44 THE CHROMOSOMES 



it is clear from recent work on the parasitic wasp 

 Habrohracon that the haploid- diploid scheme of sex- 

 determination CO -exists with a mechanism involving 

 female heterogamety.^'^ xhe females possess an X- 

 and a Y- chromosome and thus produce two kinds 

 of eggs, X-eggs and Y-eggs. If these develop with- 

 out fertilization they give rise to haploid males which 

 are accordingly of two types, X-males and Y-males. 

 These produce sperms by a modified meiosis which 

 does not involve any reduction in number of chromo- 

 somes. Normally X-eggs are only fertilized by Y 

 sperms and vice versa, giving rise to diploid XY- 

 females ; occasionally, however, X-sperms fertilize 

 X-eggs and Y-sperms Y-eggs. The result of this is 

 to produce extremely rare diploid male individuals. 

 Femaleness in Habrobracon clearly depends on an 

 interaction of genes present in the X- and Y- chromo- 

 somes ; which explains why both haploid and homozy- 

 gous individuals are males. 



Polyploidy 



Polyploid cells arise in the first place through a 

 failure of cell division ; that is to say that in a par- 

 ticular cell the chromosomes divide, but the cell fails 

 to do so.^'^' ^'^ If this happens in a diploid organism 

 a tetraploid cell will result. Such a process occurs 

 normally in a small percentage of the meristem cells 

 in the root tips of certain plants 121, 126. ^g^ result 

 patches of tetraploid tissue are produced, surrounded 

 by diploid cells. In the tomato it is possible to pro- 

 duce tetraploid shoots by repeated cutting back.*^^ 

 From such shoots it is possible to produce tetraploid 

 tomato plants. 



If the process is repeated in a tetraploid plant 

 octoploid cells will be produced. Triploids arise in 

 the main through crossing between diploids and terta- 

 ploids, hexaploids by a doubling of the chromosome 

 set in a triploid and pentaploids by crossing between 

 tetraploids and hexaploids. 



