Sex-Linked TraitsSex-linked traits are genetic characteristics determined by genes, which are located on sex chromosomes. Genes are segments of DNA found on chromosomes that carry information for protein production and that are responsible for the inheritance of specific traits. Genes exist in alternative forms called alleles. One allele for a trait is inherited from each parent. Like traits originating from genes on autosomes (non-sex chromosomes), sex-linked traits are passed from parents to offspring through sexual reproduction.
Sex CellsOrganisms that reproduce sexually do so via the production of sex cells, also called gametes. In humans, male sex cells are spermatozoa (sperm cells) and female sex cells are ova or eggs. Male sperm cells contain one of two types of sex chromosomes. They either carry an X chromosome or a Y chromosome. A female egg cell however, contains only the X sex chromosome. When the sex cells fuse in a process called fertilization, the resulting cell (zygote) receives one sex chromosome from each parent cell. The sperm cell determines the sex of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be (XX) or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be (XY) or male.
Sex-Linked GenesGenes that are found on the sex chromosomes are called sex-linked genes. These genes can be on the X chromosome or on the Y chromosome. If a gene is located on the Y chromosome, it is a Y-linked gene. These genes are only inherited by males because, in most instances, males have a genotype of (XY). Females do not have the Y sex chromosome. Genes that are found on the X chromosome are called X-linked genes. These genes can be inherited by both males and females. Since genes for a trait may have two forms or alleles, one allele is usually dominant and the other is recessive. Dominant traits mask recessive traits in that the recessive trait is not expressed in the phenotype.
In X-linked recessive traits, the phenotype is expressed in males because they only contain one X chromosome. The phenotype may be masked in females if the second X chromosome contains a normal gene for that same trait. An example of this can be seen in hemophilia. Hemophilia is a blood disorder in which certain blood clotting factors are not produced. This results in excessive bleeding that can damage organs and tissues. Hemophilia is an X-linked recessive trait caused by a gene mutation. It is more often seen in men than women. The image above depicts the inheritance pattern of the hemophilia trait when the mother is a carrier and the father does not have the trait. The sons have a 50/50 chance of inheriting the trait and the daughters have a 50/50 chance of being carriers of the trait. If a son inherits an X chromosome with the hemophilia gene, the trait will be expressed and he will have the disorder. If a daughter inherits the mutated X chromosome, her normal X chromosome will compensate for the abnormal chromosome and the disease will not be expressed.
In X-linked dominant traits, the phenotype is expressed in both males and females who have an X chromosome that contains the abnormal gene. If the mother has one mutated X gene (she has the disease) and the father does not, the sons and daughters have a 50/50 chance of inheriting the disease. If the father has the disease and the mother does not, all of the daughters will inherit the disease and none of the sons will inherit the disease.
There are several disorders that are caused by abnormal sex-linked traits. In addition to hemophilia, color blindness, Duchenne muscular dystrophy, and fragile-X syndrome are examples of X-linked recessive disorders. A common Y chromosome linked disorder is male infertility.