An X-linked trait is one that is controlled by alleles located on the X-chromosome. The X-chromosome contains roughly 4% of all genes but is responsible for nearly 10% of all single-gene or “Mendelian” diseases, also known as sex-linked disorders. Thomas Hunt Morgan, an American geneticist, is widely regarded as the scientist behind the theory of X-linked traits and sex-linked inheritance. He first began his research in 1910 while working with Drosophila (fruit flies). One day, Morgan noticed that one fly did not display the usual scarlet eye color but rather had white eyes. Perplexed, Morgan performed a test cross between the white-eyed male and several scarlet-eyed females, resulting in an F1 generation with only scarlet eyes. Still suspecting that perhaps the white eyes were a recessive trait and there were carriers of the alleles in the scarlet-eyed population; Morgan crossed the F1 generation to produce an F2 with a 3:1 ration of scarlet eyes to white. Curiously enough however, all of the white-eyed Drosophila were male. Morgan toyed with the idea of the white-eyed trait being lethal in females but more test crosses proved that to be incorrect. After this, Morgan turned to the relatively new idea of a sex chromosome presented by Nettie Stevens and E.B. Wilson, and based on further testing with Drosophila concluded that traits could indeed be linked to the X-chromosome and developed the well regarded theory of sex-linked inheritance.2
Sex-linked inheritance works differently in males and females, because of their ratio of X-chromosomes. Due to the fact that females posses two X-chromosomes, traits linked to them are inherited much like autosomal traits. Two recessive alleles are required for expression of a recessive trait whereas only one dominant allele is required for expression of a dominant trait. In males, however, there is only one X-chromosome, inherited from the mother, paired with a Y-chromosome inherited from the father. Because of this, only one copy of an affected allele is needed to produce an affected male. This makes males more likely to display recessive X-linked traits than females. 3
A well known X-linked recessive disorder is Hemophilia, a condition that drastically slows blood clotting. Males only require one recessive allele in order to get hemophilia whereas in females, two are required. Due to this, it is often found that affected females pass on the disease to 50% of their sons. Affected females are much less common due to the fact that they must be the product of an affected male and either an affected or heterozygous, carrier mother.1
Much less common are X-linked dominant disorders. A good example of this is Rickets, an X-linked disorder that leads to softening and weakening of bones. In the case of an X-linked dominant trait, an affected father will pass on the trait to 100% of his offspring whereas an affected mother could pass on the trait to 50%-100% of her children, depending on her mates allele on his X-chromosome. 4
X-linked traits are a fascinating example of how genetics shapes species based on gender. X-linked inheritance makes it astronomically more likely for males to become affected with a variety of disorders exclusively linked to the X-chromosome, than females. Although there are some biological mechanisms in place that correct for this phenomenon, they do not fully counter its affects.
1 Hemophilia (2014). Genetics Home Reference Your guide to Understanding Gentic Conditions from http://ghr.nlm.nih.gov/condition/hemophilia
2 Ilona Miko, P. D. (2008). Thomas Hunt Morgan and Sex Linkage 2014, from http://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452
3 Mark F. Sanders, J. L. B. (2012). Genetic Analysis: An Integrated Approach: Library of Congress Cataloging-in-Publication Data 2012.
4 Neil K. Kaneshiro, M. (2012). Sex-linked dominant, from http://www.nlm.nih.gov/medlineplus/ency/article/002050.htm