A chromosome is DNA folded until it can be moved
You asked about human chromosomes because a report printed 46,XX or a school diagram made them look like extra objects beside DNA. They are not a second substance. They are DNA, wrapped with protein, folded until a cell can drag the library through division.
Think of a library of 23 volumes. Each volume is one chromosome: a single long DNA molecule plus the proteins that pack it. In a working cell the volumes are unpacked as chromatin. When the cell divides, they condense into the shapes on a karyotype.
The real term is chromosome. If you can see it on a stained metaphase spread, you are looking at packed DNA, not at something other than DNA.
Twenty-three pairs is the usual human count
Typical human cells carry 23 pairs of chromosomes, 46 in all. One member of each pair comes from the egg, the other from the sperm. That is two copies of a book for each nuclear volume, with a special case on the sex chromosomes.
Twenty-two of those pairs are autosomes. They are numbered 1 to 22. The remaining pair is the sex chromosomes, X and Y. A karyotype is the picture of that set lined up.
Eggs and sperm are haploid: 23 chromosomes, not 23 pairs. Fertilization restores the pairs. If you only remember one number, remember 46 for most body cells and 23 for gametes.
- Typical body cell: 23 pairs, 46 chromosomes
- Autosomes: numbered 1 to 22
- Sex chromosomes: X and Y
- Egg or sperm: 23 chromosomes, one of each pair
Autosomes numbered 1 to 22, chromosome 1 the largest
The autosomes were numbered roughly by size. Chromosome 1 is the largest, about 249 million bases. Chromosome 21 is the smallest autosome, which is why the numbering and the actual lengths are not a perfect scale.
Larger chromosomes hold more DNA and, in general, more genes. They are not more important in a moral sense. A small chromosome can still carry a gene that a family knows by name. Chromosome 21 is small and still famous because an extra copy is visible on a karyotype and changes development. Scale and consequence are not the same variable.
When a report says a variant is on chromosome 7, it is naming one volume of the nuclear library. It is not naming a separate kind of inheritance. Autosomes follow the two-copy pattern: one from each parent.
46,XX, 46,XY, and the karyotypes that also exist
The typical shorthand is 46,XX or 46,XY: forty-six chromosomes, with two X chromosomes or one X and one Y. Those are common karyotypes. They are not the whole catalog of human sex chromosomes.
Sex chromosomes also vary. People are born with 47,XXY, with 45,X (sometimes written XO), with extra X or Y chromosomes, and with mosaicism, meaning different cells in one body carry different counts. This is a fact about chromosomes, not a lecture on anyone's life.
X and Y are not a simple male/female switch in every case. They are chromosomes with genes. SRY on Y is one important gene among others. If a report printed a karyotype you do not recognize, that is a question for a clinician, not for a webpage.
Short arm, long arm, and a label like 7q31
Each chromosome has a short arm (p, for petit) and a long arm (q). The centromere is the constriction that separates them. Band stains produce the numbers that follow: 7q31 is a region on the long arm of chromosome 7.
Those labels are how older maps, cytogenetic reports, and many gene pages still talk. A modern file may prefer chr7 and a GRCh38 base position. Both are addresses. They are not the same resolution.
A cytogenetic address can hold millions of bases. An rsID names a much smaller site. If you came from a microarray report, you are often looking at chunks. If you came from a SNP file, you are looking at letters.
- p: short arm
- q: long arm
- Centromere: the constriction between arms
- 7q31: a band address, coarser than a base position
Mitochondria keep their own small circle
Mitochondrial DNA is not one of the 23 nuclear volumes. It is a small circle inside mitochondria, present in many copies, inherited almost always along the maternal line.
People still say mitochondrial chromosome in some textbooks. The honest picture is a separate genome with its own inheritance pattern. Mixing it into a 46-count karyotype is a category error.
If you want that pamphlet in detail, it has its own guide. The only job of this paragraph is to keep it off the karyotype without pretending it is not DNA.
What a karyotype shows and what it hides
A karyotype can show extra or missing chromosomes, large deletions or duplications, and some rearrangements. Down syndrome is an extra chromosome 21. Turner syndrome is a missing X. Those are chromosomal findings, visible at a scale of millions of bases.
A karyotype cannot see a SNP. It cannot see most of the variants in a 23andMe file. It cannot read a gene. Different tests look at different scales of the same library.
Most chromosomal conditions are not inherited as a tidy Mendelian trait. They often arise as a new event when eggs or sperm form. Some structural rearrangements can be inherited. The pattern is not one pattern.
How chromosomes are dealt into eggs and sperm
Meiosis is the deal. Homologous chromosomes pair, swap segments by recombination, then separate so each gamete gets one of each pair. The child is not a photocopy of either parent. The volumes are shuffled.
Nondisjunction is a failure of that separation. It can produce a gamete with an extra chromosome or a missing one. That is a mechanical fact about cell division, not a verdict on a family.
Once you can see chromosomes as packed DNA with a count, inheritance stops being a slogan. It becomes a process with two copies, a shuffle, and a chance of error. The next guide is that process.
A chromosomal location is a public address
You can look up a gene by chromosome and position, or by symbol, without sending a karyotype or a VCF anywhere. genome.sh treats those names as public identifiers. The file that says which letters you carry can stay on disk.
A location is not a phenotype. 7q31 is an address. CFTR is a gene at an address. A person's trait is later, and leakier, than both. This page is informational, not a diagnosis.
NCBI Gene will also take a symbol and show you the chromosome. That is still a public address. A personal karyotype belongs with a genetics professional, not with a lookup box.
Questions
Are chromosomes the same as DNA?
Chromosomes are DNA packed with protein. They are not a second substance. Unpack them and you still have DNA. The nuclear genome is stored as chromosomes. Mitochondrial DNA is a separate circle.
How many chromosomes do humans have?
Typically 23 pairs, 46 in all, in body cells. Eggs and sperm have 23. Counts also vary: extra or missing chromosomes, and mosaicism, exist. 46 is the usual number, not the only number.
What are autosomes?
The numbered chromosomes, 1 to 22. They look the same in typical 46,XX and 46,XY karyotypes. Chromosome 1 is the largest. The remaining pair is the sex chromosomes, X and Y.
What is 46,XX and 46,XY?
Karyotype shorthand: 46 chromosomes with two X chromosomes, or 46 with one X and one Y. Those are typical. Other karyotypes, including XXY, 45,X, and mosaics, also occur.
Which human chromosome is the largest?
Chromosome 1. The autosomes were numbered roughly by size. Chromosome 21 is the smallest autosome. Y is small compared with X.
What is a karyotype?
A picture or report of the chromosome set, often written as 46,XX or 46,XY. It can show large extra, missing, or rearranged pieces. It cannot see single-letter variants.
Do mitochondria have chromosomes?
Mitochondria have their own small circular DNA, not one of the 23 nuclear pairs. Some texts call it a mitochondrial chromosome. Count it as a separate genome with maternal inheritance.
Can chromosome number vary?
Yes. Extra or missing chromosomes, and mosaicism, occur. Many of those changes arise when eggs or sperm form, rather than being passed as a simple family trait. A clinician, not a webpage, interprets a personal karyotype.