SEX CHROMOSOME INACTIVATION

=X Chromosome Inactivation= Sex chromosome aneuploids are actually relatively common especially when compared with autosomal aneuploids. Because many autosomal chromosomes contain so many genes that are crucial to the normal development of an individual; changes in the number of autosomal chromosomes are not generally tolerated (= lethal). Human chromosomes are numbered according to size, chromosome 1 being the largest. You will notice that those autosomal aneuploids which are tolerated are some of the smaller chromosomes, presumably containing fewer genes. If we look at the sex chromosomes however, changes are relatively tolerated and can have little affect (when compared with other cases of aneuploidy). Two common sex chromosome aneuploids are listed on the previous page, however, it is also possible for females to have an extra X chromosome or males to have an extra Y chromosome and never even know it! Even more remarkable is the fact that the loss of an entire chromosome X (Turners Syndrome XO) is not lethal. There are two reasons why changes to the number of sex chromosomes are so tolerated. Firstly, the Y chromosome contains very few genes and none of these are crucial for normal development (remember half the population do not have a Y chromosome). Secondly, an individual only requires one X chromosome to survive (remember half the population only have one X chromosome). Additional X chromosomes can play a slight developmental role but are soon switched off. Regardless of how many X chromosomes an individual has, all but one are compacted down into a dense mass called a barr body. These barr bodies can be seen under a microscope and represent the X chromosomes that have been switched off. A normal male will contain no barr bodies and a normal female will contain one barr body (as all but one X chromosome are switch off). [image:http://i.imgur.com/eruhOkO.png]