Edwards syndrome, also known as Trisomy 18, is a genetic condition affecting development and growth. It is characterized by an extra copy of chromosome 18, leading to physical and intellectual challenges. Babies often have low birth weight, heart defects, and distinctive features like a small head, small jaw, and clenched fists. The condition impacts multiple organ systems, contributing to a poor prognosis.
Understanding Genetic Inheritance
Genetic traits are passed from parents to offspring through genes, which come in different versions called alleles. Two primary patterns are dominant and recessive inheritance. A dominant trait requires only one copy of a specific gene variant for it to be expressed. If an individual inherits one altered gene from a parent, they will exhibit the trait.
A recessive trait only appears if an individual inherits two copies of the gene variant, one from each parent. If a person has only one copy of a recessive allele, they are considered a “carrier” and typically do not show symptoms. These Mendelian patterns describe how single genes on non-sex chromosomes affect traits.
Edwards Syndrome and Chromosomes
Edwards syndrome is a chromosomal disorder, not following typical dominant or recessive inheritance patterns. The defining characteristic is the presence of an extra copy of chromosome 18, meaning individuals have three copies instead of the usual two; this condition is therefore referred to as Trisomy 18. The additional genetic material from this extra chromosome disrupts normal development, leading to the various features associated with the syndrome. Since it involves an entire extra chromosome rather than a specific gene mutation, it does not fit the Mendelian inheritance model. The issue is an abnormal number of chromosomes, specifically chromosome 18.
How Trisomy 18 Develops
Trisomy 18 most commonly occurs through an error in cell division called non-disjunction. This error typically happens during the formation of egg or sperm cells (meiosis). During non-disjunction, chromosomes fail to separate properly, resulting in a reproductive cell that carries an extra copy of chromosome 18. If such a cell contributes to conception, the resulting embryo will have three copies of chromosome 18 in every cell. This non-disjunction event is random and not inherited, though the likelihood of this error increases with maternal age.
When Edwards Syndrome is Inherited
While most Edwards syndrome cases are not inherited, rare instances can occur due to a chromosomal rearrangement called a Robertsonian translocation. A parent might be a “balanced carrier” of a translocation involving chromosome 18; a balanced carrier has the correct amount of genetic material, but a piece of chromosome 18 has become attached to another chromosome. Balanced carriers typically do not show symptoms because they have all necessary genetic information, just rearranged. However, when they produce reproductive cells, this rearrangement can lead to an unbalanced form in offspring, resulting in an extra copy of chromosome 18 material, or partial Trisomy 18. Even in these cases, the underlying mechanism is a chromosomal structural abnormality rather than a dominant or recessive gene mutation.
Understanding Recurrence Risk
For families with a child with full Trisomy 18 from non-disjunction, the risk of recurrence in future pregnancies is generally low, estimated to be around 0.5% to 1%. While low, this risk is slightly higher than the general population’s background risk. The chance of recurrence increases with maternal age, but the event remains random. If Edwards syndrome is due to a parental balanced translocation, the recurrence risk is significantly higher, potentially up to 20% depending on the specific translocation and parent’s carrier status. Genetic counseling becomes important for families to understand their specific risks and explore options for future pregnancies, and genetic testing can help determine if a translocation is present, guiding accurate risk assessment.