BREEDING SYSTEMS 



cross-fertilization. This habit is the breeding system of the population, 

 or group, or species. We now want to know what breeding systems 

 exist, how thcv arc controlled, and what arc their effects. 



Breeding Systems 



Breeding systems work at different levels with different methods 

 of reproduction. In some fungi, such as the phycomycete Allomyces 

 javatiicus, two cells of a single haploid hypha fuse in sexual repro- 

 duction. This is called homothally and we can see that its result is 

 haploid self-fertilizatioth All the haploid gametes produced by a 

 haploid are identical and, mutations apart, this type of mating must 

 therefore give homozygosis, complete and immediate. In other 

 fungi, such as Mucor, there is a special device, hetcrothally, which 

 prevents self-fcrtiHzation, and ensures that every fusion shall be the 

 result of a cross between different haploid individuals. We shall 

 examine hetcrothally in more detail later. 



In the higher plants and animals, all sexual reproduction demands 

 the crossing of different haploid individuals, since each haploid indi- 

 vidual produces, or even consists of, only one germ cell. But these 

 may or may not come from the same diploid individual. In peas 

 and barley they usually do: diploid selj-fertilizatioti is the rule, even 

 if the parent is heterozygous. This process, like haploid self-fertiliza- 

 tion, leads to homozygosis ; but it does not do so immediately. On 

 mating at random, the haploid gametes produced after meiosis by 

 a heterozygous diploid give zygotes half of which are heterozygous, 

 and half homozygous, in respect of each gene pair which is hetero- 

 zygous in the parent diploid. They give Fo proportions of a half 

 AA and aa taken together, and a half Aa. Diploid self-fertihzation, 

 therefore, does not give immediate homozygosis. Instead it moves 

 (on the average) half-way towards it in each generation. 



In the flowering plants devices exist which encourage self- 

 fertilization. Far more exist, in both plants and animals, which 

 discourage or prohibit it. Of these the most striking, especially in 

 animals, is sexual differentiation, or dioecy, of the diploid organism. 

 Even in these conditions, however, with regular brother-sister 

 mating, heterozygotes will give place to homozygotes just as they 

 do when self-fertilized, though again not so rapidly. For example, 



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