In about 20 per cent of men with fewer than 20 million sperm cells per millimetre a genetic abnormality is found. A genetic investigation is called for if during a physical examination abnormalities are found which might be consonant with a genetic disorder (for example, the absence of seminal ducts or certain genetic disorders occurring in the family). Investigation of genetic abnormalities in subfertile men has only taken off in the last few decades, partly as a result of new repro­duction techniques. Genetic investigation consists of two parts: karyo­typing and dna examination. In karyotyping, all the chromosomes in a cell, generally a blood cell, are coloured and counted under the micro­scope and subsequently examined separately. The main abnormalities that may be found are: an abnormal number of chromosomes (46 is the norm) or an abnormality in one or more chromosomes, like the lack of a section of chromosome. The section may have transferred to another chromosome (translocation) or been lost (deletion). Kline­felter’s syndrome is the best-known example of an abnormal number. At least і in every 1,000 newborn boys have this syndrome. In the great majority of cases these men are infertile.

One of the most striking phenomena in Klinefelter’s syndrome is the marked underdevelopment of the testicles, which usually do not grow beyond the size of a pea. Other characteristics are a relatively short penis, the formation of mammary glands, greater than average height, sparse beard growth and little pubic hair, which in addition often has a female growth pattern, namely with a horizontal upper limit. A man with an extra x chromosome is bound to be effeminate, would seem to be the obvious assumption, but that is totally wrong. And it is equally wrong to imagine that such a man is necessarily bisexual. If boys with Klinefelter’s syndrome receive testosterone treat­ment in puberty, they grow into very masculine men, quite able to hold their own sexually!