Albinism refers to a group of inherited genetic conditions that result in reduced or absent production of melanin, the pigment responsible for coloring the skin, hair, and eyes. This lack of pigmentation can lead to a characteristic pale appearance in affected individuals. Beyond visible physical traits, albinism also impacts eye development and function, often resulting in various visual impairments.
The Genetics of Albinism
The most common forms of albinism, known as oculocutaneous albinism (OCA), are inherited in an autosomal recessive manner. This means the genes responsible for OCA are located on autosomes, which are the 22 pairs of non-sex chromosomes. For an individual to develop OCA, they must inherit two altered copies of a specific gene, one from each parent.
A “recessive” inheritance pattern indicates that a single functional copy of the gene can compensate for a non-functional one, meaning individuals with only one altered gene copy do not show symptoms of albinism. These individuals are referred to as carriers. Genes such as TYR, OCA2, TYRP1, and SLC45A2 are associated with different types of OCA, each playing a role in the complex process of melanin synthesis within specialized cells called melanocytes.
How Albinism Is Inherited
Understanding the inheritance patterns of oculocutaneous albinism helps clarify the likelihood of a child being affected. When both parents are carriers, each child has a 25% chance of inheriting two altered genes and thus having albinism. There is also a 50% chance that their child will be a carrier, and a 25% chance the child will inherit two functional genes and not have albinism or be a carrier.
When one parent has albinism and the other parent is a carrier, their children have a 50% chance of inheriting the condition. This occurs because the affected parent will always pass on an altered gene copy, and the carrier parent has a 50% chance of passing on their altered copy. Conversely, if one parent has albinism and the other parent is not a carrier, none of their children will have albinism; however, all of their children will inherit one altered gene copy from the affected parent, making them carriers. When both parents have the same type of albinism, all their children will have albinism.
Distinguishing from Sex-Linked Traits
The most common forms of albinism, oculocutaneous albinism (OCA), are not sex-linked traits. Sex-linked traits involve genes located on the sex chromosomes, specifically the X and Y chromosomes. Red-green color blindness, for example, is a common X-linked trait that affects males more frequently due to their single X chromosome.
In contrast, OCA affects males and females equally because the genes involved are on autosomal chromosomes. While the majority of albinism cases are autosomal recessive, a very rare form known as Ocular Albinism (OA) can be X-linked. This form, caused by mutations in the GPR143 gene on the X chromosome, primarily affects males and results predominantly in vision impairments, with little to no impact on skin or hair pigmentation.