Genetics and Types of Albinism
Albinism is an inherited genetic condition. Most forms of albinism are recessive, which means that a child inherits one abnormal gene from each parent. Most children with albinism are born to parents with normal pigmentation. These parents — like one in every 75 people — are carriers for albinism. A carrier is someone who has one abnormal gene and one that functions normally. Because the functional gene overrides the abnormal one, these people do not have albinism themselves. However, they are still able to pass the abnormal gene on to their child. This type of inheritance is called “autosomal recessive”. When two people are carriers of the albinism gene, they have a one in four chance that the child will have albinism, one in two chance that the child will not have albinism but be a carrier and a one in four chance that the child will neither be a carrier nor have albinism.
Simply put, DNA is a molecule that carries genetic information or instructions telling our bodies how to function and develop. The DNA in the albinism gene (different for each type of albinism explained below) does not contain the instructions for the body to create as much pigment needed for various functions – including vision.
Types of Albinism
To date, there are several identified types of albinism. Below you will find a brief description of each:
Ocular Albinism (OA) affects only the eyes, not the skin or hair. It results from an X-linked chromosomal inheritance and so occurs mostly in boys.
Oculocutaneous Albinism (OCA) affects the eyes, hair and skin and includes several different forms. The first form, OCA1 involves the tyrosinase enzyme, which converts tyrosine (an amino acid) into melanin. Melanin is a chemical that colors our skin, eyes and hair. OCA1 includes two sub-groups:
- OCA1a: Those with this type of albinism have no pigment and no active tyrosinase
- OCA1b: Those with this type of albinism have some residual tyrosinase activity and so have some pigment. Although they have decreased pigmentation at birth, it may increase slightly throughout their lives. People in this group may even have the ability to sun tan.
Another form of OCA, OCA2, affects the p-protein, another protein involved in melanin production. It is much more common in persons of African descent. People in this group have some pigment, but typically do not tan. Yet another form of OCA, OCA4, is clinically indistinguishable from OCA2. However, it involves the SLC45A2 gene, which produces another protein involved in melanin production.
Hermansky-Pudlak Syndrome (HPS) is a rare type of albinism, which has an increased incidence among people of Puerto Rican descent. It may involve bleeding disorders, bruising, lung problems and intestinal disorders. For more information, please go to hpsnetwork.org.
Chediak-Higashi Syndrome is another rare form of albinism. It is characterized by an abnormality in certain types of white blood cells, lowering resistance to infection. For more information, please go to emedicine.com/derm/topic704.htm.
Griscelli Syndrome is an extremely rare type of albinism. In fact, there are only 60 known cases worldwide. Like Chediak-Higashi Syndrome, it involves immunodeficiency and may also have associated neurological problems. You can learn more at emedicine.com/derm/topic926.htm.