The key difference between trisomy 18 and 21 is that trisomy 18 is a chromosomal disorder caused due to the presence of an extra chromosome 18 while trisomy 21 is a chromosomal disorder caused due to the presence of an extra chromosome 21.
A healthy human cell contains a total of 46 chromosomes (23 pairs). There are 22 autosomal chromosome pairs and one sex chromosome pair. Cells divide and produce more cells and gametes. During the cell divisions, the genome replicates and segregates into daughter cells. Cell division is a well-organized and regulated process. However, random errors can also occur. As a result, three chromosomes can be copied instead of two. This phenomenon is known as trisomy. The total number of chromosomes per cell becomes 2n+1. In other words, the total number increases to 47. An extra copy of one particular chromosome is created in trisomy.
Trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are three types of trisomy. Trisomy 21 is the most common, while trisomy 18 and 13 are less common. Trisomy makes a serious impact on the further development of an unborn baby.
CONTENTS
1. Overview and Key Difference
2. What is Trisomy 18
3. What is Trisomy 21
4. Similarities Between Trisomy 18 and 21
5. Side by Side Comparison – Trisomy 18 vs 21 in Tabular Form
6. Summary
What is Trisomy 18?
Trisomy 18, also known as Edwards syndrome, is a chromosomal disorder caused due to the presence of an extra chromosome 18. In this condition, the genome of the cell has three chromosomes of chromosome 18. The total number of chromosomes in the cell increases into 2n+1 or 47. Trisomy 18 is less common than trisomy 21, but it gives a serious impact on the development of an unborn baby. The baby is born with low birth weight, an abnormally shaped head, a small jaw, a small mouth, frequently with a cleft lip or cleft palate in this chromosomal disorder. Moreover, these babies also suffer from breathing and feeding problems. They are prone to develop heart diseases and kidney problems, as well.
Figure 01: Trisomy 18
Edwards syndrome pregnancies are at a higher risk of miscarriage. If the baby is born, their life span will be less than one year. Therefore trisomy 18 is more life-threatening than Down syndrome. This disease affects 1 in every 5000 babies. Trisomy 18 can be diagnosed by a noninvasive prenatal screening.
What is Trisomy 21?
Trisomy 21, also known as Down syndrome, is a genetic disorder which arises due to the presence of an extra chromosome 21. It is a chromosomal abnormality. The cell has three copies of chromosome 21. In other words, the cell has a total of 47 chromosomes instead of 46. This is the most common form of trisomy. Approximately 30% of Down syndrome pregnancies end in miscarriage. Two thirds will end in normal childbirth. Trisomy 21 affects the baby’s overall growth and wellbeing seriously. Moreover, the shape of the body will also be affected by this syndrome. Down syndrome is characterized by distinct facial features such as wide-set eyes, etc. and different levels of physiological and mental dysfunction. The child may get frequent immune and circulatory disorders or gastrointestinal disturbances. Down syndrome symptoms may vary from child to child. Some children may need special health care, while some Down syndrome children are able to enjoy a relatively long life.
Figure 02: Down Syndrome
Similar to trisomy 18, trisomy 21 is not an inherited genetic disorder. It occurs as a result of a random error during the cell division. Mothers at age over 35 are at a higher risk of having Down syndrome babies since their eggs can divide abnormally. Moreover, if the mother or father is a carrier, then also there will be a risk of having a Down syndrome baby. This disease affects 1 in every 700 babies.
What are the Similarities Difference Between Trisomy 18 and 21?
- Both trisomy 18 and 21 are chromosomal disorders.
- They occur as a result of a random error in cell division.
- In both cases, there are 47 chromosomes in a human cell.
- They are not inherited genetic disorders.
- The age of the mother is a big risk factor for trisomy. Risk increases with the age of the mother.
- Trisomy 18 and 21 pregnancies show a high risk of miscarriage.
- Affected babies have an intellectual disability.
What is the Difference Between Trisomy 18 and 21?
Trisomy 18 (Edwards syndrome) is a genetic disorder caused due to the presence of an extra chromosome 18. Trisomy 21 (Down syndrome) is a genetic disorder caused due to the presence of an extra chromosome 21. So, this is the key difference between trisomy 18 and 21. Trisomy 21 is the most common form of trisomy, while trisomy 18 is the second most common form. Moreover, trisomy 18 is more life-threatening than trisomy 21.
The below infographic summarizes the differences between trisomy 18 and 21 in tabular form.
Summary – Trisomy 18 vs 21
Chromosomal abnormalities take place due to an error in the cell division. They cause an extremely serious impact on the development of an unborn baby. Children affected with trisomy, show development delays, birth defects and intellectual disabilities. Down syndrome is a result of trisomy of chromosome 21. Edward syndrome is a result of trisomy of chromosome 18. There is an extra chromosome 18 in Edwards syndrome while there is an extra chromosome 21 in Down syndrome. Thus, this is the key difference between trisomy 18 and 21.
Reference:
1. “Default – Stanford Children’s Health.” Stanford Children’s Health – Lucile Packard Children’s Hospital Stanford, Available here.
2. “Edwards’ syndrome (trisomy 18)” NHS Choices, NHS, Available here.
Image Courtesy:
1. “Trisomia 18” By Serra Amoros – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Down Syndrome (Trisomy 21)”By NIH Image Gallery (CC BY-NC 2.0) via Flickr