Chromosomes and karyotype
Human DNA is divided across 46 chromosomes, that is 23 pairs. 22 pairs are autosomes, the last pair determines sex (XX or XY). For each pair one copy comes from the mother, one from the father. The ordered overall picture of all chromosomes is called the karyotype.
23 pairs
The autosomes are numbered by size from 1 to 22, the 23rd pair are the sex chromosomes. That there are exactly 46 was only established with certainty in 1956. Because every pair is present twice, you carry two editions of almost every gene, which may be identical or different.
The karyotype
Arranging all chromosomes by size and banding pattern yields the karyotype. Every chromosome has a short arm (p) and a long arm (q), separated by the centromere. This picture makes large changes visible, such as an extra chromosome or missing pieces. Genome works at the finer level of single bases, the chromosome picture provides the map for it.
What lies outside the 46
Two portions of the genome follow their own rules. The mitochondria carry a tiny genome of their own, passed on almost exclusively through the mother. The Y chromosome is inherited unchanged from father to son. Both are therefore suited to tracing maternal and paternal ancestral lines.
What Genome measures. Which chromosome and which position a marker sits on is given in each gene article, for example the HLA region on chromosome 6. Because every autosome is present twice, you carry two alleles per position.
Related topics
Sources
- 1Tjio & Levan, 1956 The chromosome number of man. Hereditas 42:1–6. doi.org/10.1111/j.1601-5223.1956.tb03010.x
- 2Nurk et al., 2022 The complete sequence of a human genome (T2T-CHM13). Science 376:44–53. doi.org/10.1126/science.abj6987