CYTOLOGY AND MENDELIAN HEREDITY 



173 



character pairs are tall vs. dwarf, and green pod vs. yellow pod, the 

 first-named character in each pair being the dominant one. A plant 

 dominant and homozygous for both differential gene pairs (7" T G-p Gp) 

 is crossed to one that is recessive and homozygous for both pairs (/ / gp gp). 

 The Fi plants are heterozygous for both pairs (T t Gp gp). When these 

 plants are testcrossed to a, t t gp gp plant, plants of four kinds appear in 

 equal numbers in Fo: tall-green, dwarf -yellow, tall-yellow, and dwarf- 



T AND Gp INDEPENDENT 



/\ and 

 Somatic nucleus MefaphoseX i 



1^ 



t t gp gp rTGpjftgpj (Tgp)(tGp) 



Mgp),^cr". Gametes 



1^ 



1^ 



T t Gp gp 



Tt gpgp t t Gp 



IF T AND Gp WERE LINKED 



T t Gp gp 



t t gp gp 



Fig. 123. — Diagram illustrating the cytological basis of independent and of linked 

 inheritance of characters. Vertical lines represent plants; green pods stippled; yellow pods 

 unshaded. Actually, these particular characters are independent, not linked. Further 

 explanation in text. 



green. This shows that the Fi plants must have produced spores and 

 gametes of four kinds in equal numbers: T Gp, t gp, T gp, and t Gp. The 

 explanation for this lies in the fact that the two gene pairs concerned 

 lie in different chromosome pairs which have two possible relative orienta- 

 tions at metaphase / in the sporocytes of the Fi plants. Hence Mendel's 

 second law expressed the results of random assortment of chromo.some 

 l^airs at meiosis. 



An important qualification has had to be made in Mendel's second 

 law, now that more character pairs have been studied. Organisms ha\-e 



