Hereditary predispositions to dizygotic twin-births in Norwegian peasant families

The sex ratio and multiple births in cattle1)

“…If genetic differences exist between breeds in this respect one would also expect to find such differences between herds and individuals within a breed, because the breeds are by no means homogeneous for all their characters. In man it has been shown that hereditary differences in the predisposition to twinning exist between the individuals within a population (Weinberg 1902 a, 1909, Bonnevie and Sverdrup 1926, Wehefritz 1925, Davenport 1920, 1928, and others). Breeds of sheep differ from one another in their tendency towards multiple births (Heape 1899, Nichols 1924, Marshall and Potts 1924), and within a breed there is also found a correlation between closely related individuals in regard to the frequency of such births (Rietz and Roberts 1915, Wriedt 1925).…”

“…Dahlberg (1926), working on Australian statistics, did not find any influence of the number of previous births, in addition to the influence of age, on the frequency of twin births when he arranged the data in classes with one year age interval. Bonnevie and Sverdrup (1926) conclude from a study of this particular problem, that „not much room is left for any influence other than that of age of the mother”, although they leave open the question whether parity may be of some minor importance.…”

“…of twin mothers, and to compare the percentage obtained with the frequency of twinning in the whole population (Weinberg 1902 a and 1909, and others). Bonnevie (1919) and Bonnevie and Sverdrup (1926) have also made extensive genealogical studies of twin families in order to trace the inheritance of the twinning tendency. These investigators have arrived at the conclusion that dizygotic twinning only is hereditary, and that the twin mother must be homozygous for the character if it is to manifest itself; in other words, the tendency for simultaneous ovulation of two or more eggs is a recessive character.…”

Variations in the Human Twinning Rate

“…The m ajority of research workers have considered the ultim ate cause of dizygotic tw inning (dissimilar twinning, polyovulation, or the release of extra eggs) to be genetic in nature (2,4,5,6,8,13). The increase in incidence becomes more manifest w ith a rise in m aternal age an d /o r parity (birth rank).…”

Twinning Rates in Isolates

“…The convergence between the TWRs is obvious. Bonnevie and Sverdrup (1926) presented a detailed study of the TWRs in Norway. They paid special attention to extreme TWRs in an isolated Norwegian valley consisting of the three villages of Meldal, Rennebo, and Ringebu.…”