We have known for the greater part of the 20th century that mammalian females and males differ in both number and kind of sex chromosomes. While females have two X-chromosomes (XX), males have one X- and one Y-chromosome (XY). Both the X and Y carry genes that enhance female and male reproductive function, respectively. Because the Y-chromosome is relatively small and carries only a few genes, this sex chromosome difference means that females essentially have double the number of sex-chromosome genes compared to males. Yet, biochemical evidence indicates that this difference in chromosome number does not result in sex-specific differences in the total amount of X-encoded RNAs or proteins. In other words, female cells with two copies of every X-linked gene synthesized the same amount of protein products as males cells with half the number of genes. For years, this equation puzzled biologists. Clearly, some form of dosage compensation akin to those found in other sexual organisms must exist in mammals.
KeywordsFemale Cell Xist Gene Barr Body Xist Expression Cell BioI
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