Learn About Sex-Linked Traits and Disorders

Human Sex Chromosomes X and Y
This is a scanning electron micrograph (SEM) of human sex chromosomes X and Y (Pair 23). The X chromosome is much larger than the Y chromosome. Power and Syred/Science Photo Library/Getty Images

Sex-linked traits are genetic characteristics determined by genes located on sex chromosomes. Sex chromosomes are found within our reproductive cells and determine the sex of an individual. Traits are passed on from one generation to the next by our genes. 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 that originate from genes on autosomes (non-sex chromosomes), sex-linked traits are passed from parents to offspring through sexual reproduction.

Sex Cells

Organisms that reproduce sexually do so via the production of sex cells, also called gametes. In humans, male gametes are spermatozoa (sperm cells) and female gametes are ova or eggs. Male sperm cells may carry one of two types of sex chromosomes. They either carry an X chromosome or a Y chromosome. However, a female egg cell may carry only an X sex chromosome. When 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 Genes

Sex-linked Traits
Hemophilia is a sex-linked trait caused by a gene mutation. The image shows the inheritance pattern of the hemophilia trait when the mother is a carrier and the father does not have the trait. Darryl Leja, NHGRI

Genes that are found on sex chromosomes are called sex-linked genes. These genes can be on either the X chromosome or 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. Genes for a trait may have two forms or alleles. In complete dominance inheritance, 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.

X-Linked Recessive Traits

In X-linked recessive traits, the phenotype is expressed in males because they only have 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 inheritance pattern for the hemophilia trait differs depending on whether or not the mother is a carrier for the trait and if the father does or does not have the trait. If the mother carries the trait and the father does not have hemophilia, the sons have a 50/50 chance of inheriting the disorder and the daughters have a 50/50 chance of being carriers for the trait. If a son inherits an X chromosome with the hemophilia gene from the mother, 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. Although she will not have the disorder, she will be a carrier for the trait.

If the father has hemophilia and the mother does not have the trait, none of the sons will have hemophilia because they inherit a normal X chromosome from the mother, who does not carry the trait. However, all of the daughters will carry the trait as they inherit an X chromosome from the father with the hemophilia gene.

X-Linked Dominant Traits

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.

Sex-Linked Disorders

Color Blindness Test Plates
Color Blindness Test Plates. Dorling Kindersley/Getty Images

There are several disorders that are caused by abnormal sex-linked traits. A common Y-linked disorder is male infertility. In addition to hemophilia, other X-linked recessive disorders include color blindness, Duchenne muscular dystrophy, and fragile-X syndrome. A person with color blindness has difficulty seeing color differences. Red-green color blindness is the most common form and is characterized by the inability to distinguish shades of red and green.

Duchenne muscular dystrophy is a condition that causes muscle degeneration. It is the most common and severe form of muscular dystrophy that quickly worsens and is fatal. Fragile X syndrome is a condition that results in learning, behavioral, and intellectual disabilities. It affects about 1 in 4,000 males and 1 in 8,000 females.

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Bailey, Regina. "Learn About Sex-Linked Traits and Disorders." ThoughtCo, Nov. 7, 2017, thoughtco.com/sex-linked-traits-373451. Bailey, Regina. (2017, November 7). Learn About Sex-Linked Traits and Disorders. Retrieved from https://www.thoughtco.com/sex-linked-traits-373451 Bailey, Regina. "Learn About Sex-Linked Traits and Disorders." ThoughtCo. https://www.thoughtco.com/sex-linked-traits-373451 (accessed November 18, 2017).