Jlq

Xist inactive

Coating by Xist RNA spreads outwards from Xiïigerie

Active Inactive

FIGURE 10.17 X-inactivation Involves the Xist Gene and Xist RNA

A) Originally, both X chromosomes transcribe Xist RNA from the Xist gene. B) The X chromosome that will remain active is methylated in the Xist gene region, which inactivates the Xist gene. The Xist RNA coats the other X chromosomes and inactivates it. C) The inactive X chromosome is almost entirely methylated, except for the Xist gene (together with a few aberrant loci that are exempted from X-inactivation and are not shown here). This causes the X chromosome to transform largely into heterochromatin. Only the Xist gene itself remains active.

FIGURE 10.18 X-Inactivation Causes Skin Patterning in Mice

A female mouse is shown that is heterozygous for an X-linked gene involved in hair pigmentation.

A) All cells contains two X chromosomes, one with a functional copy (+) of the hair color gene and the other with a defective copy (-).

B) During development, different X chromosomes are inactivated at random in different ancestral cells. Each ancestral cell divides and gives rise to a patch of cells on the body surface. C) The result is a mixture of zones of different skin colors. The white (mutant) zones appear when the active X chromosome carries the defective hair color gene. The dark zones appear when the active X chromosome carries a wild-type hair color gene.

A) Stem cell - heterozygous

FIGURE 10.19 A Calico Cat

Calico coloration is seen only in female cats. It occurs because genes for fur color are carried on the X chromosomes. If a female cat is heterozygous for mutant and wildtype alleles, random inactivation of the two X chromosomes in different regions creates the pattern. White patches are due to cells where the activated X chromosome contains the mutant allele.

FIGURE 10.19 A Calico Cat

Calico coloration is seen only in female cats. It occurs because genes for fur color are carried on the X chromosomes. If a female cat is heterozygous for mutant and wildtype alleles, random inactivation of the two X chromosomes in different regions creates the pattern. White patches are due to cells where the activated X chromosome contains the mutant allele.

In any given cell, only one X chromosome is active and so only one of the two alleles will be expressed. The descendents of a particular ancestral cell stay together and form regions of skin with the same color. Hence, some regions of the coat are wildtype and others show the mutant color.A similar effect is seen in calico cats (Fig. 10.19).

Was this article helpful?

0 0

Post a comment