476 



BOTANY OF THE LIVING PLANT 



rectangles, the recessive being black. As applied to the Peas of Mendel's 

 experiment, the original parents (Pi) are pure tall and pure dwarf, the latter 

 being the pollen-parent. In (Fr) the heterozygote contains factors of both 

 tallness and dwarfness, but the plants are all tall like the tall parent. The 

 factors being unsplittable, on producing gametes these plants yield equal 

 numbers of two sorts, bearing tall and dwarf factors respectively. Every 

 ovule which bears the factor for tallness has an equal chance of being fertilised 

 by a " tall " or a " dwarf " pollen-grain, and " tall " ovules will therefore 

 give rise to equal numbers of homozygous and heterozygous tails. Similarly 

 with " dwarf " ovules. Hence of every four zygotes in (F2), one will be 



f? 



Gametes 



of F? 



Fj(heteTozySt>te) 



F-^ zygtites 



FIG. 396. 



Scheme illustrating the segregation of characters of a heterozygote in tetrad 

 division. See Text. (After Punnett.) 



homozygous for tallness, another homozygous for shortness, and the remain- 

 ing two heterozygous. These conditions, which correspond to the number 

 resulting from experiments, are represented by the middle column of the (F2) 

 zygotes. 



Since by Mendelian segregation the racial characters are thus 

 being constantly sifted out, and kept pure, one chief effect of sex 

 upon heredity will be the perpetuation of new heritable characters. 

 A plant resulting from a mutation would be liable to interbreed with 

 the more numerous normal forms. There would thus be a proba- 

 bility of the mutation being swamped by promiscuous crossing, 

 were it not for the fact that the segregation will tend in each genera- 

 tion to sift the pure heritable mutation out again. An innovation 



