SHORTER ARTICLES AND DISCUSSION 



AN AUTOSOMAL BRISTLE MODIFIER, AFFECTING A 

 SEX-LINKED CHARACTER 



A recessive gene in the third chromosome of Drosophila mela- 

 nogastcr (ampelophila) affects the bristles on the thorax and 

 scutelhim of females which are heterozygous for a recessive sex- 

 linked character, forked in such a way as to make forked semi- 

 dominant. This latter character has been described by Morgan 

 and Bridges (Carnegie Publ. 237). The bristles of "stock" 

 forked flies are shortened, twisted, and heavier than normal. 

 This applies to the bristles of the head, thorax and scutelhim. 

 Flies heterozygous for forked, that is, females, since the gene is 

 in the X chromosome, have normal bristles unless the fly is also 

 homozygous for the third chromosome modifier here recorded. 

 Females with one forked gene and one normal allelomorph, 

 which an- homozygous for a modifying gene in the third chromo- 

 some, are intermediate in appearance between forked and normal 

 flies, and are designated as " semif orked. " The males never 

 show the character since they are never heterozygous for sex- 

 linked factors. The forked appearance is limited to a few of 

 the thoracic and seutellar bristles and the bristles in general are 

 less affected than are those of the homozygous forked flies as 

 regards thickening of bristles and twisting. Both of the third 

 chromosomes must bear the modifying gene in order to affect 

 the bristles. In the absence of the forked gene, the semiforked 

 genes, even when homozygous, are nearly always without effect, 

 but occasionally a few individuals may be detected which have 

 shorter and heavier bristles, but this is not pronounced and is 

 rarely found. Flies which are known to be pure both for forked 

 and for the modifier, semiforked, can not be distinguished from 

 the simple forked individuals without the modifier. 



Origin op Semiforked 

 The semiforked character was first observed in February, 

 1918, in the heterozygous Bar forked daughters resulting from 

 culture 668, a cross of a Bar male from stock to a forked female 

 of a non-disjunction strain which had been used to observe non- 

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