Pregnancy is a complex process and can sometimes result in the development of an abnormal fetus. In fact, nearly 3% of all deliveries are associated with a major birth defect. Abnormal fetal development can result from either developmental factors or genetic disorders.

Developmental Factors

Developmental factors are associated with 10% of all birth defects. Developmental factors can be due to a teratogenic agent, infectious agent (e.g. rubella virus, herpes virus), or metabolic disorder (eg. diabetes, phenylketonuria). Teratogenic agent exposure is seen in less than 50% of all birth defect cases. Information important to teratogenic exposure defects includes the agent type, dose, length of exposure, and developmental stage at the time of exposure.

Genetic Disorders

Genetic disorders are a much more common cause of abnormal fetal development and are associated with 20-25% of birth defects. In fact, 42% of spontaneously aborted fetuses are associated with a genetic disorder. Genetic disorders are also the cause of nearly 4000 different human diseases.

A genetic disorder is a medical condition caused by abnormalities in an individual’s DNA, which can result in various health problems. In a typical human, there are 22 pairs of autosomes (non-sex chromosomes) and 1 pair of sex chromosomes, with one chromosome from each pair inherited from each parent. When a genetic disorder occurs, there is an alteration in either the number or structure of these chromosomes. This alteration can affect either the autosomes or the sex chromosomes. However, it is worth noting that autosomal disorders often tend to be associated with more severe birth defects than abnormalities in the sex chromosomes. In the following discussion, we will explore several types of genetic number disorders.

Chromosomal Number Disorders 

There are several different types of chromosomal number disorders that can occur amongst the autosomes. The general term for an abnormal number of chromosomes is aneuploidy. If nondisjunction occurs, where chromosomes fail to separate or disjoin properly, two specific types of aneuploidy can occur: trisomy and monosomy. Trisomy occurs when there is an extra chromosome (2n+1), while monosomy results from a missing chromosome (2n-1), where ‘n’ represents the number of chromosomes in a single set.

Monosomy

Monosomy, which involves the presence of a single copy of a particular chromosome in a diploid organism, typically leads to severe developmental abnormalities and is often not compatible with life. This situation can indeed result in spontaneous abortion (miscarriage) in many cases because the embryo’s genetic imbalance and missing genetic material are not viable.

Trisomy

Trisomy, which involves the presence of an extra copy of a particular chromosome, is more commonly observed and can lead to genetic disorders. Some trisomies, such as Trisomy 21 (Down’s Syndrome), are survivable, although they can result in a range of developmental and health challenges. The impact of trisomy can vary depending on the specific chromosome involved and the extent of the genetic imbalance.

Two additional important terms related to chromosomal number disorders are polyploidy and triploidy.

Polyploidy

Polyploidy refers to the presence of more than two complete sets of chromosomes in an organism’s cells. While most humans are diploid, meaning they have two sets of chromosomes (2n), polyploid organisms may have three (triploid, 3n), four (tetraploid, 4n), or more sets of chromosomes. Polyploidy is relatively less common in animals, including humans.

Triploidy

Triploidy is a specific type of polyploidy where an individual has three sets of chromosomes (3n). It often results from the fertilization of an egg by two sperm cells or, less commonly, from the fusion of a haploid sperm with a diploid egg. Triploidy is associated with significant developmental abnormalities and usually leads to miscarriage or stillbirth in humans. It’s considered a type of aneuploidy, where there is an abnormal number of chromosomes.

An Example of Autosomal Chromosome Number Disorder 

Down’s Syndrome

Down’s Syndrome can result from two main causes:

• Nondisjunction: Trisomy 21 is typically related to nondisjunction during meiotic division. If nondisjunction is related to the maternal side, maternal age is a significant factor. Older mothers have a higher risk of giving birth to a child with Trisomy 21 because the chances of nondisjunction increase with maternal age.
• Translocation: This form of Down’s Syndrome is less common and occurs when a portion of chromosome 21 breaks off and attaches itself to another chromosome (usually chromosome 14). This type can be hereditary and may involve a rearrangement of genetic material between different chromosomes. It’s important to note that translocation Down’s Syndrome is not typically associated with maternal age but may have a familial genetic component.

Down’s Syndrome can result in a variety of physical, intellectual, and developmental expressions. The specific features and their degree of severity may differ from one individual to another.