326 



ELEMENTS OF BIOLOGY 



is, the number of pairs o£ characters is unknown and may be very 

 great. 



Dihybrid Cross. If a cross is made between two individuals 

 involving two pairs of alternative characters located either on the 

 same or on diilerent chromosomal pairs, it is known as a dihybrid 



PARENTS: 



YYSS 



Y Y S S 



GERM CELLS: 



ys, Ys. 



r,: 



Ff GERM CELLS- 



yr, Ss, 



ys, ys, ys, y s, 



EGGS 



SPERM i 



TYPES OF PROGENY 

 F^ GENERATION 



Fig. 205. — Scheme to show the segregations of yellow color (Y) and its alternative 

 character, green (y); and of smooth seed (5) and its alternative, wrinkled (■>■), in a 

 dihybrid cross in peas. Capital letters represent the dominant, and small letters the 

 recessive characters. Posterior germ cells and their combinations are shown as ova across 

 the top of the table and as sperms along the side. Verify the Mendelian 9:3:3:1 

 ratio. 



cross; when three alternative characters are employed, it is termed 

 a trihybrid; when many are used the cross is a polyhybrid. We con- 

 tinue to follow Mendel's original work as examples; the next case 

 is that of a dihybrid cross (Fig. 205). For this he used two indi- 

 viduals which differed in two characters, the color and smoothness 

 of the seed. Alternativt characters for seed color were Yellow (Y) 



