572 BOTANY OF THE LIVING PLANT 



must " carry " the tall character ; for from an impure tall three- 

 quarters of the offspring are tall. If the race of tall Peas proves 

 experimentally to be pure for that character, all the gametes must 

 " carry " that character, and that alone. The union of two such 

 gametes will give a zygote carrying the gene for tall character only. 

 Such a zygote is known as a homozygote. But a zygote formed by the 

 union of two dissimilar gametes — e.g. in the case of Peas where one 

 •'carries " the tall and the other the dwarf character — is termed a hetero- 

 zygote. The plant produced from a heterozygote frequently shows the 

 form of the pure dominant and can be distinguished from it only by the 

 test of breeding. That, nevertheless, the recessive character is present 

 in it is shown when such heterozygotes are bred together, one-quarter 

 of the progeny proving recessive. It is the elements in the gametes cor- 

 responding to the differentiating characters of the zygotes that are now 

 known as genes (Johannsen). Pairs of genes corresponding to pairs of 

 contrasting characters, such as tallness and dwarfness in the Pea, are 

 called allelomorphs or alleles. The heterozygote is formed by the union 

 of two dissimilar gametes, and consequently the cells of the individual 

 into which it grows must contain both allelomorphs. In order to recon- 

 cile these statements it must be supposed that at some cell-division 

 previous to the formation of gametes a primitive germ-cell divides into 

 two dissimilar portions. Instead of the two allelomorphs passing in 

 association into both daughter-cells, the gene corresponding to the 

 " dominant " characterpasses into one, and the " recessive " geneinto the 

 other. From this it follows that every gamete contains one of a pair of 

 allelomorphs only, i.e. it is pure for that gene. In other words, a simple 

 heterozygote produces gametes of two kinds, and produces them in equal 

 numbers. Each gamete is pure for one of each pair of allelomorphs. The 

 genes are said to segregate in the formation of the gametes. 



If we now return to the details of the tetrad-division described 

 above (pp. 563-567), it is seen that the segregation postulated as the 

 result of Mendel's experiments is actually effected. Nuclei of two 

 types are there segregated, each tetrad having two of each type. 

 From the cells containing these nuclei (tetraspores) the gametes 

 which share their genes are ultimately derived. Two types of gametes 

 are thus produced, as the Mendelian experiments require for their 

 explanation. The results arrived at first by the actual experiments 

 in crossing, and thereafter explained on the basis of the cytological 

 details, will be made clear by the diagram (Fig. 436). 



The zygotes are represented by squares, the gametes by circles. 

 Every zygote, being formed by fusion of two gametes, is double and 



