The human karyotype contains 22 pairs of autosomal chromosomes and a pair of sex chromosomes. The normal karyotype for a female contains a pair of X chromosomes, whereas the normal karyotype for a male contains an X and a Y chromosome . 
Variations in the human karyotype can lead to genetic disorders. These can be sex linked genetic such as Klinefelter's Syndrome or Turner syndrome. These have the genotypes 47 XXY and 45 XO, respectively, and cause a change in appearance and infertility. When a Y chromosome is present, the person always primarily appears male.
Trisomies can also occur such as Down's Syndrome (trisomy 21) or Edwards Syndrome (trisomy 18). A trisomy is where three of one chromosome is present in the karyotype. Trisomies tend to occur in smaller chromosomes because the cell is more able to deal with the extra mass. 
- ↑ Hartl,D.Jones,W. (2008)Genetics: Analysis Of Genes And Genomes, 7th edition,Jones and Bartlett Publishers. Pg 256.
- ↑ http://www.nlm.nih.gov/medlineplus/klinefelterssyndrome.html