rSo MAIZE 



CHAP, and various theories have been proposed to account for it. 

 v ' Webber (i) thought that it might be due to mosaic develop- 

 ment of cell descendants of the endosperm nucleus and of the 

 second male nucleus. East and Hayes, on the other hand, 

 attribute it to incomplete dominance caused by other factors, 

 arguing that if it were due to mosaic development the same 

 cause would act in the case of heterozygous yellow endosperm, 

 whereas " such cases have never been reported ". 



135. Gametic Segregation.— -The heterozygous maize plant 

 obtained by the crossing of male and female gametes produces 

 flowers, and these develop new gametes, approximately 1,000 

 female (egg-cells) and several thousand times as man)- male 

 (pollen grains). The new gametes are formed by the division 

 of a primitive cell into pairs of daughter cells; this process is 

 constantly repeated till the requisite number is reached. 



In this cell-division for the formation of new gametes, 

 the constituent dominant and recessive characters of any one 

 allelomorphic pair (such as yellowness and absence of yellow- 

 ness) both of which were present in the heterozygous mother 

 cell, do not pass into the daughter cells in combination, but 

 the dominant passes into one gamete (e.g. either a pollen 

 grain or an egg-cell) and the recessive into the other (e.g. 

 either an egg-cell or a pollen grain), so that each gamete con- 

 tains only one of an allelomorphic pair of characters, i.e. it is 

 pure for that character. 



As regards any one allelomorphic pair of characters, e.g. 

 yellowness and absence ot yellowness, a heterozygous plant 

 produces gametes of only two kinds, dominant (yellow) and 

 recessive (non-yellow) and produces them in equal numbers ; 

 thus the heterozygous maize plant would produce approxi- 

 mately 500 ovules carrying the dominant, and 500 the recessive 

 character, while of its pollen grains one half (say a million) 

 would contain the dominant and an equal number the recessive. 

 But we find that a heterozygous parent does not produce an 

 equal number of dominants and recessives ; instead we o-et 

 for every three showing the dominant only one showing the 

 recessive character. Let us see why this is. 



136. T lie Reason for Segregation in Definite Mathematical 

 Ratios. — We have seen that half of the pollen grains of a 

 heterozygote contain the dominant character and half the 



