THE MODE OF INHERITANCE IN ISOLATED LAeVOCARDIA AND DEXTROCARDIA AND SITUS INVERSUS

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Abstract
Estimates for the incidence of situs inversus vary greatly. Torgersen finds 1 in 10,000 and Lowe and McKeown 1 in 7000. Calculations here suggest that about 1 in 4 of the subjects with situs inversus has other malformations of the heart as well. Estimates for the incidence of isolated dextrocardia and isolated levocardia, are few, dextrocardia is probably between 1 in 17,500 and 1 in 29,000, and levocardia about 1 in 22,000. Details about the families of 40 subjects with situs inversus totalis or isolated dextrocardia or isolated levocardia, all of whom had other malformations of the heart, have been reported and compared with the findings of others. As regards distribution in the family and the high incidence of cousin marriages between parents, all these groups seem to behave in the same way. Cockayne''s view that all are inherited in an autosomal recessive manner cannot be entirely right, because monozygotic twins are not always concordant. Nevertheless, their frequency of concordance is so much more frequent and the incidence of cousin marriages so much higher (5 per cent) with situs inversus than with other malformations of the heart that recessive inheritance appears to play a large part in its causation; environmental factors also must be important. This is true for situs inversus whether it is the sole lesion or whether there are other malformations of the heart as well, almost certainly for isolated dextrocardia, and probably for isolated levocardia also. Ivemark''s syndrome of splenic agenesis and cono-truncal malformations has been considered shortly because it often occurs with some form of situs inversus. The association suggests that some teratogenic factor may be at work between the 10th and 15th day and between the 31st and 36th days of intrauterine life in this syndrome, though not in all cases of situs inversus.