Sex-linked traits are carried on the X and Y, or sex, chromosomes. Sex-linked traits may be linked to either the X or the Y chromosome and may also be either dominant, semi-dominant, or recessive. Many more X-linked traits have been identified than Y-linked traits.
The sex chromosomes control the biological sex of an individual. Individuals with XY chromosomes are male, and individuals with XX chromosomes are female. The chromosome inherited from the mother is always the X chromosome, while the chromosome carried by the father's sperm may be either an X or Y chromosome.
X-LINKED DOMINANT Chromosomally normal females possess two X chromosomes; therefore, they can be homozygous or heterozygous in a trait that is caused by a gene mutation on the X chromosome. In the case of X-linked dominant traits, only one copy of the mutant gene must be present for the trait to be fully expressed. A female child affected with an X-linked trait may inherit this trait from either her mother or her father. In cases of
an affected heterozygous mother and an unaffected father, the probability that a female child will be affected with an X-linked dominant trait is 50%. In cases of an affected homozygous mother, the probability that a female child will be affected is 100%, regardless of whether or not the father is affected. In cases of an affected father, the probability that a female child will be affected is 100%. This is because the father is hemizygous for the mutant allele. His only copy is affected and he must pass that copy on to his daughters.
A chromosomally normal male child must receive his only X chromosome from his mother. He gets his Y chromosome from his father. Therefore, in cases of X-linked dominant traits, a male child has a 50% chance that he will receive the mutant gene from his heterozygous affected mother. If his mother is homozygous, this male child has a 100% likelihood of being affected with the trait. Therefore, while X-linked dominant traits are passed on equally from mothers to daughters and from mothers to sons, females may also inherit X-linked dominant traits from their fathers.
In some instances of dominant X-linked inheritance, the lack of the presence of a copy of the normal gene causes embryonic, fetal, or neonatal death. Therefore, in these cases, only very few affected males are born alive, and those that are generally die within a few hours of birth. This inheritance pattern is also known as male-lethal X-linked dominant inheritance. Since there are no affected males to contribute to the inheritance patterns of these traits, inheritance from father to daughter is not possible. Likewise, homozygous females are not possible. Only heterozygous females survive. In this form of inheritance, all affected males will inherit this trait from their heterozygous mothers. These males will either
become miscarriages, they will be stillborn, or they will die shortly after birth. Heterozygous females can inherit male-lethal X-linked dominant traits from their heterozygous mothers. Therefore, the inheritance of these traits has an overall 50% probability of occurrence.
X-LINKED RECESSIVE In cases of X-linked recessive traits, female children can only be affected if their mothers are carriers and their fathers are affected with the trait. The inheritance patterns in females of X-linked recessive traits are identical to the inheritance patterns of autosomal recessive traits. However, because the odds of a carrier mother producing offspring with an affected father are extremely low, X-linked recessive traits are characterized by the general absence of affected females. Because males are hemizygous in all X-linked traits, they have a 50% probability of inheriting an X-linked recessive trait from their carrier mothers. In the rare instances of affected mothers, males have a 100% chance of inheritance. Fathers cannot pass any X-linked trait to their XY sons. When affected fathers produce female children, 100% of these girls will be carriers of this trait. Almost all cases of females affected by an X-linked recessive trait are the result of consanguineous parents.
X-LINKED SEMI-DOMINANT A few examples of X-linked semi-dominant traits exist. In these cases, the carrier females are generally affected with a milder form of the trait than the affected males. Occasionally, some females show mosaicism of their X chromosomes that causes an activation of one of the X chromosomes in preference to the other. In these cases, heterozygous females show characteristics of the trait caused by the mutant gene that are identical, or nearly identical, to those characteristics seen in hemizygous affected males. Examples of this type of X-inactivation are a form of
hereditary mental retardation called fragile X syndrome, and both Duchenne type and Becker type muscular dystrophies.
Paul A. Johnson, The Gale Group Inc., Gale, Detroit,