Humans have 23 pairs of chromosomes and each of them carry several thousand genes. As you can see in figure 1, the chromosomes are different sizes. Humans with the standard number of chromosomes are considered to have a euploid karyotype. Any deviation in the number of chromosomes is termed aneuploidy. Due to instability in the genome, this occurs frequently in cancer cells. Chromosomes can be visualized using a microscope; the corresponding picture made is also termed karyotype.
There are over 20,000 genes in the human genome. Each gene is coded for in a specific location on each chromatid called the genetic locus. The second copy of that gene is on the other sister chromatid. This 2-copy system offers redundancy in the genetic code. Twenty-two of the 23 chromosomes are autosomes, meaning they look the same in males and females. The 23rd chromosome pair looks different in males and females. Females have a larger sex chromosome (X); men have a much smaller chromosome (Y). Thanks to the Human Genome Project every gene has been mapped to the corresponding location on the chromosome.
Image courtesy of U.S. National Library of Medicine
