124 THE FIRST PRINCIPLES OF HEREDITY 



parent the character r. Now the fertiHzed ovum which is 

 derived from the crossing of both parents will combine in 

 itself both qualities, d and r ; the hybrid will therefore 

 contain the characters d and r, but as the character r is 

 recessive to d, the hybrid will have the appearance of the 

 dominant parent only. 



We call the germ-cells containing the various characters 

 " Gametes/' and the product derived from the union of 

 two gametes — i.e., the fertilized ovum — a " Zygote." If 

 the zygote is formed by the union of two gametes having 

 the same character — i.e., both having the d or both having 

 the r character — it is called a " Homozygote " ; if, on the 

 other hand, it contains two gametes of dissimilar characters 

 — i.e., one having the character d, the other the character r — 

 it is called a " Heterozygote." Mendel now assumed that 

 in the gametes of such a heterozygote as our first hybrid a 

 segregation of the two original parent characters d and r 

 takes place in such wise that only one of the characters 

 appears in any given gamete — in our case either d or r, but 

 never both together. Such characters have therefore been 

 called by W. Bateson " allelomorphic characters- — i.e., they 

 are a pair of characters which in the constitution of the 

 gametes are alternative to each other " — one may replace 

 the other, but both cannot be contained together within 

 the same gamete. For instance, the green factor cannot 

 be contained in the same gamete together with the yellow, 

 nor is any mixture of them possible. The given hybrid 

 would therefore furnish only two kinds of gametes in equal 

 numbers, one set containing the character d, the other 

 only the character r. If now inbreeding takes place, a 

 union of these two sets of gametes is effected, and three 

 kinds of combinations are possible : a gamete d may unite 

 with another d, a gamete d may unite with one of the type ;', 

 or two gametes r may unite in the act of fertilization. 

 The result of these three possibilities can be graphically 

 illustrated in a very lucid manner, according to R. C. 

 Punnett, in the following way : Taking two female 



