Mammalian chimaeras / Anne McLaren.

  • McLaren, Anne.
Date:
[1976]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Mammalian chimaeras / Anne McLaren. Source: Wellcome Collection.

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    Sexual development XX/XY chimaeras Neither at the 8-cell nor at the blastocyst stage is it yet possible to sex a live mouse embryo. Chimaeras are therefore made without regard to the sex of the components, and since pre-implantation mouse embryos comprise about equal numbers of males and females (Vickers, 1967; Kaufman, 1973), 50% of chimaeras on average would be expected to be of the XX/XY type. The available information, such as it is, agrees well with this expectation. The four bodies of data summarized in Table 3 are consistent with one another, and give an overall proportion of XX/XY animals very close to the predicted 50 %. The paucity of the data reflects the tedium of chromosomal sexing in the mouse. No a priori reasons exist for expecting XX/XY mice to be male, or female, or intermediate. In the samples listed in Table 3, Mystkowska & Tarkowski's nine XX/XY animals comprised two females (both foetal) and seven males (including one male hermaphrodite); the four of Milet et al. included a sterile female, two males and a female hermaphrodite; our eight were all males (six fertile and one sterile male, and one male hermaphrodite); and the four of Ford et al. consisted of two females and two males. A strong tendency for development in the male direction is evident, but a calculation of the overall proportion of males (76 %) may be misleading, as it could conceal a real heterogeneity between the different samples. Mintz (1969(7) mentions nineteen XX/XY chimaeras. These included one hermaphrodite, three phenotypic females (two sterile, one not mated) and fifteen males (eight fertile, five sterile, two not mated), but since they were stated not to be a random sample of the population, they shed little light on the relative frequency of the different phenotypes. Of three overt chimaeras made in sheep by the injection technique, all were male (Tucker et al., 1974). Two proved to be XX/XY, and one XY/XY. Sex ratio Indirect but more extensive data come from a consideration of the sex ratio of chimaeras. In the initial series of aggregation chimaeras described by Tarkowski (1961, 1963) a striking preponderance of males was seen (11/13),