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ESSENTIALS OF ZOOLOGY 



To the amazement of the investigator they produced nothing but tall 

 peas ; peas as tall as the original tall parents. Dwarf ness seemed to 

 be lost and tallness was certainly dominant. A less thorough investi- 

 gator might have called his experiment finished and have proclaimed 

 as a rule, the fact that a trait can disappear when it is crossed with 

 its opposite. But not so with the patient monk. He pollinated these 

 plants with their own pollen and planted another generation. This 

 time three-fourths of the plants were tall and one-fourth of them 

 were as short as the original dwarf parents. Not being ready even 

 yet to formulate a law, he self -pollinated his plants for several more 

 generations and got results that required his best mathematical skill 

 to interpret. The dwarf peas produced only dwarf peas. Of the tall 



0J0 



TT 



Td 



Td 



dd 



Fig. 73. — Diagram to show the result of crossing- tall and dwarf peas, TT, at 

 upper left, is an individual which is pure dominant for tallness. At upper right, 

 dd is pure recessive for dwarfness. When two such individuals are bred together, 

 or crossed, all of their offspring will carry both characters, but appear tall, smce 

 tallness is dominant. The Td individual in the upper middle is representative of 

 this generation, which is the Fi (1st filial). W^hen two of these mixed, Td individ- 

 uals are crossed, the offspring (F2 or 2nd filial generation) present what is known 

 as the 1:2:1 ratio, i.e., one pure tall (TT) individual (at left), two mixed (Td) in- 

 dividuals (in center), and one pure dwarf individual (dd) at right. The bottom and 

 next to bottom rows show (at left) continued generations of crossing pure tall TT 

 with TT to consistently produce pure tall offspring. The middle of the rows shows 

 the consistent production of the 1:2:1 ratio when the mixed Td individuals are 

 crossed. The pure dwarf individuals dd at the right in these lower rows show suc- 

 ceeding generations of breeding together to produce all pure recessive dwarf (dd) 

 offspring. 



