Field work in the inheritance of human disease has brought to light two striking phenomena:
First, in regard to many of the diseases due to unit traits, that is, conditioned by a single defective gene, a wide survey of 50 to 100 families possessing such a trait usually shows a triple pattern of inheritance: dominant, sex-linked recessive and simple recessive.
How general this rule is in human, animal or plant genetics is not known, nor is it obvious why the same trait should be dominant in 1 family and recessive in a second and in a third ride the sex chromosomes down the generations, becoming manifest only in the males.
The second phenomenon, a corollary of the first, is that the pattern of inheritance determines the age of onset and the clinical severity of various morbid unitary traits, as may be seen by a study of peroneal atrophy (Charcot-Marie-Tooth type).
ALLAN W. RELATION OF HEREDITARY PATTERN TO CLINICAL SEVERITY AS ILLUSTRATED BY PERONEAL ATROPHY. Arch Intern Med (Chic). 1939;63(6):1123–1131. doi:10.1001/archinte.1939.00180230108008
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