The key difference between nullisomy and double monosomy is that nullisomy is the loss of both pairs of homologous chromosomes while double monosomy is the loss of one chromosome from each of two pairs of homologous chromosomes.
Euploidy and aneuploidy are two chromosomal variations identified in organisms. Aneuploidy refers to a variation in the total number of chromosomes in a cell by addition or deletion of chromosomes. Aneuploidy does not change the number of chromosome sets. It changes the normal total number of chromosomes in a cell or organism. This variation affects the genetic balance of the cell or organism since it changes the amount of genetic information or products. Aneuploidy is an abnormal condition which can lead to different syndromes such as Down syndrome, Edwards syndrome, triple X syndrome, Klinefelter syndrome, Turner’s syndrome and Cri du chat syndrome, etc. Nullisomy and tetrasomy are two types of aneuploidy conditions.
CONTENTS
1. Overview and Key Difference
2. What is Nullisomy
3. What is Double Monosomy
4. Similarities Between Nullisomy and Double Monosomy
5. Side by Side Comparison – Nullisomy vs Double Monosomy in Tabular Form
6. Summary
What is Nullisomy?
Nullisomy is an abnormal chromosomal composition which occurs due to the loss of both chromosomes in a homologous chromosome pair. It can be represented as 2n-2. It is a genomic mutation. When compared to the total number of chromosomes, two chromosomes are less in nullisomy. Individuals who show nullisomy are called nullisomics. The main reason for nullisomy is nondisjunction during cell division, especially during meiosis. Nondisjunction takes place when two sister chromatids or homologous chromosomes fail to separate. As a result, one gamete lacks one homologous chromosome pair (nullisomic) while the other gamete gains that pair (disomic). When nullisomy occurs in higher animals, they cannot survive. In diploids, nullisomy is a lethal condition. In plants, nullisomy produces viable polyploid plants.
What is Double Monosomy?
The word monosomic means ‘one chromosome’. The term monosomy is used to explain the aneuploid condition in which one member of a homologous chromosome pair is missing. Due to this condition, cells will contain only 45 chromosomes, instead of the usual 46 chromosomes.
Figure 01: Nondisjunction in Mitosis
Cells show 2n-1 chromosomes in each cell of the body. Sometimes, it can involve more than one pair of homologous chromosomes. Double monosomy is such a condition. In double monosomy, one chromosome from each of two pair of homologous chromosomes is missing. It can be represented as 2n-1-1.
What are the Similarities Between Nullisomy and Double Monosomy?
- Nullisomy and double monosomy are two aneuploidy conditions seen in organisms.
- Both produce abnormal chromosome numbers.
- They change the total number of chromosomes in a set.
- Overall, gene balance in the chromosome set is disrupted due to nullisomy and double monosomy.
- Both occur due to nondisjunction during meiosis.
- In both conditions, two chromosomes are missing from the total number of chromosomes.
What is the Difference Between Nullisomy and Double Monosomy?
Nullisomy is the loss of both chromosomes in a pair of homologous chromosomes. Double monosomy is the loss of one chromosome from each of two pairs of homologous chromosomes. So, this is the key difference between nullisomy and double monosomy. Nullisomy occurs in one pair of homologous chromosomes. Double monosomy occurs in two pairs of homologous chromosomes.
Below infographic tabulates the differences between nullisomy and double monosomy.
Summary – Nullisomy vs Double Monosomy
Aneuploidy is a mutation in which chromosomal number is abnormal. It changes the total number of chromosomes, which can be either due to the loss of one or more chromosomes or due to the addition or deletion of one or more chromosomes. Nullisomy and double monosomy are two such conditions. In nullisomy, both chromosomes of a homologous chromosome pair are missing. In double monosomy, one chromosome from each of two pairs of homologous chromosomes is missing. Nullisomy is represented as 2n-2 while double monosomy is represented as 2n-1-1. Thus, this is the summary of the difference between nullisomy and double monosomy.
Reference:
1. Griffiths, Anthony et al. “Aneuploidy”. Ncbi.Nlm.Nih.Gov, 2020, Available here.
Image Courtesy:
1. “Nondisjunction in Mitosis” By Alex Federspiel – Own work (CC BY-SA 4.0) via Commons Wikimedia
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