In reptilian times, the X and the Y were an ordinary pair of chromosomes until the male-determining gene landed on the Y. Thereupon the Y started shedding the genes it held in common with the X and shriveled to a fraction of its former size.
Both sex chromosomes evolved from an ordinary pair of chromosomes more than 200 million years ago (Science, 29 October , 1999 p. 964).
It is interesting that tumors diagnosed as ordinary lipomas occasionally display rings and/giant chromosomes, which were found in 3% [ 3], 6% [ 5], and 2% [ 6] of ordinary lipoma samples in three different studies.
Mammalian sex chromosomes arose from an ordinary pair of autosomes after the proto-Y acquired a male-determining gene and degenerated due to suppression of X-Y recombination.
The X and Y chromosomes evolved from an ordinary pair of autosomes via a process of gene loss on the Y [ 1], which resulted in an imbalance of X gene dosage with the autosomes (1X: 2A gene dosage) in males.
In an ordinary cell of the human body, there are 46 chromosomes arranged in 23 pairs, 22 pairs of which are autosomes (chromosomes other than sex chromosomes), with the remaining pair being the sex chromosomes, designated XX in females and XY in males.
In ordinary lipoma, however, the presence of a supernumerary ring chromosome is a rare finding [ 3, 7, 11].
Sex chromosomes are generally thought to have evolved from ordinary "autosomal" chromosomes.
Heteromorphic sex chromosomes have arisen independently in many species from ordinary autosomes (Bull 1983; Charlesworth 1996).
First, monozygotic (MZ) twins share all their inherited parental chromosomes and are therefore genetically identical, whereas dizygotic (DZ) twins, like ordinary full siblings, share on average only half of their parental chromosomes and therefore 50% of inherited genetic variation.
