EVERYDAY SCIENCE

Detailed explanation-1: -The correct determination of the human diploid chromosome number as 46, by J-H Tjio and A Levan, at the University of Lund, Sweden, occurred 50 years ago, in December 1955; the finding was published in April 1956, ending a period of more than 30 years when the number had been thought to be 48.

Detailed explanation-2: -Some 60 years ago, two researchers, Joe Hin Tjio and Albert Levan, discovered that the number of chromosomes (karyotype) in humans was 46 chromosomes, that is, 23 pairs and not 48 as was thought previously (1).

Detailed explanation-3: -In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females.

Detailed explanation-4: -In plants and animals (including humans), chromosomes reside in the nucleus of cells. Humans have 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes (XX or XY), for a total of 46.

Detailed explanation-5: -Human chromosomes were probably first observed in cancer cells by Arnold in 1879. Hansemann in 1881 and Flemming in 1898 attempted to count the number in serial sections of mitotic cells producing crude estimates of approximately 24. Quite different results were produced in 1912 by de Winiwarter.