HEREDITY AND VARIATION 



573 



contains a pair of allelomorphs corresponding to a pair of contrasting 

 characters. These genes arc represented by rectangles, the ' es- 

 sive " gene being shown in black. As applied to the Peas of Mend- 

 experiment, the original parents (P x ) are pure tall and pure dwarf, 

 the latter being the pollen-parent. In F x the heterozygote contains 

 genes for both tallness and dwarf ness, but the plants are all tall like 

 the tall parent. On producing gametes, these plants yield equal 



f? 



Gametes 



of F? 



rY(heterozygbte) 



Lr 





/ s 



\ P 



&«.& 



©-' 





a- 



~^ 



^ 



(Z=3 



9, 



i 



^ 



o 



t5 



^1 



Ch- 



B^ BL** 



R zygotes 



Fig. 436. 



Scheme illustrating the segregation of characters of a heterozygote in tetrad- 

 division. See Text. (After Punnett.) 



numbers of two kinds, containing "tall" and "dwarf" ger 

 respectively. Every ovule which contains the -cue for tallness may 

 be assumed to have an equal chance of being fertilised by a " tall " 

 or by a " dwarf " pollen-grain, so that " tall " ovules will giv to 



equal numbers of homozygous and of heterozygous tails. Similarly 

 with "dwarf" ovules. Hence of every four zy. in 1.,. on I 



average one will be homozygous for talkn nother homozygous 



for dwarfness, and the remaining two heterozy These pro- 



portions, which correspond to those actually observed in breed- 

 ing experiments, are represented in the middle column of the F, 

 zygotes. 



