306 



INTRODUCTION TO CYTOLOGY 



there has been at hand genetic evidence that crossing-over usually, if not 

 always, involves only two of the four chromatids at a given locus, and 

 it has also been shown that the same two chromatids are not always 

 involved in all the crossovers in one tetrad. ^^ Recent data from Droso- 

 phila indicate further that crossing-over occurs only rarely, if at all, 

 between sister chromatids. ^^ 



Mendelism in Monoploid Plants. — The principles enunciated by 

 Mendel were based on phenomena observed in organisms with diploid 



A BC 



ZY&OTE. 



SYNAPSIS 



ABC aire 



Aa Bt Cc 



abc 



ntioTic niTOSLs 



Fig. 176. — Diagram illustrating chromosome cycle of plant with monoploid (haploid) 

 body. Owing to independent orientation of the several chromosome pairs in the meiotic 

 mitoses, a given monoploid individual may have any one of eight possible chromosome 

 assortments when there are three pairs of chromosomes. 



somatic nuclei and monoploid gametes. Since that time most genetic 

 investigations have been carried on with such organisms. It will now 

 be of interest to see how the same principles hold in the case of organisms 

 having the monoploid number of chromosomes in their somatic nuclei. 

 Genetic work of this nature has been done with haplonts (in which only 

 the zygote is diploid; p. 278) and with the gametophytes of diplohaplonts, 

 especially among bryophytes. 



The chromosome cycle of a haplont (e.g., certain green algae) is shown 

 in Fig. 176. The monoploid plants produce monoploid gametes which 



12 Bridges (1916), Bridges and Anderson (1925), Anderson (1925c, 1929), L. V. 

 Morgan (1925), Rhoades (1932), Redfield (1930), Anderson and Rhoades (1931). 

 " Weinstein (1932), L. V. Morgan (1933). 



