Not a Fact Anymore
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Not a fact anymore

The normal human chromosome number is 48.

What we know now

Most human body cells contain 46 chromosomes, arranged in 23 pairs. Eggs and sperm normally contain 23.

Why it changed

Better cell preparation and microscopy made chromosomes easier to separate and count. In 1955, Joe Hin Tjio found 46, correcting a number that had been accepted for about 30 years.

Status
Corrected
Category
Genetics
Accepted for
≈31 years
Accepted approximately
1920s–1955
Changed approximately
1955–1956

The correction applies to the normal chromosome number in most body cells. Mature red blood cells have no nucleus, eggs and sperm have 23 chromosomes, and cancers or chromosome disorders can produce other counts.

Tjio published the result with Albert Levan in 1956. Clearer chromosome preparations made the decisive recount possible.

Evidence

Sources and what they establish

Historical context

  • 1955: 46 Human ChromosomesNational Human Genome Research Institute

    States that 48 had been the accepted human chromosome number for about thirty years and describes Tjio's correction.

Current evidence

  • Chromosomes Fact SheetNational Human Genome Research Institute

    Explains that humans normally have 23 chromosome pairs, for a total of 46.

Related entries

Genetics

RNA cannot serve as hereditary material.1957 · OverturnedGenes occupy permanently fixed positions in chromosomes and cannot move around the genome.1950 · Overturned
Browse Genetics →

Around 1956

Immune tolerance was thought to prevent self-directed immune reactions from becoming a major cause of disease.1956 · ImmunologyThe laws of physics always treat a process and its mirror image identically.1957 · Physics
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