INHERITANCE 



293 



Ft 



YR Yr yR 9 r 



in turn, at fertilization there are sixteen possible combinations of 

 gametes, since there are four different kinds of sperm and four dif- 

 ferent kinds of eggs 

 with respect to the 

 characters in question. 

 Accordingly the F 2 

 generation, which is 

 produced by the union 

 of these gametes, is 

 represented by one 

 pure dominant 

 (YRYR), one pure re- 

 cessive (yryr), four YR 

 homozygotes includ- 

 ing the two just men- 

 tioned, and twelve het- 

 erozygotes. These six- 

 teen individuals form 

 nine genotypes but, 

 since only the domi- 

 nant character is ex- 

 pressed when dominant 

 and recessive genes 



Yr 



J/R 



y 



vfi/ 



E 



z 



phenotypes (YR, Yr, 

 yR, yr) in the ratio 

 9YR:3Yr:3yR:lyr. 

 Thus the 9:3:3:1 



Fig. 183A. — Diagram of a Mendelian dihy- 

 brid. Results of crossing yellow round seeded 

 combine, they are re- (YR) Peas with green wrinkled seeded (yr). The 

 solvable into four circles represent the zygotes and the characters 



of the soma (phenotype); the letters within the 

 circles, the germinal constitution (genotype). The 

 letter groups outside the recombination square 

 represent gametes. The hybrids of the Fi gen- 

 eration are all yellow round seeded since green 

 and wrinkled are recessive. The Fi plants form 

 Mendelian ratio for four types of gametes which afford sixteen pos- 



two pairs of alterna- sible tyP es of z yg°tes, representing four pheno- 



. types (shown graphically) and nine genotypes 

 tive Characters IS ( num bered). There is one pure dominant (1) and 

 merely the monohy- one pure recessive (9). The zygotes numbered 4 

 brid 3 ■ 1 expanded are identical with the Fi generation. Four are 

 tj . , ' ., homozygotes (1, 7, 8, 9) and the rest are hetero- 

 tfotll rest on the same zygotes Those numbered 7 and 8 are new homo- 

 fundamental assump- zygous combinations resulting from independent 

 tion that the genes for assortment. The relative number of individuals 



._. , . ! composing the phenotypes is 9 : 3 : 3 : 1. 

 alternative characters 



are segregated during gamete formation — both members of a pair 

 of allelomorphs never occur in the same gamete. (Fig. 183R.) 



