Cell Physiology : Females Have Two X Chromosomes; Males Have Only One
The history of X chromosome research has been dominated by efforts in two areas: the X-linked recessive pattern of inheritance and the inactivation of one X chromosome.
The first major area of X chromosome research is of the X-linked recessive pattern of inheritance (see figure below). This pattern of inheritance arises because males, who have a single X chromosome, are unable to complement mutations in genes that reside on the X chromosome and for which there is no homologue on the Y chromosome. Females, on the other hand, have two X chromosomes and so are able to complement the defective allele by using a functional copy on the other X chromosome.
In the case of a mutation, half of the sons of mothers who carry an X-linked mutation will receive the X chromosome bearing the defective gene and will be affected. The other half of the sons will receive the X chromosome bearing the functional gene and will not be affected. None of the daughters will be affected, although half will be carriers like their mother.
Basic Mendelian Pedigree Patterns (A) Autosomal dominant; (B) Autosomal recessive; (C) X-linked recessive; (D) X-linked dominant; (E) Y-linked. The risk for the individuals marked with a query are (A) 1 in 2, (B) 1 in 4, (C) 1 in 2 males or 1 in 4 of all offspring, (D) negligibly low for males, 100% for females. Source: http://www.ncbi.nlm.nih.gov © BIOS Scientific Publishers Ltd, 1999 Key: = female, O = male
