Hypotrichosis is a condition of abnormal hair patterns - predominantly loss or reduction. It occurs, most frequently, by the growth of vellus hair in areas of the body that normally produce terminal hair
Terminal hair
Terminal hairs are thick, long, and dark, as compared with vellus hair. During puberty, the increase in androgenic hormone levels causes vellus hair to be replaced with terminal hair in certain parts of the human body...

. Typically, the individual's hair growth is normal after birth, but shortly thereafter the hair is shed and replaced with sparse, abnormal hair growth. The new hair is typically fine, short and brittle, and may lack pigmentation. Baldness
Baldness implies partial or complete lack of hair and can be understood as part of the wider topic of "hair thinning". The degree and pattern of baldness can vary greatly, but its most common cause is male and female pattern baldness, also known as androgenic alopecia, alopecia androgenetica or...

 may be present by the time the subject is 25 years old.

Hypotrichosis is a common feature of Hallermann-Streiff syndrome
Hallermann-Streiff syndrome
Hallermann-Streiff syndrome is a congenital disorder that affects growth, cranial development, hair growth and dental development.There are fewer than 200 people with the syndrome worldwide...

 as well as others. It can also be used to describe the lack of hair growth due to chemotherapy.

The opposite of hypotrichosis is hypertrichosis
Hypertrichosis is an abnormal amount of hair growth on the body; extensive cases of hypertrichosis have informally been called werewolf syndrome. There are two distinct types of hypertrichosis: generalized hypertrichosis, which occurs over the entire body, and localized hypertrichosis, which is...

, where terminal hair (thick) grows in areas that would otherwise normally have vellus hair (thin), for example abnormally thick facial hair growth in women.

Genetic forms of localized autosomal recessive hypotrichosis include:
Type OMIM Gene Locus
Desmoglein-4 is a protein that in humans is encoded by the DSG4 gene.-Further reading:...

Lipase member H is an enzyme that in humans is encoded by the LIPH gene.-Further reading:...

Lysophosphatidic acid receptor 6 also known as LPA6 and P2RY5 is a protein that in humans is encoded by the LPAR6 gene. LPA6 is a G protein-coupled receptor that binds the lipid signaling molecule lysophosphatidic acid ....

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