12 



HEREDITY AND EUGENICS 



one of the four combinations, it follows that only 

 25 per cent, of the offspring will be likely to have 

 normal lingers. Of the other 75 per cent, which are 

 brach3'dact3^1ous, two out of three will be heterozygous 

 {i.e., with half their germ cells of each type), while 

 one-third will be homoz^^gous, carrying the determiner 

 for brach^'dactyl}' in all their germ cells. 



We may similarh' consider the case where a hetero- 

 zygous brach3'dactyl marries a homozygous brachy- 

 dact\d.* 



From the diagram (Fig. 3) it will be seen that the 

 offspring from such a mating would be all brachy- 



Heberozygous 

 brachydacbyl 

 parent. 



Germ cells 

 50% B 



ZvQotes 



/homozygous 



brachydacbyl 



parent. 



50% B 



Fig. 3. — Theoretical Results of Cross between a 

 Heterozygous and a Homozygous Parent. 



dact\dous, half of them heteroz3^gous and half homo- 

 Z3^gous. So long as the former continued mating 

 wdth homoz3^gous brach3^dact3ds the normal condition 

 would be completeh" suppressed, and the strain would 

 appear to be pure for brach3^dactyly. But if in 

 any generation two heterozygous individuals mated, 

 there would be one chance in four of the recessive 

 condition reappearing. The sudden appearance of 

 a reversion or throw back in a pedigree strain, for 

 example, of cattle, is no doubt often to be accounted 

 for in this wa3^ A recessive character ma3^ thus be 



* There is some evidence that the homozygous brachy-phalan- 

 gous (related) condition is non-viable and therefore cannot exist 

 (see p. 90). 



