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