356 PERMANENT HYBRIDS 



chromosome from the opposite complex was covering, as it were, 

 the recessive lethal of the homozygous complex. But this is not 

 the case. Different chromosomes are equally effective. Catcheside, 

 therefore, concludes that the lethal effect is not the property of 

 specific genes, but is due to a general unbalance which may be 

 rectified by different extra chromosomes. In considering the half- 

 mutants of CE. Lamarckiana, we must now say, therefore, not that 

 subvelans has lost the lethal genes of velans and gaudens, but that 

 it is a viable combination of parts of these two complexes. 



The lethal properties of a complex are therefore due to unbalance, 

 to an exchange of materials between the two complexes of such a 

 kind that each becomes unsatisfactory by itself, while both together 

 remain a working combination. Such a condition can arise by the 

 differential segments becoming altered by the loss, gain or inter- 

 change of parts. Although other changes no doubt occur, inter- 

 change between chromosomes inside the ring will by itself lead to 

 such a differentiation and will thus account for all the genetical 

 properties of complex heterozygotes (D., 1933 a). 



3. SEX HETEROZYGOTES 



(i) Types of Sex Chromosomes. The difference between the sexes 

 is inherited, as a rule, as a single alternative — male or female and no 

 intermediate. When it is determined by genetic segregation it 

 must therefore depend on a single gene difference or on a group of 

 differences acting as a unit in inheritance. Such an inheritance of 

 sex is often bound up with the inheritance of " sex-linked " 

 characters, which also act as a unit in the heterozygote. It is then 

 clear that the whole block must be determined by differences 

 between one pair of chromosomes or between chromosomes in a 

 configuration of the (Enothera type. This is found to be the case 

 in all organisms with unitary genotypic sex determination. The 

 chromosomes concerned are known as sex chromosomes. Other 

 pairs of chromosomes whose differences bear no relation to the 

 differentiation of sex are known as autosomes. 



In the dioecious Bryophyta the diploid sporophyte produces 

 two kinds of haploid spores, those giving gametophytes with only 

 male organs and gametes, and those giving gametophytes with only 



