How many chromosomes do humans have?
Pairs, chromatids, the cells that differ, and why apes have 48
Matic Broz, PhD9 min
Humans have 46 chromosomes, arranged in 23 pairs, in almost every cell that has a nucleus. Twenty-two of the pairs are autosomes, numbered 1 to 22, and the 23rd pair is the sex chromosomes: usually XX in females and XY in males. Sperm and egg cells carry one chromosome from each pair, 23 in total, and fertilization restores 46.
Both familiar answers are therefore right: 23 counts the pairs, or the chromosomes in a sperm or egg, and 46 counts the chromosomes in a body cell. The number also shifts with the cell and the moment. A cell that has copied its DNA still has 46 chromosomes but 92 chromatids, a red blood cell has none, and some liver cells carry 92 or more.
Why the answer is 46, or 23 pairs
A typical human body cell has 46 chromosomes in 23 pairs. Twenty-two pairs, the autosomes, look the same in males and females, while the 23rd pair, the sex chromosomes, differs: two X chromosomes in females, one X and one Y in males.[1]
Each pair holds one chromosome inherited from the mother and one from the father. A cell with two complete sets like this is called diploid, written 2n, and a cell with a single set is haploid, written n. For humans, n is 23 and 2n is 46.
Meiosis, the division that makes sperm and eggs, halves the count by sending one member of each pair into each cell.[2] A sperm therefore carries 22 autosomes and either an X or a Y, and an egg carries 22 autosomes and an X. Fertilization brings 23 from each parent together, and every later division in the embryo copies the full set of 46.
Females and males have the same total. Some people have other combinations of sex chromosomes, described below, and sex development also depends on genes on other chromosomes, so XX and XY describe the usual karyotypes rather than every person.
Chromosomes and chromatids: where 92 comes from
Before a cell divides, it copies its DNA, and each chromosome becomes two identical sister chromatids held together at a constriction called the centromere.[3] The joined pair still counts as one chromosome. From replication until the chromatids are pulled apart in mitosis, a human cell has 46 chromosomes made of 92 chromatids.
Counted the other way, a cell that has not yet copied its DNA has 46 chromosomes of one chromatid each, so 46 chromatids. The chromosome number stays at 46 through the whole cycle; only the number of DNA copies doubles and halves.
Eggs show how far the two counts can drift apart. Meiosis takes two divisions, and a vertebrate egg is released partway through the second, arrested at a stage called metaphase II until fertilization.[2] A human egg at ovulation already has its final 23 chromosomes, but each still consists of two chromatids, 46 in all. A sperm has finished both divisions and carries 23 chromosomes of one chromatid each.
Which human cells do not have 46 chromosomes?
By number, most cells in the body have no chromosomes at all. Hatton and colleagues counted about 36 trillion cells in a reference adult man, and red blood cells and platelets, neither of which has a nucleus, made up about 29 trillion of them.[4] Only about 7 trillion cells carry nuclear chromosomes, which is why multiplying 46 by the total number of cells in the body overstates the chromosomes a body holds.
| Cell or stage | Chromosome sets | Chromosomes | Chromatids and notes |
|---|---|---|---|
| Mature red blood cell | 0 | 0 | No nucleus |
| Platelet | 0 | 0 | A fragment of megakaryocyte cytoplasm, no nucleus |
| Sperm | 1 (haploid) | 23 | 23 chromatids; 22 autosomes plus X or Y |
| Egg at ovulation | 1 (haploid) | 23 | 46 chromatids until meiosis finishes at fertilization |
| Typical body cell before DNA replication | 2 (diploid) | 46 | 46 chromatids; 22 autosome pairs plus XX or XY |
| Body cell after DNA replication, before division | 2 (diploid) | 46 | 92 chromatids |
| Skeletal muscle fiber | 2 in each nucleus | 46 in each of thousands of nuclei | One very large cell; each nucleus is diploid |
| Tetraploid liver cell (hepatocyte) | 4 | 92 | In one nucleus, or in two nuclei of 46 each |
| Octoploid or higher liver cell | 8 or more | 184 or more | About 5% of hepatocytes in normal livers |
| Mature megakaryocyte | 16 (most common) | 368 | One large lobed nucleus |
Cells with extra chromosome sets are rare overall: Hatton and colleagues estimated that only about 1% of nucleated cells are polyploid.[4] The liver is the main exception among ordinary tissues. In 13 normal adult livers, Bou-Nader and colleagues classified 73.5% of hepatocytes as diploid, 21.7% as tetraploid and 5.4% as octoploid or higher. Just over half of the polyploid cells had two nuclei rather than one enlarged nucleus.[5]
Megakaryocytes, the bone marrow cells that shed platelets, go further. They copy their DNA several times without completing division, ending most often at 16N: sixteen chromosome sets, or about 368 chromosomes, in a single lobed nucleus. The platelets they release are pieces of their cytoplasm and inherit none of those chromosomes.[6]
Is mitochondrial DNA a 47th chromosome?
