Edwards syndrome is not a duplication in the traditional sense of a segment of a chromosome being copied. Instead, it's caused by trisomy 18, which means there are three copies of chromosome 18 instead of the usual two. While technically not a segmental duplication, the overall effect is an increase in genetic material from chromosome 18.
Here's a breakdown:
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Normal Chromosomes: Humans typically have 23 pairs of chromosomes (46 total), one set inherited from each parent.
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Edwards Syndrome (Trisomy 18): In Edwards syndrome, an individual has three copies of chromosome 18 in each cell. This extra genetic material disrupts normal development.
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Mechanism: This usually occurs randomly during the formation of the egg or sperm cell. The chromosomes fail to separate correctly (nondisjunction), resulting in a gamete (egg or sperm) with an extra copy of chromosome 18. When that gamete fertilizes a normal gamete, the resulting embryo has three copies of chromosome 18.
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Why Not "Duplication"? While a segment of a chromosome can be duplicated within the chromosome itself, Edwards Syndrome is due to having a whole extra chromosome. Therefore, using the term "duplication" can be misleading, even though the overall effect of more chromosome 18 material is present.
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Impact: The presence of this extra chromosome 18 material significantly affects development, leading to a range of health problems, including heart defects, kidney problems, and severe developmental delays.
In summary, while it's not technically a "duplication" of a chromosome segment, Edwards syndrome involves extra genetic material from chromosome 18 because the affected individual has three copies of the entire chromosome.
