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Every species is characterised by a specific number of chromosomes in the somatic cells. For example, onion cell has 6 pairs of chromosomes.
But if we talk about the human cells, there are 23 pairs or 46 chromosomes. Out of this set of 23 pairs/46 chromosomes, 22 pairs (or 44) chromosomes are called as autosomes. The remaining one pair is of the sex chromosomes (as we know, in males this is XY and in females it is XX).
It is to be noted here that the 23 pairs/46 chromosomes represent the diploid number (2n) in somatic cells. After meiosis, the four sex cells (sperms/ova) formed contain the haploid number (n) of chromosomes, which is the half of 46 = 23. In these 23 chromosomes, 22 are autosomes and one sex chromosome.
This haploidy is important because when sperms and ova unite during sexual reproduction, they form zygote, which again has the diploid number of chromosomes.
n (sperm) + n (ova) = 2n (zygote)
Haploid state Diploid state
Now, there is another case in which through genetic mutations, abnormalities in cell division etc. ; there could be extra or less chromosomes than the normal number. For example, in case of Down Syndrome, the 21st set of chromosome has 3 instead of the normal two chromosomes (trisomy), making the total 47 (instead of the normal 46).
But, if we consider the normal circumstances, the human cells have 46 chromosomes in the somatic cells and 23 chromosomes in the gamete/sex cells.
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