When individuals reproduce, they pass on characteristics to their offspring through a process called heredity. These inherited characteristics, known as traits, determine many aspects of an organism’s appearance and function. While many traits follow predictable patterns of inheritance, some exhibit unique patterns because they are linked to an individual’s sex.
The Fundamentals of Inheritance
Most traits, like eye color or blood type, are determined by genes located on chromosomes not involved in sex determination. These are called autosomes, and humans have 22 pairs of them. Genes are segments of DNA that carry instructions for building and maintaining an organism, and each gene exists in different versions called alleles. For any given gene, an individual inherits one allele from each parent.
These two inherited alleles determine the trait expressed. Some alleles are dominant, meaning they express their trait even if only one copy is present. Other alleles are recessive, meaning their trait will only be expressed if an individual inherits two copies of that specific allele, one from each parent.
The Unique Nature of Sex Chromosomes
Humans possess a pair of sex chromosomes that differ significantly from the autosomal chromosomes. These chromosomes determine an individual’s biological sex. Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
The X chromosome is considerably larger than the Y chromosome and carries many more genes. It contains hundreds of genes responsible for various functions throughout the body, not just those related to sex. In contrast, the Y chromosome is much smaller and contains relatively few genes, primarily those involved in male development, such as the SRY gene which initiates male sexual characteristics.
How Sex-Linked Traits Are Inherited
Traits determined by genes on the X chromosome are known as X-linked traits. Since males have only one X chromosome, they express any allele present on that X chromosome, even if it is recessive. This means a single recessive allele on the X chromosome is sufficient for a male to exhibit the associated trait.
Females, having two X chromosomes, generally need to inherit two copies of a recessive allele—one on each X chromosome—to express an X-linked recessive trait. If a female inherits one normal, dominant allele and one recessive allele on her two X chromosomes, she will not express the trait but will be a carrier. She can then pass the recessive allele to her offspring. Traits on the Y chromosome, known as Y-linked traits, are directly passed from father to son and are much rarer due to the Y chromosome’s limited gene content.
Common Examples of Sex-Linked Traits
Red-green color blindness is a common X-linked recessive trait. Males are far more likely to be colorblind because they only need one affected X chromosome, while females require two affected X chromosomes to exhibit the condition. This explains why approximately one in twelve males are affected, compared to about one in 200 females.
Hemophilia A, a disorder that impairs the blood’s ability to clot, is another X-linked recessive condition. Affected males experience prolonged bleeding, while females are carriers with normal clotting function. Duchenne muscular dystrophy, a severe muscle-wasting disease, is also an X-linked recessive disorder. Its inheritance pattern closely mirrors that of hemophilia and color blindness, predominantly affecting males due to their single X chromosome.