Heredity 177 



Results involving three pairs of characters (trihybrid). 

 When three allelomorphic pairs are concerned, the num- 

 ber of forms in the second and subsequent generations is 

 greatly increased. For illustration, let us take a hypo- 

 thetical case. Suppose we cross together a tomato having 

 red fruit, dwarf vine, and hairy stems and leaves (the 

 latter is hypothetical) with a variety having yellow fruit, 

 tall vine, and smooth stems. Their formulae would be as 

 follows, using capitals again to represent dominant units 

 and small letters to represent recessive units : red, 

 dwarf, hairy (RtH) X yellow, tall, smooth (rTh) = red, 

 tall and hairy (in appearance) RrTtHh. F 2 generation 

 will be as shown in table on page 178. 



In order to get a better understanding of the probable 

 union of gametes of various kinds of crosses, the student 

 should carefully master the method of squares, always 

 having in mind that the use of formulae is only a con- 

 venient method of representing plants. Each square 

 represents a plant. (See methods as already outlined on 

 page 169.) Capital letters will be used for dominant 

 units and small letters for recessives as formerly. Plants 

 having as their formulae large and small of any letter, i.e. 

 Rr, are hybrids (heterozygous) for that character, and those 

 in which the letters are the same, i.e. RR, are pure (homo- 

 zygous) for that character. 



It will be seen that when three pairs of characters are 

 involved, at least 64 squares are necessary to allow for 

 the theoretically possible number of combinations to be 

 formed. A very careful study of the table will show that 

 there are produced 8 visible types (2 n ) with proportions 

 as follows : 27 Red Tall Hairy, RTH ; 9 Red Tall smooth, 



