THE SEX RATIO IX HYBRID RATS. 



HKLEN DEAN KING, 

 THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY. 



It has frequently been stated that the sex ratio in mammals 

 can be altered by hybridizing, although there are but few series 

 of observations that give support to such a view. Buffon (1709- 

 1788) seems to have been one of the first to note the apparent 

 excess of males among hybrid offspring; but as his records com- 

 prise only a comparatively small number of cases they cannot be 

 considered to afford conclusive evidence that hybridizing changes 

 the normal proportion of the sexes. 



In recent times little attention has been paid to the question 

 of the sex ratio in hybrid offspring. Davenport ('06) ascertained 

 the sex of 377 out of a total of 950 hybrid fowls obtained in an 

 extensive series of investigations on inheritance in poultry, and 

 found 204 males and 173 females. Taking the sex ratio for 

 any given lot of individuals as the number of males to each 100 

 females, it is found that among these hybrids the sexes exist 

 in the ratio of 117.91 males to 100 females. This sex ratio seems 

 to indicate that there is a pronounced excess of males among 

 hybrid fowls, yet Davenport states that the proportion of the 

 sexes in hybrids is normal, and that "the exceptions to the law 

 of equality of sexes in hybrid offspring are individual and not of 

 general significance." Davenport attributes the excess of males 

 among his 377 hybrid fowls to a difference in the death rate of 

 the two sexes, yet he gives no figures to show that among young 

 poultry more female^ (lie- than do males. Human statistic-. 

 as well as the records I have been collecting for the albino rat, 

 indie, ue that mortality i> greater among young males than among 

 \ on ii- female--. I )a\ enport'> conclusion does not appear to apply 

 to hybrid birds in general, as < iuyer ('03, '09) found a great excess 

 of male^ among hyl>rid pigeon^, and the data which he ha> 

 collected regarding the sex of other hybrid binl- ^ho\\ a \ er\ 

 much greater number of male- than of female^ in practical!) 



e\ cry case. 



104 



