structural:summarization
Expression of X-linked disorders in heterozygous females

Since males only have one copy of the X chromosome, all expressed X-chromosomal genes (or alleles, in the case of multiple variant forms for a given gene in the population) are located on that copy of the chromosome. Females, however, will primarily express the genes or alleles located on the X-chromosomal copy that remains active. Considering the situation for one gene or multiple genes causing individual differences in a particular phenotype (i.e., causing variation observed in the population for that phenotype), in homozygous females it does not particularly matter which copy of the chromosome is inactivated, as the alleles on both copies are the same. However, in females that are heterozygous at the causal genes, the inactivation of one copy of the chromosome over the other can have a direct impact on their phenotypic value. Because of this phenomenon, there is an observed increase in phenotypic variation in females that are heterozygous at the involved gene or genes than in females that are homozygous at that gene or those genes. There are many different ways in which the phenotypic variation can play out. In many cases, heterozygous females may be asymptomatic or only present minor symptoms of a given disorder, such as with X-linked adrenoleukodystrophy.