326 



HEREDITY 



When hybrids are crossed with pure dominants, the offspring 

 will still retain the dominant appearance, but the recessiveness is 

 not destroyed, for it may be made to reappear in later generations. 

 These offspring are found to be, approximately, one half pure domi- 

 nant and one half hybrid, although they may all look dominant. 



When hybrids are crossed with pure recessives, segregation again 

 takes place. One half are recessive and one half appear to be 

 dominants, but are hybrids. When hybrids are crossed with 

 hybrids, or with pure dominants, or with pure recessives, the 

 original parental characters tend to sort out. This, Law of Segre- 

 gation, is perhaps the most important discovery that Mendel made, 

 although it grew out of his discovery of dominance. 



Problem. What combinations of chromosomes result from maturation 

 and fertilization f 



I. Place two black beans (to indicate chromosomes carrying black color) 

 in the circle representing the egg mother ceH, and two white beans (indicating 



mature eg^« Sperms 



€g^Tnother cell 



sperm jnother^cell 



the chromosomes carrying white color) in the sperm mother cell. Assume 

 that the cliromosomes have the character-determiners or genes for color only. 



A. Move the chromosomes from the primary sex cell to the gametes to 

 show the change that takes place during maturation (reduction division). 



B. Move the beans in the circles representing the sperms, so as to indicate 



