(Figure 2-23). This phenotypic difference is determined by two alleles of a gene located on the differential region of the X chromosome. In Drosophila and many other or-ganisms, the convention is to name a gene after the ﬁrst mutant allele found, and then designate the wild-type allele with the mutant symbol plus a superscript cross. Hence the mutant allele in the present case is w for white eyes (the lowercase shows it is recessive), and the corresponding wild-type allele is w ± . When white-eyed males are crossed with red-eyed females, all the F 1 prog-eny have red eyes, showing that the allele for white is re-cessive. Crossing these red-eyed F 1 males and females produces a 3:1 F 2 ratio of red-eyed to white-eyed ﬂies, but all the white-eyed ﬂies are males. This inheritance pattern is explained by the inheritance of a gene on the differential region of the X chromosome, with a domi-nant wild-type allele for redness, and a recessive allele for whiteness. In other words, this is a case of X linkage.
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.