CHROMOSOMES AND MENDELIAN HEREDITY 293 



as units of some kind in the two members of the pair of chromosomes. In 

 the disjunction of these members in meiosis is recognized a cytological 

 mechanism adequate to accomphsh the factorial segregation underlying 

 Mendel's first law. In Fig. 165 the symbols T and t may be taken to 

 represent either a pair of factors or a pair of chromosomes. The same 

 is true of the letters in Fig. 167. 



The cytological basis for Mendel's second law is seen in the inde- 

 pendent distribution of the various chromosome pairs. Different factor 

 pairs are independently distributed, as Mendel held, if they are associated 

 with different chromosome pairs ; but this will not be the case if they lie in 

 the same pair. The discovery of "linkage," to be described below, has 

 accordingly made necessary an important qualification of Mendel's 

 second law, but this has served to show even more strikingly the significant 

 relation between chromosome behavior and Mendelian heredity. 



Mendelian studies are often rendered more difficult by a number of 

 phenomena which tend to alter the characteristic ratios obtained. It 

 frequently happens that among sister individuals with unlike genie 

 combinations some do not proceed far with development, so that an 

 expected class may be partially or wholly absent from the progeny. The 

 germination of pollen grains with certain combinations is also sometimes 

 deficient; even when all the grains germinate it may be found that the 

 pollen tubes carrying certain combinations outgrow the others and so 

 reach all or most of the ovules before the slower tubes arrive. Further- 

 more, there are known a number of "lethal factors" which in certain 

 combinations retard or prevent development at certain stages, notably in 

 the gametes and zygotes. As would be expected, occasional abnormal 

 chromosome behavior results in abnormal ratios also. Because of these 

 phenomena there is often a marked "developmental selection" which 

 favors the multiplication of certain genetic types at the expense of others. 

 In such cases the analysis of the genetic data becomes a more complicated 

 problem, but in this analysis the fundamental assumptions regarding the 

 association of chromosomes and factors are found to be adequate.* 



Linkage and Crossing-over. — When two pairs of Mendelian char- 

 acters are due to differential factors located in different chromosome 

 pairs they are inherited independently, that is, either character of one pair 

 has an equal chance of appearing with either character of the other pair 

 because of the independent distribution of the two chromosome pairs. 

 On the contrary, when their differential genes are located in the same 

 chromosome pair, certain character combinations tend to appear much 



5 Buchholz and Blakeslee (1922, 1927a, 1929, 1930ac, 1932) on Datura pollen 

 tubes; Buchholz (1922) on developmental selection in general; Brink and MacGillivray 

 (1924), Brink (19256, 1927a), Brink and Burnham (1927), Mangelsdorf (1929), 

 Brieger (1926), Nishiyama (1928) and D. F. Jones (1928) on selection among pollen 

 and male gametes. For lethal factors, see Muller (1917, 1918), Li (1927), B. M. 

 Davis (1923), and Morgan et al. (1922). 



