GENETICS AND EUGENICS 



241 



that there are nine different combinations of letters and that in only 

 one case as manj^ as four are exactly similar. Individuals whose genes 

 cause them to look alike are called phenotypes ; those whose genes are 

 exactly alike are called genoty,pes. 



A trihybrid cross, such as one between a tall yellow-podded pea with 

 a wrinkled seed-coat (TYw) and a dwarf green-podded pea with a 

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

 sixty-four squares. There are eight different kinds of male and female 

 gametes found in this arrangement. The ratio of phenotypes of a tri- 

 hvbrid cross is 27 :9 :9 :9 :3 :3 :3 :1. 



Fig-. 77A. 



Fig. TIB. 



Fig. nA. — A cross between heterozygous (Td) and homozygous tall peas (TT) 

 produces a 2:2 ratio. The resulting individuals are two pure tall (TT) and two 

 phenotypically tall or mixed (Td). 



Fig-. 77B. — A cross between heterozygous tall (Td) and homozygous dwarf peas 

 (dd) also produces a 2:2 ratio. The resulting individuals here are two pure dw.arf 

 (dd) and two phenotypically tall or mixed (Td). 



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 animal 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. 



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

 sive parental stock a 2 :2 ratio is also produced, but between heterozy- 

 gous and homozygous recessive individuals as : Td, Td, dd, dd. 



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 elementary. Unfortunately this is 

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



