La réponse courte
Your DNA is not one continuous thread but 46 separate ones, each wound up with proteins into a structure called a chromosome. They come in 23 pairs — one of each pair from each parent. Twenty-two pairs look the same in everyone; the last pair, the sex chromosomes, differ.
Chromosomes are DNA molecules complexed with histone and non-histone proteins to form chromatin. Humans typically have 46 chromosomes in 23 pairs: 22 autosome pairs and one sex chromosome pair. Each has a centromere dividing it into a short (p) and long (q) arm, and telomeres capping its ends. Cytogenetic location — the notation used on this site's gene pages, such as 11p15.4 — refers to a chromosome, an arm and a banding position.
How to read a gene address
Gene locations are written in a fixed format. HBB is at 11p15.4: chromosome 11, short arm (p), band 15, sub-band 4. The gene pages on this site draw that position on a schematic chromosome so you can see roughly where in the genome a gene sits.
Sources
- National Human Genome Research Institute
Talking Glossary of Genomic Terms ↗ - MedlinePlus Genetics, U.S. National Library of Medicine
Help Me Understand Genetics ↗