As a consequence, if a female is heterozygous for a sex-linked trait, about half of her cells will express one allele and the other cells well express the alternate allele. Examples of this type of mosaicism are coloration in calico cats and normal sweat gland development in humans. X chromosome inactivation is associated with DNA methylation. A recently discovered gene, XIST is active only on the Barr body. The product of the XIST gene, X-inactive specific transcript , is an RNA that interacts with the X chromosome and maintains its inactivation.
Many questions are yet to be answered.
Alterations of chromosome number or structure cause some genetic disorders Meiotic errors and mutagens can cause major chromosomal changes such as altered chromosome numbers or altered chromosomal structure. Meiotic or mitotic error during which certain homologous chromosomes or sister chromatids fail to separate. Results in one gamete receiving two of the same type of chromosome and another gamete receiving no copy. The remaining chromosomes may be distributed normally.