Chromosomes and karyotype
Most human body cells have 46 chromosomes in 23 pairs: 22 pairs of autosomes and usually two sex chromosomes, often XX or XY. Chromosome number and composition can vary. An ordered view of the chromosomes examined is called a karyotype.
Key points
- Most body cells have 46 chromosomes, though natural variations occur.
- Diploid means that chromosomes are typically present in pairs.
- Outside the 46 lie two special cases: the mitochondrial DNA and the Y chromosome as the paternal lineage.
23 pairs
Autosomes are numbered from 1 to 22; the 23rd pair comprises the sex chromosomes. Most people have a diploid set of 46 chromosomes, but natural differences in number and structure occur. The copies in a chromosome pair can carry the same or different alleles at a given position.
The karyotype
A karyotype arranges chromosomes by size and banding pattern. Many chromosomes have a short arm (p) and a long arm (q) on either side of the centromere. The method can reveal larger changes such as extra chromosomes or large missing segments; smaller variants require other methods. Genome analyses sequence data and does not produce a karyotype.
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