Triploidy is a rare genetic disease that often becomes a focus during deeply emotional times, such as following an unexpected pregnancy loss or upsetting prenatal testing results. It occurs when an embryo inherits three full sets of chromosomes instead of the usual two, resulting in the fetus having 69 chromosomes instead of 46. Whilst this may seem like an insignificant difference, Triploidy has a profound effect on development, causing restriction in growth and birth defects.

Overview

Triploidy is a rare disease which affects the amount of genetic material that makes up the individual. This disorder affects 1-2% of clinically recognised pregnancies and is responsible for around 10% of miscarriages.^1,2 To gain an understanding of what the disease is, we can break down the word into “tri”, meaning three, and “ploid”, which refers to the number of sets of chromosomes in a cell. In a normal cell, there are a total of 46 chromosomes, 23 of which are maternal and 23 are paternal. In Triploidy, this number becomes a total of 69 chromosomes. Whilst this disease is typically associated with foetal or infant death, it can also occur in plants and animals, making it a significant genetic topic to focus on. Whether you are looking for answers to specific questions or just want to learn more about this rare genetic disorder, this article aims to give an insight into Triploidy- its definition and genetic basis.  

Definitions of related terms

Genetic topics may be intimidating to learn about, as they feature many keywords that may be unfamiliar. Hence, below is a glossary of keywords which will be used throughout the article and their definitions:

• Chromosomes - Structures of packed DNA, which encode all of the proteins that are needed for survival. One set of chromosomes is inherited from each parent, and each set consists of 23 chromosomes
• Gamete - Reproductive cell (egg or sperm)
• Diploid - A cell consisting of the usual two sets of chromosomes
• Haploid - A cell which has only one set of chromosomes. These are usually the gametes
• Meiosis - The process by which gametes are made in the body

Definition of triploidy

Triploidy is defined as a chromosomal disorder that causes a foetus to inherit three copies of each chromosome, as opposed to the usual two. This results in inheriting 69 chromosomes instead of 46. Triploidy is almost always fatal in humans, as it results in developmental problems and birth defects, which make sustaining life impossible. 

Triploidy vs trisomy 

Triploidy can sometimes be confused with trisomy, which involves inheriting an additional copy of only one of the chromosomes. For example, trisomy 21 (often referred to as Down’s syndrome) is caused by inheriting three copies of chromosome 21, trisomy 18 (also known as Edward’s syndrome) is caused by inheriting three copies of chromosome 18, and so on. Trisomy syndromes tend to have a negative impact on life expectancies, but are not all immediately fatal like triploidy. The genetic difference between the two is that individuals with trisomy have 47 total chromosomes, and those with Triploidy have 69. 

Occurrence in other organisms

Humans are not the only living organisms that experience triploidy. This condition can also affect animals and plants, with various outcomes. Triploidy is induced in some fish in aquaculture and produces sterile but otherwise minimally affected fish.³ In agriculture, triploidy can cause changes in crop yield and can even make varieties of seedless fruits.⁴ 

Genetic basis of triploidy

In order to understand how a foetus with triploidy can form, we must understand the genetic mechanisms that can lead to the presence of three sets of chromosomes in one cell. 

Mechanisms of triploidy formation

Triploidy can occur via three potential mechanisms, caused by issues during egg fertilisation or meiosis (production of the egg or sperm). The mutations that cause triploidy tend to occur randomly and are not inherited from parents. The causes are outlined below. 

1. Dispermy

Dispermy happens when two sperm cells fertilise one egg cell. Each of the sperm cells has one set of chromosomes, which results in three sets being present in the fetus. Two of these are paternal and one is maternal.. This is the most common cause of human triploidy, being responsible for around two-thirds of all cases.⁵ 

2. Diandric triploidy

Accounting for over 20% of triploidy cases, diandric triploidy occurs when a haploid egg cell is fertilised by a diploid sperm cell.⁵ Although there are only two cells involved in this type of triploidy, the sperm has two sets of chromosomes and the egg has one. A diploid sperm cell is formed when there is an error during its production  (in meiosis), resulting in an incorrect or failed division of a cell with 46 chromosomes into two cells with 26 chromosomes. 

3. Digynic triploidy

10% of triploidy cases arise from a diploid egg cell.⁵ In this case, one sperm with 26 chromosomes fertilises an egg with 46 chromosomes. In this case, like with diandric triploidy, there is an error during meiosis which causes the gamete to have two sets of chromosomes. 

Chromosomal composition 

The inherited sex chromosomes can have different karyotype; XXX, XXY and XYY, where X can come from either the egg or sperm and Y can only be inherited from the sperm cell. These are usually used to categorise triploidy, but don’t really tell us much about the cause of the effects. In all these cases, the placenta does not form correctly, and the pregnancy is rarely viable. If the placenta is less affected, and the pregnancy is carried to full term, newborns and infants with triploidy are usually lost to the birth defects associated with the condition.⁶ 

Diagnosis and detection

Diagnosis of triploidy is usually obtained early during pregnancy. Certain aspects of the ultrasound may prompt the healthcare practitioner to order additional tests to check for the disorder in the fetus. These include karyotyping (analysis of chromosomes) of foetal cells obtained via amniocentesis or chorionic villus sampling. The diagnosis is usually confirmed after birth when cells can be collected from the baby. 

Summary 

Triploidy is a rare and typically fatal genetic condition. In this disorder, the embryo inherits three full sets of chromosomes. This means that their cells have 69 chromosomes instead of the usual 46. This leads to early pregnancy loss or severe developmental issues. Triploidy occurs in around 1-2% of pregnancies, being the cause of around 10% of miscarriages. This condition can arise via three possible mechanisms which are: dispermy (the fertilisation of one egg by two sperm cells), diandric triploidy (the fertilisation of a normal egg by a sperm with an extra set of chromosomes) or digynic triploidy (fertilisation of an egg with an extra set of chromosomes by a normal sperm cell). Unlike similar disorders like trisomy (where there is an extra copy of one of the chromosomes), triploidy affects all chromosomes and, hence, is usually fatal for the fetus or infant. This is due to things like abnormal placenta development, developmental issues and birth defects, which are not compatible with life. Diagnosis of triploidy occurs early on in the pregnancy by chromosome analysis of foetal cells following any abnormal ultrasound scans. It can be definitively confirmed after birth. 

Despite its devastating effects on humans, triploidy is sometimes exploited in fishing and agriculture. It is responsible for producing sterile fish or varieties of seedless fruits.