Animal genetics / Frederick B. Hutt.

  • Frederick Hutt
Date:
[1964]
Catalogue details

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

Credit: Animal genetics / Frederick B. Hutt. Source: Wellcome Collection.

    468/578 page 446
    446 ANIMAL GENETICS that conditions affecting sex ratio in one species, or one breed, do not necessarily influence others in the same way. Among dog breeders, it is a common belief that the proportion of males depends to some extent upon the season of the year when the. litter is conceived. There are extensive figures showing that for litters of warm and cold months the sex ratios are 126 and 122 ¿ ê per 100 9 9, respectively, but in Whitney's German Shepherds the corresponding figures were 116 and 143. In rats and mice the feasibility has been demonstrated of breed¬ ing strains that differ in sex ratio. In her famous inbred rats at the Wistar Institute, Helen Dean King selected (after six generations of brother X sister matings) two distinct lines, one (A) bred from litters having an excess of males, the other (B) from those with a majority of females. From the eighth to the twenty-fifth genera¬ tions, the average number of males per 100 females was 122.3 in line A but only 81.8 in line B. Most of the differentiation was effected during the first two generations of selection. While it is under¬ standable that sex-linked genes affecting viability of male fetuses could reduce that sex in line B, a satisfactory explanation for the excess of males in line A has not vet been adduced. In the mouse, Weir found that selection to differentiate two lines differing in the pH ( alkalinity ) of the blood was effective, and that the two strains differed consistently in sex ratio. The one with the higher pH (7.48) showed the higher proportion of males, the other (with pH 7.43) a remarkably low one. In reciprocal crosses be¬ tween the two strains and in matings of males of both to females of other stock, sex ratios in the progeny were always like that of the sire's strain (Table 19-5). The degree to which males of these two lines differ in sex ratio of their progeny is more conspicuous if the averages in Table 19-5 are converted to the other measure of sex ratio, i.e., males per 100 females. On that scale, the figure for the high strain is 114, against only 67 for the low one. This is very striking, because, as the data show, the consistent deficiency of males in the low strain was not matched by any corresponding decrease in litter size. One cannot attribute the difference, therefore, to any sex-linked lethal genes killing males in the low strain. The difference between the high and low strains is clearly a genetic one, but the physiological basis for the effect has yet to be determined. The difference between the strains in pH appears slight, but is consistent. Other investigators have been similarly able to breed lines of mice differing in pH of the blood, but have not found therein correspond-
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