58 AN INTRODUCTION TO MODERN GENETICS 



also occur in straightforward diploid inheritance if the character 

 brought in by the male shows early enough. 



4. Haploid Phase Characters in Animals 



Few genes are known which affect the gametes of animals.^ Self- 

 incompatibility, apparently similar to that described in plants, is known 

 in some hermaphrodite animals (e.g. the ascidian Ciona)^ but nothing 

 is known of its genetic basis. In some animals, it has been stated that 

 the sperm fall into two size classes, or have a bimodal size-frequency 

 curve.^ The two sizes are usually supposed to be associated with the 

 two classes of sex-determining sperms, since bimodal curves are only 

 known in forms with male heterogamety. 



Cases in which the eggs fall into two classes in size have also been 

 reported and in some of these the size differences have been supposed 

 to be the causes of sex determination (p. 230); the genetic basis of the 

 dimegaly is, however, unclear. The best known cases of variation in the 

 egg characters of animals have been shown to be due to factors which 

 act in the diploid tissue of the mother causing all her eggs to be of one 

 specific kind, e.g. egg-shape and colour in silk worms, type of coiling in 

 Limnea (p. 143). These genes must be clearly distinguished from genes 

 which act during the haploid phase itself. 



B. APOMIXIS^ 



The normal process of sexual reproduction, on which Mendelian 

 inheritance depends, has two essential features : reduction of chromo- 

 some number and fertilization. In the reproduction of some organisms 

 one or other or both of these features may be absent. In cases where 

 reproduction occurs simply by the development of some of the somatic 

 cells into a new organism we speak of budding, or vegetative propaga- 

 tion; the genetic result is of course that the offspring exactly resemble 

 the parent, with rare exceptions due to somatic mutation. Genetic 

 series of organisms derived vegetatively from one parent constitute a 

 clone. Reproduction of this kind occurs in few animal species but is 

 frequent in plants where it is of great economic importance in the 

 propagation of varieties (hybrids, imbalanced polyploids, etc.) which do 

 not breed true. Many of the best varieties of cultivated apples, oranges, 



1 Cf. Muller and Settles 1927 ^ Morgan 1938. 



^ Wodsedalek 1913, Zeleny and Faust 1915, but see Krallinger 1928. 

 * General references: Darlington 1937, chap. 11, Peacock 1925, Rosenberg 

 1930, Stem 1928. 



