White-eyed, sex-linked mutant

Sex-linkage in Drosophila melanogaster (TH Morgan, 1915)

    TH Morgan was disinclined to think that Mendel's laws were of general application, and was also critical that Mendel's "elementen" had no material basis, until he encountered an exceptional white-eyed male fly [top row, left-hand fly, and below]. Morgan crossed the white-eyed male to a typical red-eyed female [top-row, right hand fly]. As expected, given Mendel's laws of dominant and recessive alleles, the first generation (the first filial, or F1) comprised entirely red-eyed flies, the expectation if Red were dominant to White. When Morgan crossed two flies from the F1 generation to produce an F2 generation [third row], as expected there was a 3:1 ratio of Red:White eyed flies. However, Morgan observed that this ratio was obtained because all females were red-eyed, and that half of males were red-eyed and the other half white-eyed. 

    Morgan was aware that in Drosophila, females had two "X" chromosomes and males only one, plus a much smaller chromosomes shown as "0". Sex is determined by whether a fly inherited a X or an O chromosome from the male parent. Morgan reasoned that, if the "White" allele were physically present on the single X chromosome in the original fly, it would have passed to all of the females flies in the F1, all of which would then be heterozygous. In the F2, when these females were crossed to a red-eyed male, all of the female offspring receive the dominant "Red" allele from the male parent and are therefore red-eyed, whereas half of the male offspring receive from their female parent an X chromosome with the "Red" allele and would be red-eyed, and half an X chromosome with the "White" allele and would be white-eyed.

    That is, controlled genetic crosses show that the behavior of the novel white-eyed mutant accords with the expectations of Mendel's Laws, allowing for the new phenomenon of "sex-linked inheritance". Equally important, "genes" are physically present on chromosomes and therefore have a material basis. Further proof of the concept rapidly arose from demonstration of other genes "linked" to other chromosomes, from which "linkage maps" of their physical order could be calculated.

     Morgan is quoted as having said of this experiment, "Treasure your exceptions." Whereas others might have discarded the unusual fly as a contaminant, Morgan took the opportunity to use it as experimental material. [The case has similarities to the discovery of penicillin from mold contaminating a petri dish].

White-eyed male & Red-eyed-female flies

Male Drosophila [left] have blunt abdomens with dark tips, females Drosophila [right] have more lightly colored, pointed abdomens.

All text material ©2018 by Steven M. Carr