328 



HEREDITY 



that particular variety differs from others. It is known, however, 

 that the individual organism is a combination of thousands of small 



sperm 



R 



carries 



Cq 



Q) 



u 

 u 



d 

 



Rr 



Rr 



^T 



r 



^ 



all hybrid 



An experimenter, Punnett, devised a method 

 of predicting how genes combine after crossing. 

 He used boxes known as Punnett squares. 



Turn to diagram on page 321. The gametes 

 of the parent flowers carry only red, R, or no 

 color, r. Note in the above diagram, all the 

 possible combinations of gametes and observe 

 the identical result Rr — a hybrid. 



sperm 



-R 



Carries 



0) K 



U 



R]$ 



•eB^ 





hylgriol 



y^ 



i-H 



lavl^rid. 



r 



NN^hiiiex 



The gametes of hybrid, pink four-o'clocks, 

 carry either the dominant R or the recessive r. 

 When fertilization occurs, the law of chance 

 determines whether a pure dominant, a hybrid 

 or a pure recessive is formed. Notice how the 

 combinations are described with the Punnett 

 squares. 



differences, some structural, 

 and some physiological or the 

 result of physiological activity. 

 The problem of heredity has to 

 do with studies in respect to 

 each of these individual char- 

 acters. 



After Mendel had assured 

 himself that there was domi- 

 nance and, later, segregation 

 for each of several pairs of 

 characters, he took up the 

 problem of organisms that dif- 

 fered in two characters. For 

 example, some tall plants were 

 hairy and some were smooth ; 

 some hairy plants had yellow 

 seed coats and some had green 

 seed coats ; some of the yellow 

 seed coats were wrinkled and 

 some were smooth. By experi- 

 ments in crossing for several 

 generations, Mendel found that 

 dominance and segregation oc- 

 curred for each character inde- 

 pendenth^ of the other charac- 

 ters, and so he formulated the 

 J.aw of Unit Characters. 



Simply stated, it means that 

 a pair of characters behaves in- 

 dependently of any other pair 



