23 pairs are called autosomes and one pair is called sex chromosomes. The female has 22 pairs of autosomes and 2 X - chromosomes where as the males have 22 pairs of autosomes and one X chromosome and one Y chromosome. The sex is determined by the sex chromosomes. It is the male that determines the sex of the baby and not the female as often believed. The chromosome number is fixed for each species and in human it is 23 pairs.
Alterations do occur in chromosomes either in sets or in their structure.
With regard to gene mutations that is gene based genetic disorders the number complied are about 5,000. Many have yet to be detected since of there are about 1,00,000 genes in humans. The effect of genome mutations like Down syndrome, Turners syndrome or Klinefelter syndrome are diagnosable at birth or even in prenatal period at 12 weeks pregnancy. The disorders due to chromosomal changes are Down syndrome. Klinefelter and turner syndrome Down - translocation and mosaics to be brief.