PROBLEM 2. Differences Betiveeii Varents and Offspring 



Fig. 414 (Pj) ///eiiiis the irirst 

 parental generation; that is, 

 the first mating in this par- 

 ticular experiment. The X 

 means that matings occur. 

 What kinds of gametes are 

 provided by each parent? 

 (F^) means the first filial 

 generation, the children of 

 the first generation. The 

 next generation woidd be 

 (FJ. 



Pi 



fi 



RW 



50% 

 Gametes 



50% 



.0 



25% 



25% 



467 



RW 



.0 



i'y% 



R spemi Tiiigbt meet R egg, producing 

 RR offspring. 



W spenn might meet W egg, producing 

 WW offspring. 



R sperm Tiiight meet W egg, producing 

 RW offspring. 



W sperm jnight meet R egg, producing 

 RW offspring. 



The results of these matings are dia- 

 gramed in Figure 414. To get some prac- 

 tice in such crossings, do Exercise 2. 



Since these four kinds of fertilizations 

 are a matter of chance, the first one to 

 occur may be the WW cross. If, because 

 of some accident, no more fertilizations 

 were to take place there would be only 

 one offspring, TI^TF. But when there are 

 large numbers of fertilizations, a thou- 

 sand or more, there are on an average 

 25 per cent of RR produced, twice as 

 many or 50 per cent RW, and 25 per 

 cent WW. This is the 1:2:1 ratio of 25 

 per cent red-flowered plants, 50 per cent 

 pink, and 25 per cent white. By using 

 beads or beans to represent genes you 

 can demonstrate the ratio obtained in 

 crossing hybrids. Do Exercise 3. You 

 can test your understanding by doing 

 Exercises 4, 5, and 6. 



When contrasting characters do not 

 blend. Most of the characters in plants 

 and animals do not blend as red blends 

 with white in the four-o'clock, or as 

 black and white blend in the Andalusian 

 fowl (see Fig. 415). For example, when 

 pure black guinea pigs {BB) are crossed 

 with pure white (iviv) all the offspring 

 are as black as the black parent. There is 

 no blending (see Fig. 419, page 470). Yet 

 the F, black guinea pigs contain a gene 

 for blackness (B) and one for \\ hiteness 

 (11'); they are hybrids just as the pink- 

 flowered four-o'clock. Notice that the 

 gene for whiteness is represented by a 

 small letter. You will soon find out why. 



Almost a centurv^ ago an Austrian 

 monk, Gregor Mendel (1822-1884), ex- 

 perimented with garden peas in his mon- 

 astery^ garden. His experiments are mod- 

 els of clear thinking, accurate observa- 

 tion, and careful recording. His work is 

 considered particularly brilliant, in part 

 because he knew nothing about chromo- 

 somes, which had not yet been seen. He 

 crossed tall pea plants six to seven feet 

 high, with short ones less than eighteen 

 inches high. All the offspring were as tall 

 as the tall parent. As in guinea pigs, there 

 was no blending of the two contrasting 

 characters. Mendel called the character 



