208 



FUNDAMENTALS OF CYTOLOGY 



borne on the diploids. Similarlj^ yellow kernels produced by tetraploid 

 maize plants have more vitamin A than those from diploids. 



Chromosome behavior in autotetraploid plants is normal throughout 

 somatic development. At meiosis, however, certain irregularities arise 

 when all or some of the chromosomes form quadrivalent groups at 

 synapsis (Fig. 154). This may lead to some irregularity in distribution 

 at anaphase / and add thus to the sterility attributable to pther genetical 

 causes. In colchicine-induced autotetraploids, fertility ranges from a 

 fairly high value comparable to that in many natural tetraploids down to 

 complete sterility. Later it will be pointed out that in allotetraploid 



Fig. 155. — Selted ears from diploid (above) and tetraploid (below) plants heterozygous 

 for the color-factor pair Rr. Segregation for color is about 3:1 in the diploid and 35: 1 in 

 the tetraploid. Note difference in size of kernels. {After L. F. Randolph.) 



plants the fertility may be much higher than in the diploids from which 

 they are derived (page 221). In tetraploid Unes, diploid individuals 

 appear on rare occasions as a result of parthenogenesis, just as haploids 

 sometimes appear among diploid organisms. 



Genetical ratios for characters of autotetraploid plants tend to be 

 unlike those of diploids, for each chromosome is present in quadruplicate. 

 Assuming a random distribution of the four chromosomes bearing a 

 given gene, the expected phenotypic ratios after selfing are 1:0 for a 

 plant with A AAA or AAAa, 35: 1 for a plant with AAaa,^:l for a plant 

 with Aaaa, and 0:1 for one with aaaa. The corresponding tcstcross 

 ratios are 1:0, 5:1, 1:1, and 0:1. These expectations are for chaiacters 

 controlled by genes in regions near tlie kinetochore, where the four 

 chromosomes are distributed at random but the eight chromatids are 

 not, owing to the fact that sister clu-omnlids tend to pass regularl>' to 



