In which type of Trisomy 21 (Down Syndrome) are chromosome numbers 47 in all cell?

Answer 1

Someone with Down syndrome has 47 chromosomes because they have an extra copy of chromosome 21. Normally, people just have 2 copies of every chromosome for a total of 46 chromosomes. In Down syndrome, there are 3 copies of chromosome 21 instead of 2 copies, which leads to an extra chromosome.

Sperm and egg should each have one copy of every chromosome. If the sperm has one copy of chromosomes 1-23 and the egg has one copy of chromosomes 1-23, then when the sperm and egg fuse, the resulting zygote (fertilized egg cell) will have two copies of chromosomes 1-23 for a total of 46 chromosomes.

However, if there is an error called nondisjunction when making the sperm or the egg, this could lead to the sperm or egg having too few or too many chromosomes. In the case of trisomy 21, either the sperm or the egg had 2 copies of chromosome 21 instead of just one copy like it's supposed to. So then when the sperm and egg fused, there were 3 copies of chromosome 21 instead of just 2, which leads to Down syndrome.

Answer 2

Humans have 23 pairs of chromosomes, or 46 chromosomes. Somatic cells (body cells) have 46 chromosomes, arranged into 23 homologous pairs. Thus, trisomy 21 (the addition of an extra chromosome) causes the somatic cells to have 47 chromosomes. The gametic cells (sperm and egg cells, used for reproduction) have 23 single chromosomes, as they are not yet paired up.