❓ why
Why do some people have an extra copy of chromosome 21?
Human BiologyChromosomal Disorders
Quick Answer
An extra copy of chromosome 21 usually comes from a meiotic error called nondisjunction. This causes trisomy 21, the genetic cause of Down syndrome.
The Full Story
During the formation of eggs or sperm, chromosomes are supposed to separate evenly. If chromosome 21 fails to separate properly, a gamete may end up with two copies instead of one. After fertilization, the embryo then has three copies of chromosome 21 in its cells, a condition called trisomy 21. Down syndrome was first described in medical literature by John Langdon Down in 1866, and the chromosomal cause was identified by Jérôme Lejeune in 1959.
Key Facts
1.Trisomy 21 means three copies of chromosome 21
2.The chromosomal cause was identified in 1959
3.John Langdon Down described the condition in 1866
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