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Molecular Biology, 4/e
Robert F. Weaver

Genomics and Proteomics

X Inactivation

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.



1

A 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.
2

Which 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
3

A 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.
4

Once a cell undergoes X chromosome inactivation, all of its daughter cells will have the same X chromosome inactivated.
A)True
B)False
5

X chromosome inactivation equalizes the gene dosage between mammalian males and females.
A)True
B)False