A very relatable incident in our world today, with a term that sounds abstract, is the term “Aneuploidy”
DEFINITION
Aneuploidy is a chromosomal anomaly that occurs when the number of chromosomes is not precisely a multiple of the haploid number (n). This indicates that one or more chromosomes, but not the entire set, have been gained or lost.
For context,
The normal human karyotype has 46 chromosomes (23 pairs), while
Aneuploid karyotype has 45 (monosomy), 47 (trisomy), etc.
ANEUPLOIDY VARIATIONS
There are four possible cases of aneuploidy. They are:
- Monosomy: This occurs when one chromosome is absent (2n-1). A prevalent example is Turner’s syndrome, where females only have one X chromosome.
- Trisomy: This is the presence of an extra chromosome (2n+1). A common example is Down syndrome—an additional copy of chromosome 21.
- Tetrasomy: This occurs when a person gains two homologous chromosomes (2n+2).
- Nullisomy: Loss of both homologous chromosomes (2n-2). This case often results in death.
CAUSES OF ANEUPLOIDY
- Nondisjunction: This is the most common factor causing aneuploidy, occurring when chromosomes fail to separate properly during cell formation (meiosis) and cell division (mitosis). It can happen due to various reasons such as maternal age, environmental factors like radiation and toxins, and genetic mutations.
Case example: Trisomy 21 (Down syndrome) results from nondisjunction of chromosome 21.
The diagram illustrates the distinction between normal and abnormal chromosome separation during cell division. Nondisjunction occurs when chromosomes fail to separate properly, leading to aneuploidy.
2. Anaphase Lag: Anaphase lag occurs during cell division, specifically during anaphase, when one chromosome fails to migrate to the spindle pole, resulting in a chromosome lagging. The lagging chromosome could be lost (2n-1) or end up in the wrong cell (2n+1).
The first diagram illustrates the process of cell division. In the first part, chromosomes are lined up, but in the second part, one chromosome, called “anaphase lag”, moves slowly, leading to aneuploidy, the wrong number of chromosomes in a cell.
3. Mosaicism in aneuploidy refers to a condition where an individual has two or more cell lines with different chromosomal numbers derived from a single fertilized egg.
EFFECTS OF ANEUPLOIDY
Some common resulting problems include:
- Autosomal effects e.g Down Syndrome, Edwards Syndrome and Patau syndrome leading to defects in intellectual ability and facial features.
- Sex chromosome effects e.g Turner Syndrome (45,X), Klinefelter Syndrome (47,XXY), Triple X Syndrome (47,XXX) causing defects such as stature disorders, infertility, learning disabilities.
WAYS TO MANAGE ANEUPLOIDY
The disorder has no cure; however, there are various ways to manage the condition.
- Early intervention (therapy, education)
- Surgical correction of congenital anomalies
- Hormonal therapy (e.g., growth hormone in Turner Syndrome)
Some methods of prevention include:
- Preimplantation genetic diagnosis (PGD) during In Vitro Fertilization (IVF)
- Maternal age considerations