Extra chromosomes primarily arise from errors during cell division, specifically in meiosis. Here's a breakdown:
Meiosis and Chromosome Distribution
Meiosis is the process of cell division that creates sperm and egg cells (gametes). During meiosis, chromosome pairs normally separate, ensuring each gamete receives one chromosome from each pair.
Nondisjunction: The Root Cause
The main cause of extra chromosomes is a phenomenon called nondisjunction. This occurs when chromosomes fail to separate correctly during meiosis. This can happen in either of the two meiotic divisions (meiosis I or meiosis II).
According to the provided reference, "During meiosis (cell division), there is a chance that your cells do not divide as they should, causing an additional copy of a cell to join a pair."
Types of Nondisjunction
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Meiosis I Nondisjunction: Homologous chromosomes fail to separate. This results in two gametes with an extra chromosome and two gametes missing a chromosome.
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Meiosis II Nondisjunction: Sister chromatids fail to separate. This results in one gamete with an extra chromosome, one gamete missing a chromosome, and two normal gametes.
Trisomy: The Result of an Extra Chromosome
When a gamete with an extra chromosome (due to nondisjunction) fertilizes a normal gamete, the resulting zygote will have three copies of that chromosome instead of the usual two. This condition is called trisomy.
The reference notes that normally, each pair would contain two chromosomes, but a third chromosome forms and attaches to the pair, creating a trisomy, which literally translates to “three bodies.”
Examples of Trisomy
- Down Syndrome (Trisomy 21): Individuals with Down syndrome have an extra copy of chromosome 21.
- Edwards Syndrome (Trisomy 18): Individuals with Edwards syndrome have an extra copy of chromosome 18.
- Patau Syndrome (Trisomy 13): Individuals with Patau syndrome have an extra copy of chromosome 13.
Summary Table: Causes of Extra Chromosomes
| Cause | Description | Stage of Meiosis | Resulting Gametes |
|---|---|---|---|
| Nondisjunction | Failure of chromosomes to separate properly during cell division (meiosis). | Meiosis I or II | Some gametes with extra chromosomes, some with missing chromosomes. |
| Trisomy | The presence of an extra chromosome in the zygote, resulting from fertilization with an affected gamete. | After Fertilization | Offspring has three copies of a chromosome instead of the normal two |
