HUMAN CHROMOSOMES

The normal diploid (2N) chromosome number in human being is 46.

It was given by T.H. Tjio and A. Levan in 1956.

The chromosome complement of a cell depicting the number, size and form of the chromosome as seen in metaphase of mitosis is called karyotype.

The chromosomes are morphologically numbered into 7 groups. (size and position of centromere) by Denver and Colorado (1960)

Group (A)            : 1-3 chromosomes of largest size and submetacentric or metacentric centromere.

Group (B)            : 4-5 chromosomes with less larger size, submetacentric

Group (C)            : 6-12 chromosomes with medium sized and submetacentric centromere.

Group (D)            : 13-15 chromosomes, shorter than group 'c' with centromere near the end (Arcocentric). They are sat chromosomes or satellite.

Group (E)            : 16-18 chromosomes, short sized, with median or submedian centromere.

Group (F)            : 19-20 chromosomes, short sized with median centromere.

Group (G)           : 21-22 chromosomes, smallest in size, Acrocentric and are also satellites.

X chromosome is placed in 'C' group due to its larger size and submedian centromere and Y-chromosome is placed in group 'G' due to its short size and terminal centromere.