It is often stated that acne is an inherited disease but, although the  circumstantial evidence is convincing, definite proof is lacking. In an investigation of ninetyfive pairs of identical twins with acne, in 97.9 per cent of pairs, both twins were affected. Amongst dizygotic twin pairs, in 45.8 per cent only one of each set had acne. It has recently been  shown that identical twins have identical rates of sebum excretion but,  although both may have acne, the severity is not identical. This suggests that some external factor - such as  bacterial colonization - determines the acne severity. In nonidentical twins, acne severity and rate of sebum excretion are not identical.

  Some dermatologists consider acne to be one disease whereas it probably constitutes a spectrum, not just in its severity but in the type of lesions present and the site involved.  Some patients have  acne limited to certain parts of the body such as  the face, and none on the trunk, while others may have problems only on the trunk. Some patients have predominantly noninflamed lesions and few inflamed lesions and other patients the reverse clinical appearance. Many people have a greasy skin and yet other patients with severe acne have much less seborrhoea. These variables may be related to the age of the patient and the duration of the acne, but they could also have a genetic basis.

  Thus, when investigating the inheritance of acne, it is probably necessary to consider particular subgroups of the condition. Four studies of identical twins with acne support this approach. These sets of identical twins developed their severe disease at approximately the same time. One set had severe nodulocystic acne, and three sets had acne fulminans. Each set had a more or less identical clinical  distribution and one set had a reduced response to dinitrochlorobenzene (DNCB) and Mantoux antigens.

  Studies have shown that some patients with severe nodulocystic acne may have the abnormal XYY chromosome complement.  These patients did not possess the other classical features of the XYY syndrome, such as  increased height, and mental deficiency. It was suggested by the investigators that nodulocystic acne may be  an as yet unrecognized but important  aspect of the XYY syndrome, but this author's studies on twenty patients with severe nodulocystic disease failed to confirm these findings.

  Eighty-two  per cent of  patients  with acne in one series gave a history of acne in at least one sibling and in 60 per cent a history of acne was obtained from one  or both parents.2 Von Schleicher found that patients with severe acne gave a strong family history.16 There is still a need for a well-planned genetic study to investigate, in particular, the possibility of a genetic predisposition to certain  types of acne. The fact  that  acne is now a well-treated disease may make such a study impossible.

  The incidence of eczema in acne patients and their first-degree relatives has been assessed and a statistically decreased incidence of endogenous eczema in the acne patients and relatives (3-7 per cent) was found, compared to non-acne controls (16-20 per cent).17 In part, these observations may relate to the significantly reduced sebum excretion rate of atopics compared with acne subjects.