Autosomes and Sex Chromosomes 133 



the same way as for one. If a plant phenotypically dominant 

 for both characters is suspected of being a double heterozygote, 

 it can be tested with the double recessive. If the unknown 

 dominant is heterozygous for both genes the ratio will be 

 1:1:1:1, but if it is homozygous for both, all the offspring 

 will be dominant for both characters. If it is homozygous for 

 one pair and heterozygous for the other, the testcross ratio will 

 be 1 : 1. For example, if the "unkno^Ti" plant is genotypically 

 Plpl CrCr, the offspring of the testcross will be 1 purple, normal- 

 leaved : 1 green, normal-leaved. 



Fz Generation. When the various F2 plants are self- fertilized 

 to produce the F3 generation, all the plants of the same pheno- 

 type do not produce the same results as they are not geno- 

 typically alike. For example, in the F2 of the cross between 

 homozygous purple, normal-leaved and green, crinkly-leaved 

 plants, three-sixteenths are purple and have crinkly leaves. 

 When they are self-fertilized, one-third of them breed true and 

 produce only purple, crinkly-leaved plants; but the other two- 

 thirds produce both purple, crinkly-leaved plants and green, 

 crinkly-leaved plants in the ratio of 3 : 1. AVhen all the various 

 kinds of Fo plants are selfed, the results are: 



F2 Phenotypes F2 Genotypes F3 Phenotypes 



%Q purple, normal-leaved : }^{PIPI CrCr) gives all purple, normal 



% {PlPl Crcr) gives 3 purple normal : 1 purple 



crinkly 

 %{Plpl CrCr) gives 3 purple normal : 1 green 



normal 

 %{Pl'pl Crcr) gives 9 purple normal : 3 purple 

 crinkly : 3 green normal : 1 

 green crinkly 

 ^16 purple, crinkly : }/^{PlPl crcr) gives all purple crinkly 



% {Plpl crcr) gives 3 purple, crinkly : 1 green, 



crinkly 

 /1 6 green, nqrmal : }/^{plpl CrCr) gives all green, normal 



% {plpl Crcr) gives 3 green, normal : 1 green, 

 crinkly 

 H 6 green, crinkly : all {plpl crcr) gives all green, crinkly 



Autosomes and Sex Chromosomes 



Independent segregation of chromosomes and their genes oc- 

 curs between the sex chromosome and a pair of autosomes as 



