To Identify a HeTerozgyote. — In practical breeding it is 

 often necessary to know definitely whether the organism is 

 homozygous or heterozygous as to a certain character. Trie 

 easiest way of identification is to cross with a recessive. If 

 the organism is a homozygote all the progeny will show 

 dominance, if a heterozygote only fifty per cent, will show 

 dominance, according to the following diagram:— 



(1) 



female 

 gametes 



(2) 



female 

 gametes 



D 



D 



= male gametes 



= 4RD,or all showing dominance. 



= male gametes 



= 2RD + 2RR, or 50% domi- 

 nant. (Fig. 15) 



How Unit Characters may be Determined. — The only 

 method of determining unit characters is by crossing. Men- 

 delian investigators have revealed the presence of pairs of 

 contrasting characters when varieties are crossed both in 

 the Fi and Fo generations. 



(1) The hybrids of crosses (Fi) frequently show one of 

 the pairs of characters when dominance exists. 



(2) Segregation or splitting occurs in the Fo generation. 



Simple Mendelian Segregation 



Before beginning the study of segregation in di-hybrids 

 and tri-hybrids it is advisable to become familiar with 

 the results of simpler crosses: (1) a homozygote, such as 

 a "pure yellow" pea, with a heterozygote, such as an impure 

 yellow. 



All the male gametes will contain the factor for vellow, 

 and of the female gametes half will contain the factor for 

 yellow and half the factor for green. The possible matings 

 may be represented as follows. — 



Y 



Female 

 Gametes 



J Y 

 1 

 I G 



= Male Gametes. 



= 2YY -I- 2GY; 

 Phenotypically all yellow, but 

 genotypically half are pure 

 yellow and half impure 

 yellow. 



86 



