Difference Between Monosomy and Trisomy

Main Difference – Monosomy vs Trisomy

Aneuploidy is a condition with variable chromosome number in the genome. Monosomy and trisomy are two types of aneuploidy. The numerical chromosome abnormalities lead to certain birth defects. The main difference between monosomy and trisomy is that monosomy is the presence of only one chromosome in a particular homologous pair whereas trisomy is the presence of an additional chromosome. Monosomy is represented as 2n-1 whereas trisomy is represented as 2n+1. 2n is the regular number of chromosomes in the human genome, which is diploid.

Key Areas Covered

1. What is Monosomy
     – Definition, Facts, Examples
2. What is Trisomy
     – Definition, Facts, Examples
3. What are the Similarities Between Monosomy and Trisomy
     – Outline of Common Features
4. What is the Difference Between Monosomy and Trisomy
     – Comparison of Key Differences

Key Terms: Aneuploidy, Chromosomal Abnormalities, Chromosome Non-disjunction, Monosomy, Trisomy, Turner Syndrome

What is Monosomy

Monosomy is the condition of having a diploid chromosome complement in which one chromosome lacks its homologous partner. Thus, monosomy defines the state of missing chromosome. Typically, monosomy is lethal, causing spontaneous abortions or leading to severe developmental abnormalities. The monosomy of the sex chromosomes in humans leads to Turner syndrome or monosomy X (45, X) in humans. Most human conceptions with monosomy X may abort in early pregnancy. Individuals with maternal X chromosome exhibit difference in cognition. A karyotype with Turner syndrome is shown in figure 1.

Figure 1: Turner Syndrome

Cri du chat syndrome occurs as a partial monosomy caused by the deletion of the end of the short arm of chromosome 5. The 1p36 deletion syndrome is another type of partial monosomy caused by the deletion of the end part of the chromosome 1 short arm.

What is Trisomy

Trisomy refers to the condition of having an additional copy of a chromosome in the genome. It arises as a result of non-disjunction of homologous chromosomes during meiosis. This forms gametes with 24 chromosomes in humans rather than 23 chromosomes. Typically, trisomy may occur in any chromosome in the genome. But, trisomy except in chromosomes 13, 18, 21, X and Y are lethal. Maternal age is a risk factor of non-disjunction. The commonest, viable trisomy is trisomy 21 or Down syndrome. A karyotype with Down syndrome is shown in figure 2.

Figure 2: Down Syndrome

The much rare trisomies are trisomy 13 or Patau syndrome and trisomy 18 or Edward syndrome, which cause severe congenital malformations. They also cause mental retardation, resulting in death within a few months of birth.

Similarities Between Monosomy and Trisomy

• Monosomy and trisomy are two types of aneuploidy.
• Both monosomy and trisomy consist of numerical chromosome abnormalities.
• Both monosomy and trisomy lead to birth defects in humans.

Difference Between Monosomy and Trisomy

Definition

Monosomy: Monosomy refers to the condition of having a diploid chromosome complement in which one chromosome lacks its homologous partner.

Trisomy: Trisomy refers to the condition of having an additional copy of a chromosome is present in the genome.

Type of Variation

Monosomy: Monosomy is the presence of a single chromosome in the homologous pair.

Trisomy: Trisomy is the presence of an additional chromosome.

Abbreviation

Monosomy: Monosomy is represented as 2n-1.

Trisomy: Trisomy is represented as 2n+1.

Examples

Monosomy: Turner syndrome is an example of monosomy.

Trisomy: Down syndrome is an example of trisomy.

Conclusion

Monosomy and trisomy are two types of aneuploidy with an abnormal number of chromosomes in the genome of a particular organism. In monosomy, one chromosome of the homologous pair is lost. Thus, it can be represented as 2n-1. In trisomy, an additional chromosome is present in the genome. It is represented as 2n+1. Turner syndrome is an example of monosomy while Down syndrome is an example of trisomy. The main difference between monosomy and trisomy is the type of variation in the chromosome number.

Reference: 

1.“Monosomy.” Encyclopedia of Genetics, Available here.
2.“Trisomy.” ScienceDirect Topics, Available here.

Image Courtesy:

1. “متلازمة تيرنر” By جنان العبدالمحسن – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Trisomy13” By CarloDiDio – Own work (CC BY-SA 3.0) via Commons Wikimedia

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