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X Inactivation
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Why does X-inactivation result in chimeras, individuals with genetically distinct areas of their bodies? In female mammals one X chromosome in every cell is inactivated. The inactivation occurs early in development when the embryo consists of only a few cells. The inactivation is random – one X chromosome may be turned off in one cell and the other X chromosome inactivated in a neighboring cell. Once a chromosome is turned off it remains turned off in all descendent cells. With respect to their X chromosomes, female mammals are chimeras. A chimera is an organism composed of different genotypes. Some areas of their bodies have one X chromosome turned off and other areas have the other X chromosome inactivated. An example of X-linked chimeras that can be seen visually are female cats that are heterozygous at the orange coat color locus. Orange is a locus on the X chromosome. Females with one allele for orange color and one for non-orange (tortoise shell and calico cats) have blotches of orange and other colors all over their bodies. Each blotch of color is composed of cells descended from a single cell in the embryo at the time of X-inactivation.

View the animation below, then complete the quiz to test your knowledge of the concept.

1A Barr body is typically found in the nucleus of
A)female gametes only.
B)male gametes only.
C)female somatic cells only.
D)male somatic cells only.
E)both male and female somatic cells.

2Which of the following statements about the Xist gene is TRUE?
A)It is located in the Xic region of the X chromosome.
B)It encodes an RNA that will coat an X chromosome to promote its compaction.
C)It functions in the initiation phase of X inactivation.
D)A and B
E)A, B and C

3A cell with three X chromosomes will contain
A)no Barr bodies.
B)1 Barr body.
C)2 Barr bodies.
D)3 Barr bodies.
E)4 Barr bodies.

4Once a cell undergoes X chromosome inactivation, all of its daughter cells will have the same X chromosome inactivated.

5X chromosome inactivation equalizes the gene dosage between mammalian males and females.

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