AP BIOLOGY

Detailed explanation-1: -The presence and/or absence of the Y chromosome determine the maleness of the organism, while the inequality of the gene expression between males and females is balanced by the mechanism of X-inactivation.

Detailed explanation-2: -X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. Because X-inactivation is random, in normal females the X chromosome inherited from the mother is active in some cells, and the X chromosome inherited from the father is active in other cells.

Detailed explanation-3: -Mammalian females have two X chromosomes, while males have only one X plus a Y chromosome. In order to balance X-linked gene dosage between the sexes, one X chromosome undergoes inactivation during development of female embryos. This process has been termed X-chromosome inactivation (XCI).

Detailed explanation-4: -Inactivation of the single X chromosome also occurs in the male, but is transient and is confined to the late stages of first meiotic prophase during spermatogenesis. This phenomenon has been termed meiotic sex chromosome inactivation (MSCI).

Detailed explanation-5: -X-chromosome inactivation is responsible for sex chromosome dosage compensation in females (XX), and ensures that X-chromosomal genes are not expressed at twice the levels of expression in males (XY) [1]. It occurs during early female embryonic development [2], but the exact timing in humans is still elusive.