GENETICS AND EUGENICS 



827 



smooth seed-coat (dgS) can be plotted by using a checkerboard of 

 sixty-four squares. The ratio of phenotypes of a trihibrid cross is 

 27:9:9:9:3:3:3:1. 



The Back Cross. — The checkerboard is also useful in computing 

 the results of a cross between a heterozygous and a homozygous in- 

 dividual as would be the case in which the offspring of a cross between 

 a homozygous dominant and a homozygous recessive individual is 

 bred back to either of the parental stocks ; a procedure often followed 

 in practical animals and plant breeding. When such a cross is made 

 to the homozygous dominant stock the results, in terms of pea traits, 

 are TT, TT, Td, Td. (Fig. 430.) All four will look alike, i.e., they 

 are phenotypes, but there are two pairs of genotypes. 



T T d d 



d 



Fig. 430. Fig. 431. 



Fig. 430. — A cross between heterozygous and homozygous tall peas produces a 

 2:2 ratio. 



Pig. 431. — A cross between heterozygous tall and homozygous dwarf peas also 

 produces a 2:2 ratio. 



When the heterozygous stock is bred back to the homozygous reces- 

 sive parental stock, a 2 :2 ratio is also produced. This time the two 

 kinds are not only genetically unlike, but they also appear unlike 

 (Fig. 431). 



Complications of Mendelian Inheritance 



If all inheritable characters followed Mendel's law in as simple a 

 fashion as do the pea traits that Mendel first investigated, the science 

 of genetics would be much more easily mastered. Unfortunately this is 

 not the case. Although Mendel's law is found so consistently as the 

 underlying principle of heredity that it can be said to be practically 

 universal in its application, it is often modified and complicated in 

 various ways. 



Multiple Allelomorphs. — Instead of the allelomorph always consist- 

 ing of the usual two factors, a larger number of alternatives sometimes 