Not in any standard count. Mitochondria, the compartments that make most of a cell's energy, carry their own small circular DNA molecule, 16,569 base pairs long in the human reference sequence.[7] It holds 37 genes, and most cells contain thousands of mitochondria, each with one or more copies of it.[8]
Sequence databases sometimes call this molecule the mitochondrial chromosome, but it is a poor fit for the count of 46. It sits outside the nucleus, a nuclear chromosome set is more than 180,000 times longer, and a cell holds thousands of copies rather than two. Counting it would make the total depend on how many mitochondria a cell happens to have.
Can a human have 45 or 47 chromosomes?
Yes. Laboratories describe a person's chromosomes as a karyotype, written as the total count, a comma, the sex chromosomes, and then any change. 46,XX and 46,XY are the usual female and male karyotypes. 47,XXY means 47 chromosomes including two X chromosomes and a Y, 47,XY,+21 means a male with an extra chromosome 21, and 45,X means 45 chromosomes with a single X and no second sex chromosome.
| Karyotype | Total | Condition | How common |
|---|---|---|---|
| 47,XX,+21 or 47,XY,+21 | 47 | Down syndrome from trisomy 21 | About 95% of people with Down syndrome |
| 46, with extra chromosome 21 material attached to another chromosome | 46 | Translocation Down syndrome | About 3% of people with Down syndrome |
| 45,X | 45 | Turner syndrome | About half of people with Turner syndrome, which affects about 1 in 2,000 female newborns |
| 47,XXY | 47 | Klinefelter syndrome | About 1 in 650 male newborns |
| 45, with two chromosomes joined (balanced Robertsonian translocation) | 45 | Usually no health effects | About 1 in 800 people |
About 5,775 babies with Down syndrome are born each year in the United States. About 95% of people with Down syndrome have three separate copies of chromosome 21 in every cell, and so 47 chromosomes. In about 3%, the extra chromosome 21 material is attached to another chromosome and the total stays at 46; the remaining 2% are mosaic, with some cells of 46 and some of 47.[9]
The sex chromosomes vary too. About half of people with Turner syndrome have a 45,X karyotype; the others have a partly missing or rearranged X, or a mosaic mix of cells.[10] Klinefelter syndrome, usually 47,XXY, affects about 1 in 650 male newborns, and about 10% of affected people are mosaic, with some 46,XY and some 47,XXY cells.[11]
A count of 45 does not always mean missing genes. Chromosomes 13, 14, 15, 21 and 22 have short arms that carry repeated ribosomal DNA, and two of them can fuse into one in a Robertsonian translocation. About 1 in 800 people carry one, with 45 chromosomes and no health effects, but they face higher risks of infertility, pregnancy loss and, when chromosome 21 is involved, a child with Down syndrome.[12]
Autism is not defined by chromosome count either. Autistic people typically have 46 chromosomes; clinicians diagnose autism from development and behavior, and research points to many genetic and environmental factors rather than one cause.[13] Small deletions, duplications and sequence variants can change genes without changing the visible total of 46.
Why 46 and not 48?
For more than 30 years, textbooks said humans had 48 chromosomes. Theophilus Painter reached that number in 1923 from thin slices of testicular tissue, in which crowded, overlapping chromosomes were hard to tell apart.[14][15]
The correction came from better preparation rather than sharper eyes. In Lund at the end of 1955, Joe Hin Tjio and Albert Levan treated cultured cells from human embryos with colchicine, which halts division when chromosomes are most compact, and squashed the cells flat instead of slicing them. Their 1956 paper reported 46 in the cultures from all four embryos, with only single cells deviating.[16][15]
The old number survives in our closest relatives. Chimpanzees, bonobos, gorillas and orangutans all have 48 chromosomes, as the chromosome counts by species show, and human chromosome 2 explains the difference.
In 1991, IJdo and colleagues found two runs of the telomere repeat TTAGGG, the sequence that normally caps chromosome ends, facing each other head to head in band 2q13 of human chromosome 2. They concluded that the site marks where two ancestral ape chromosomes fused end to end.[17]
That fusion also shows what the number 46 does not measure. It lowered the count by two without discarding the DNA it joined. One set of 23 holds about 3.1 billion base pairs, chromosome 1 carries about 2,000 protein-coding genes while the Y carries 61, and the 46 chromosomes of one cell hold about two meters of DNA. The count describes how that DNA is packaged, not how much there is.
