Examples of Controls in Eukaryotic Cells

X Chromosome Inactivation

Cells of a multicelled organism rarely use more than 5-10 percent of their genes at any given time. The remaining genes are selectively expressed. All cells in a multicellular organism inherited the same genes. Some of those genes are used in all cell types. During development, cells activate and suppress other genes in selective ways.

Mammalian females have two X chromosomes per cell. The gene products of only one X chromosome are needed. One X is inactivated in each cell. Inactivation is random. The other X chromosome is condensed and inactive and is called a Barr body. It is visible in micrographs. It may be either the maternal or the paternal X chromosome.

Because in some cells the paternal X chromosome is inactivated, while in other cells the maternal X chromosome is inactivated, each adult female is a mosaic of X-linked traits, called Lyonization.

This mosaic effect is seen in human females affected by anhidrotic ectodermal dysplasia in which a mutant gene on one X chromosome results in patches of skin with no sweat glands.

This animation (Audio - Important) describes how calico cats result from X Chromosome inactivation.

REVIEW: The mosaic effect in human females can be observed in

REVIEW: X chromosome inactivation is an example of _____ .

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