10 



MENDELIAN SEGREGATION 



If the factors for vestigial wings are carried by a 

 pair of chromosomes (the chromosomes carrjdng v 

 in Fig. 3) then at the ripening of the germ cells (eggs 

 and sperm) such a pair of chromosomes will come 

 together and at reduction separate; so that each 

 germ cell will have one such chromosome and not 

 the other. (See Fig. 1, e-h.) 



If such a germ cell fertilizes an egg of the wild fly 

 that contains a similar group of chromosomes, ex- 



o 

 o 



II 



Fig. 4.— (^.) Fertilization of egg by sperm. (B.) Zygote formed by 

 umon of egg and sperm. (C.) Diploid nucleus. 



cept that the corresponding chromosome carries the' 

 factor for long wings (Fig. 4, A), the result will be to 

 produce a fertihzed egg (Fig. 4, C) in which one mem- 

 ber of the pair of chromosomes in question comes 

 from the mother and carries the factor for long, and 

 the other comes from the father and carries the 

 factor for vestigial wing. Since this egg with both 

 factors present produces a fly with long wings, the 

 vestigial character is said to be recessive to the long; 

 or conversely the long is said to be dominant to the 

 vestigial character. 



A¥hen the eggs and the sperm of hybrid flies of this 

 origin come to maturity, the homologous chromo- 



