Semi-apospory — 135 — Shift 



Sex Hormones. — A group of hormones, belonging to the 

 steroids, which affect or control the sexual organs and secondary 

 sexual characters and manifestations. 



Sex-index. — In insects (particularly Drosophild), the number 

 of X-chromosomes per set of autosomes, e.g. a Drosophila male 

 has a sex index of 0.5, a female is 1.0, a super-male 0.33 and 

 a super-female 1.5. 



Sex-influenced Factors. — Factors whose dominance is de- 

 pendent on sex so that in one sex the heterozygote shows the 

 character concerned whilst in the other sex the character is 

 recessive. 



Sex Limited. — The expression of a character in only one sex 

 by reason of the essentially sexual nature of the character {e.g. 

 milk, eggs, horns in certain animals), or by reason of the gene 

 responsible being unable to express itself on the genie back- 

 ground of one or other sex. 



Sex Linkage. — The association which exists between sex and 

 the hereditary factors borne on the sex chromosomes. See 

 Partial Sex Linkage; Complete Sex Linkage. 



Sex Linked Characters. — Hereditary characters whose genes 

 are located on the sex chromosome. 



Sex Mosaic. — An organism showing attributes of both sexes; 

 intersex. 



Sex Ratio. — The number of males per hundred females, also 

 given as the percentage of males in the total population or 

 the percentage of males to total births. 



Sex Reversal. — A change of the sexual nature of an indi- 

 vidual from male to female or vice versa. 



Sex Transformation. — Sex reversal, q.v. 



Sexivalent. — See under Univalent. 



Sexual Selection. — See under Selection, Darwin's Theory 

 of Sexual. 



S.G. — Seed Generation, q.v. 



S. Genes. — Genes controlling the specificity of incompatibility 

 reactions in plants. 



Shift. — (i) The complete, permanent, masking of certain 

 characters present in one of the original parents of an allo- 

 tetraploid, due to regular homogenetic association of chromo- 

 somes in the allotetraploid. Where such homogenetic pairing 

 regularly occurs, any genes from one parent stock which are 

 recessive to their homologues from the other parent stock will 

 remain permanently masked, (ii) The removal of a segment 

 of chromosome from one position and its inclusion in a new 

 position on the same chromosome. 



