THE BEHAVIOUR OF INDIVIDUAL CHROMOSOMES 79 



is because of the injurious effects of crossing-over in the differential 

 segments of the X and Y that crossing-over has had to be suppressed 

 in all the other chromosomes of the complement. 



3. Sex Chromosomes in Polyploids 



Polyploids arising by the doubling of all the chromosomes of a 

 diploid set will be either XXXX or XXYY, These are probably 

 sexually normal females and males in most organisms. The tetraploid 

 XXXX female form is known in Drosophila but not the corresponding 

 male. Vallisneria spiralis^ is diploid and dioecious, and V, gigantea, 

 which is tetraploid and may be derived from it, has two separate sexes 

 which were probably formed by simple doubling occurring on two 

 separate occasions, once in a male and once in a female. Probably, 

 however, new polyploid forms have a low chance of survival unless 

 they are hermaphrodites and can be self-fertilized' (unless they are 

 propagated vegetatively).^ The sex chromosomes wiU probably pair in 

 equal pairs XX,XX or XX,YY and differentiated sex chromosomes 

 will therefore be difficult to detect. In fact, the only case known of sex 

 chromosomes in a polyploid is in Fragaria elatior^^ a hexaploid, where 

 it is probable that sex differentiation has been evolved de novo subse- 

 quent to the evolution of the species; the female sex is heterogametic 

 with a simple XY pair, although all other plants have the male sex 

 heterogametic. 



4. Sex-Linked Inheritance 



Some characters appear to be associated in inheritance with sex. 

 This association is of two kinds^, (i) sex-limited inheritance, which is 

 shown by some genes which have an effect in one sex only, because of 

 some physiological connection with sex differentiation (p. 162); (2) sex- 

 linked inheritance, where the association is due to the gene in question 

 lying in the X or the Y chromosome. 



A characteristic phenomenon of inheritance of genes which lie in 

 the X chromosome is a criss-cross heredity by which, assuming male 

 heterogamety, in the Fi from a cross between a homozygous recessive 

 female and the dominant male, the sons show the recessive character 

 from their mother and the daughters the dominant from their father. 

 A similar phenomenon will occur, mutatis mutandis^ in cases of female 

 heterogamety, where the phenomenon was actually first observed.* 



^ Jjj^rgensen 1927. ^ Cf. Muller 1925. 



' Kihara 1930. * Doncaster 1908. 



