352 



INTRODUCTION TO CYTOLOGY 



An 





among the offspring of plants bearing both D an d. On the other hand, 

 a single G-factor (for green stigma) is able to dominate three g-faetors 

 (for red stigma); hence only two types appear among the progeny, so 

 far as this character is concerned. 



The ratios in which the two types may be expected to appear in 

 tetraploid plants with such strong dominance may be readily computed/ 

 In Primula, for example, only green stigmas are to be expected in the 

 progeny of a GGGG plant after selfing or back-crossing to gggg, because 

 every individual has at least one dominant G; whereas in the progeny of a 



gggg plant after selfing or back-crossing to gggg, 

 only red stigmas appear, since all of the factors 

 present are recessive. In the progeny of plants 

 with GGGg, GGgg, or Gggg, characteristic ratios 

 are to be expected, these depending largely upon 

 the manner in which the four homologous 

 chromosomes carrying the factors in question 

 behave during meiosis. Thus when the four 

 form a quadrivalent with the constitution GGgg 



assume'/°bjr^he''"chromo! ^^^ ^^^ disjoined two from two at random, the 

 somes in the first meiotic ratios wiU not be the Same as when they form 



two bivalents arranged Gg and Gg. 



In tetraploids and tetrasomics a certain 

 amount of sterility may be expected to result 

 from occasional irregularities in chromosome 

 distribution, but, generally speaking, such plants 

 are much more stable and fertile than those 

 having an odd number of certain chromosomes or 

 chromosome sets as in trisomies, triploids, pentaploids, etc. This is 

 because elements present in even numbers are more regularly distributed 

 in the meiotic mitoses than are those present in odd numbers. 



Triploids and Trisomies. — In autotriploid plants the chromosomes 

 most commonly form trivalent groups in the meiotic prophase, although 

 the number of these is often rather variable.^ As described in an earlier 

 chapter (p. 274), the synaptic association in a given region ordinarily 



5 Muller (1914); Blakeslee, Belling, and Farnham (1923); Haldane (1929, 19306). 

 Such computations have ordinarily been made on the basis of the distribution of four 

 chromosomes. Calculations based on the distribution of eight chromatids should 

 be more accurate; see Haldane. 



^ Among researches on the cj^tology of triploid plants are those of Belling and 

 Blakeslee (1922, 1923, 1924a, 1926) and Belhng (19316) on Datura; Lesley (1926) on 

 Solarium; Ono (19276) and Tomowo (1927) on Primula; Newton and Darlington 

 (1927, 1929) and Darlington (1929c) on Tulipa, Belling (1929) and Darlington (19290 

 on Hyacinthus; Nagao (1929a6) on Narcissus; McClintock (1929a) on Zea; Hftkansson 

 (1926a), Capinpin (1930), and Catcheside (1931a, 1933a) on (Enothera; and Steere 

 (1932) on Petunia. 



division in diploid, triploid, 

 and tetraploid sporocytes 

 of Datura. The lowest two 

 configurations in the 3n 

 column are rare. In the 4n 

 column the commonest types 

 are the figure of eight (mid- 

 dle) and the one below and 

 to the left of it. {After 

 Belling and Blakeslee, 1923.) 



