lo HEREDITY AND EUGENICS 



tion is dominant to the normal, which is spoken 

 of as recessive. Brachydactylous individuals have 

 always married normals. Persons showing this trait 

 have therefore always been the children of one normal 

 and one abnormal parent. They are therefore hybrid 

 or heterozygous in nature as regards this character, 

 and, since they are brachydactylous in appearance, 

 this condition is said to be dominant to the normal. 

 Now the essential feature of Mendelian behaviour 

 is that the factors or determiners for such a pair of 

 characters as normal and brachydactylous fingers 

 separate in the formation of the germ cells, so that 

 half the germ cells of a brachydactylous person 

 who had one normal parent will carry the factor 



Germ cells. Unions oF Germ cells oF 



\ ■■' SO >i B ^.^^^ germ cells oFFspring. 



parent 7 """--^ ^"""--^ __— 



' 50 Z n ^ B(n).(50%) — 



Normal 



n n (50 X) 



parent. | --■^.,^ ^,^--^'^ ' ' 50 y. n 



Fig. I. — Result of Cross between Heterozygous 

 Brack YDACTYL and a Normal Parent. 



for brachydactyh' and half will carry the factor for 

 normal fingers. If such a person marries a normal 

 individual, all of whose germ cells are therefore 

 carrying the factor for normal fingers, then, on the 

 average, half the children will be brach3"dactylous 

 and half normal, for the chances for the germ cell 

 matings — (i) normal x normal and (2) normal X 

 brachy dactyl — are equal. The result will be clear 

 from the accompanying diagram (Fig. i). 



Hence we see that as long as matings of brachy- 

 dactyls wdth normals continue, half the children will, 

 on the average, be heterozygous brachydactyls 

 (transmitting this character to half their offspring), 

 while the other half of the children will be pure normal. 



