10 MENDELIAN SEGREGATION 



If the factors for vestigial wings are carried by a 

 pair of chromosomes (the cross-barred chromosomes 

 in Fig. 1) then at the ripening of the germ cells (eggs 

 and sperm) such a pair of chromosomes will come 

 together (Fig. 1, e) and then separate (Fig. 1, g)', 

 so that each germ cell (Fig. 1, h) will have one such 

 chromosome and not* the other. 



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

 that contains a similar group of chromosomes, ex- 



FIG. 4. Diagram to illustrate the fertilization of an egg by a sperm 

 A. One chromosome in the egg differs from the corresponding (ho- 

 mologous ) chromosome in the sperm . The fertilized egg (zygote) with the 

 double (duplex) number of chromosomes in B. 



cept that the corresponding chromosome carries the 

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

 produce a fertilized egg (Fig. 4, B) 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. 



When the eggs and the sperm of hybrid flies of this 

 origin come to maturity, the homologous chromo- 



