1 82 MAIZE 



CHAP, find all of the heterozygotes (i.e. 50 per cent of the total) as 



V - well as half of the homozygotes (i.e. an additional 25 per cent 



of the total, making 75 per cent in all), showing the dominant 



character. This explains why we get three dominants to one 



recessive in the F., generation. This is illustrated in Fig. 79. 



This is known as a nwnohybrid ratio because only one 

 pair of allelomorphs is concerned. 



The occurrence of definite mathematical ratios is a useful 

 guide in the breeding of animals and plants, as it indicates the 

 dominant and recessive factors, respectively, and which of the 

 progeny of a heterozygote will breed true. 



1 17. The Monokybrid Ratio. — The notation usually adopted 

 to indicate this ratio is 1:2:1 or 25 : 50 : 25 per cent, which 

 indicates that there is 1 dominant homozygote to 2 hetero- 

 zygotes and I recessive homozygote ; it is often referred to 

 as the 3 : 1 ratio, because there are three zygotes in which the 

 dominant character is visible, to one in which it is absent. 

 This ratio is due to the fact that where only one allelomorphic 

 pair (Aa J ) is concerned in the cross, there are only four 

 possible zygotic combinations of the characters involved, i.e. 

 AA, Aa, a A, and aa. 



If, instead of selfing the heterozygote Aa it is re-crossed 

 with either of the homozygous parental types (AA or aa) we 

 naturally get a different ratio, for the homozygous parent 

 carries only. one of each of the allelomorphs ; if Aa is crossed 

 with' AA for instance, it produces AA and aA in equal propor- 

 tions ; or 50 per cent homozygous for the dominant character, 

 and 50 per cent heterozygous. Or if Aa is crossed with aa it 

 produces Aa and aa in equal proportions ; or 50 per cent 

 heterozygous, and 50 per cent homozygous for the recessive 

 character. 



1 38. Dihybrid Ratios. — -Where, however, two allelomorphic 

 pairs (e.g. Aa and Bb) enter into the cross, there will be six- 

 teen possible combinations of their four allelomorphs. When 

 gametic segregation takes place in the heterozygote AaBb, 

 the chances of B being distributed to a gamete containing A, 

 or a, are equal ; hence the gametes containing B will be of two 

 sorts, AB and aB ; so, also, the gametes without B will be of 

 two sorts bA and ba, and these will be produced in equal 



1 A convenient notation, A = the dominant ; a, the recessive allelomorph. 



