnormal. A normal diploid cell (2n) contains a total of 46 chromosomes arranged in 23 pairs. A normal haploid cell (n), which is a gamete, contains 23 chromosomes. Aneuploids can have more or fewer chromosomes than the normal number. Trisomy and monosomy are two such numerical chromosome abnormalities which cause birth defects. The term monosomy is used to describe the chromosomal abnormality in which one chromosome is absent from a pair of homologous chromosomes. The term trisomy is used to describe the abnormal chromosome number in which three chromosomes (usual pair + extra chromosome) are present in a type of homologous chromosomes. This is the key difference between monosomy and trisomy. Therefore, the aneuploid condition for monosomy is 2n-1 while for trisomy it is 2n+1.
CONTENTS
1. Overview and Key Difference
2. What is Monosomy
3. What is Trisomy
4. Side by Side Comparison – Monosomy vs Trisomy
5. Summary
What is Monosomy?
The word monosomic refers ‘one chromosome’. The term monosomy is used to explain the aneuploid condition which lacks one member of a homologous chromosome pair. Due to this condition, resulting cells will contain only 45 chromosomes instead of the usual 46 chromosomes. Cells show 2n-1 chromosomes in each cell of the body. For example, monosomy of sex chromosome leads to Turner’s syndrome in which individuals bear only one X sex chromosome. Another example of monosomy is Cri du chat syndrome which is caused by partial monosomy of chromosome 5. 1p36 deletion syndrome is another syndrome which is caused due to partial monosomy of chromosome 1.
Monosomy results in deleterious effects in individuals. Missing chromosome causes changes in the overall gene balance of chromosome set. It also allows any harmful recessive allele on the single chromosome to be hemizygous and to be expressed phenotypically.
Figure 01: Karyotype of monosomy X – Turner’s syndrome
What is Trisomy?
The term trisomy describes the condition in which an extra chromosome is present in the usual pair of homologous chromosomes. Three copies are present from in one homologous chromosome pair. Every cell of the body will contain a total of 47 chromosomes instead of the usual 46 chromosomes. For example, trisomy 21 or Down syndrome occurs due to the presence of three copies of chromosome 21. Trisomy 18 is another example in which three copies of chromosome number 18 are present than the usual pair resulting in a syndrome called Edwards syndrome. Patau syndrome is caused due to the presence of three chromosomes in chromosome number 13. Trisomy of sex chromosome pair can also be seen. If this happens, several abnormal sex chromosome conditions such as XXX (triple X syndrome), XYY and XXY (Klinefelter syndrome) may occur.
Figure 02: Karyotype of 21 trisomy – Down syndrome
What is the difference between Monosomy and Trisomy?
Monosomy vs Trisomy |
|
| Monosomy is a type of aneuploidy in which only one copy is present instead of two | Trisomy is a type of aneuploidy in which three copies of a particular chromosome is present instead of two. |
| Chromosome Number | |
| Cells contain a 2n-1 number of chromosomes (total of 45). | Cells bear a 2n+1 number of chromosomes (total of 47). |
| Syndrome | |
| Turner’s syndrome, Cri du chat syndrome, and 1p36 deletion syndrome are caused by this. | Down syndrome, Edwards syndrome, Patau syndrome, Klinefelter syndrome, Warkany syndrome 2, and triple X are caused by this. |
Summary – Monosomy vs Trisomy
Cell division is a cellular process which creates new cells for various purposes. There are instances where cell division shows errors in chromosomal disjunction. It is known as chromosomal nondisjunction and it happens during mitosis and meiosis resulting in diploid and haploid cells with abnormal chromosomal numbers. Monosomy and trisomy are two types of numerical chromosomal abnormalities. Monosomy is a condition which describes the absence of one copy from the normal pair of chromosomes. Trisomy is a condition which describes the presence of three copies of a particular chromosome instead of two copies. Both situations cause deleterious effects in individuals by making imbalance in the overall genetic composition.
Image Courtesy:
1. “45,X” By No machine-readable author provided. The cat~commonswiki assumed (based on copyright claims). – (CC BY-SA 3.0) via Commons Wikimedia
2. “21 trisomy – Down syndrome” By U.S. Department of Energy Human Genome Program. – (Public Domain) via Commons Wikimedia