SEX AND HEREDITY 



of the offspring arc tall. If the strain of tall Peas provi-^ . 

 mentally to be pure for that character, all the gametes mu^' 

 that character, and that alone. The union of two such garnet < 

 give a zygote having the tall character only. Such a zygote is known 

 as a homozygote. But a zygote formed by the union of two dissimilar 

 gametes,— ^.^. in the case of Peas where one bears the tall and the 

 other the dwarf character,— is termed a heterozygole. The plant 

 produced from a hetcrozygote frequently shows the form of the pure 

 dominant, and can only be distinguished from it by the test of breed- 

 ing. That the recessive character is carried by it is shown when 

 such heterozygotes are bred together : one quarter of their offspring 

 prove then recessive. Such facts led Mendel to the conception of 

 pairs of unit-characters, of which either can be carried by any gamete 

 to the exclusion of the other. A fundamental property of the gamete 

 is that it can bear either one of such characters, but not both of them. 

 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 characters. To reconcile these statements it must be 

 supposed that at some cell-division in the formation of gametes a 

 primitive germ-cell divides into two dissimilar portions. Instead 

 of the dominant and recessive constituents passing in combination 

 to the two daughter-cells, the whole of the dominant goes into one of 

 these, and the whole of the recessive into the other. From this it 

 follows that every gamete contains only one of such pair of char- 

 acters, i.e. it is pure for that character. In other words, a simple 

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

 numbers. The characters are said to segregate in the gametes. 



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

 (pp. 466-470), it is seen that the segregation postulated as the result 

 of Mendel's experiments is actually effected in the reduction-division, 

 which precedes the formation of gametes. Nuclei of two types are 

 there segregated, each tetrad having two of each type. From the 

 cells containing these nuclei the gametes which share their char.. 

 are finally derived. Two types of gametes are thus produced, .. 

 Mendelian experiments require for their explanation. The rosull> 

 arrived at first by the actual experiments in crossing, and thereafter 

 explained on the basis of the nuclear details, will be made clear by 

 Professor Punnett's figure (Fig. 396). 



The zygotes are represented by squares, the gametes by circles. Evcr>- 

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

 factors belonging to a pair of characters. These factors arc represented by 



