CHAPTER IX 



SOME MENDELIAN TERMS AND THEIR USES 



In describing Mendelian heredity it is convenient for brevity 

 to use technical terms, some of which are already in general 

 use among biologists, but others of which have been framed 

 to meet needs not previously existing. The significance of 

 these the reader must keep clearly in mind, for which reason 

 it seems best brieflv to define them. 



A gamete is a reproductive cell capable of uniting with 

 another reproductive cell to form a new individual. In all 

 the higher animals and plants the gametes which are capable 

 of union in pairs are of two unlike sorts, eggs and sperms. 



An egg-cell (capable of fertilization) is the larger, non- 

 motile gamete, produced by the female parent, when the 

 parents are sexually different. 



A sperm is the smaller gamete, commonly motile, and pro- 

 duced by the male parent, when the parents are sexually 

 different. Exceptions to the motility of sperms occur in the 

 Crustacea among animals and in all but the lowest of the 

 flowering plants. In the lowest flowering plants motile 

 sperms are found in the pollen-tube, but in the ordinary 

 flowering plants the two gametes which are produced in the 

 pollen-tube are non-motile. The pollen-tube itself transports 

 them by its growth toward the egg-cell of the plant. 



A zygote results from the union of two gametes in fertiliza- 

 tion, an egg with a sperm. It is, potentially or actually, 

 a new individual produced by a sexual process (union of 

 gametes) . 



A homo-zygote results from the union of gametes which 

 transmit the same Mendelian character, as black joined with 

 black, or white joined with white. 



A hetero-zygote results from the union of gametes which 

 transmit alternative Mendelian characters, as black united 

 with white. 



98 



