62 CHARLES B. STOCKARD 



will be si. On crossing two such animals the only possible 

 recombinations will be ss and si, since every germ cell from 

 one parent carries an s, and from the other parent half will 

 carry s and half I; thus, half the fertilizations will give ss 

 and half si. 



Plate 7 illustrates in the case of one litter of six animals, 

 the realized expectation of three extremely short ss puppies 

 (fig. 3) and three intermediately short si (fig. 4). In none of 

 the offspring from this backcross can long legs ever appear 

 since there is no possibility of the arrangement II. In the 

 preceding section with the shepherd backcross it was equally 

 impossible to obtain the extreme achondroplasic shortness 

 since the homozygous condition necessary for this, ss, could 

 not occur. 



In all, six matings were made between the F t shepherd- 

 bassethound and the pure bassethound. Both Fj males and 

 females were used in this backcross, with similar results. 

 Thirty-six puppies were whelped in litters of from four to 

 eight individuals. On careful measurement and examination 

 from puppyhood to adult size it was estimated that very short 

 legs and intermediate shortness occurred in about equal num- 

 bers. There is the possibility that the hound leg bone and 

 the shepherd leg bone are not equally susceptible to the 

 achondroplasic reaction; the one might be more shortened 

 by this effect than the other. Since the different qualities of 

 bone from the two ancestral pure stocks are involved in an 

 estimation of the degree of shortness, it is almost impossible 



PLATE 7 



EXPLANATION OP FIGURES 



Backcross of the F t German shepherd-bassethound having intermediate legs 

 with the pure short legged bassethound. The expectancy here is equal numbers 

 of short legged and intermediate legged offspring. 



1 Bassethound 83 $. 



2 F, 248 c?. 



3-4 Litter of six backcross hybrids. 



3 (LtoR) 412 <?, 407 c?, 410^ (short legged). 



4 (LtoR) 409 <?, 408 <, 411$ (intermediate legs). 



