ROBERTSON: MULE AND HORSE TWINS. 295 



As the pedigree (plate I) will show, the mare was hetero- 

 zygous for the presence of the bay factor, since her sire was 

 black and her dam had the bay variation of chestnut, known 

 (Castle's "Genetics and Eugenics") as sorrel. This probably 

 gave her the ability to throw the black and brown coats of her 

 first twin mules (IVo and IV?) and also probably the darker 

 and lighter brown coats of her third twin mules (IV12 and 

 IV13) . That she carried the bay factor is certain from the fact 

 that by a black horse stallion (IIIo) she produced a solid bay 

 colt (IVio). Her daughter was probably heterozygous for 

 black, which gave her, when bred to a sorrel sire (IV2), the 

 ability to throw in her twins one a sorrel, the other a bay like 

 herself. This daughter must likewise have been heterozygous 

 for spotting, {. e., assuming spotting recessive. She herself 

 was solid bay, but her mother (III9) had white face and 

 white points and one of her twin colts was similarly marked. 

 These foals did not live to maturity. 



By way of summary, these facts, then, show that in all 

 probability each case of twinning in this family is one of the 

 dizygotic type, due to the simultaneous, or nearly so, fertiliza- 

 tion of two distinctly separate unlike ova by two unlike sper- 

 matoza. Of the four cases where observations were made, this 

 is shown in two by differing sexes and at the same time differ- 

 ing general coat color and widely differing markings; in the 

 two remaining cases by differing general coat color, black and 

 sorrel or black and brown, and at the same time differing spot- 

 ting with white. In addition, one of the former cases, where 

 the sex of the pair is different, one foal is a mule and the other 

 a horse. 



A second conclusion is that in these cases we have evidence 

 of the inheritance of the tendency to produce heterologous 

 twins through three generations, transmitted evidently from 

 the bald-faced sorrel Oregon mare (II4), since two of her 

 daughters (III9 and IIIio), by different sires, possessed the 

 trait. 



Likewise, it is probably to be concluded that in order to be 

 the producer of such twins, the dam herself need not have 

 been a twin, since neither IIIo, IIIio nor IVs, all of which pro- 

 duced twins, were themselves twins. It must not be forgotten, 

 however, that possibly the twin in each of these may have 

 been formed but failed to develop. 



