Morgan’s experiment: proof of chromosomal theory of inheritance

 Morgan worked on the eye color of Drosophila. He observed that the pattern of transmission of the white eye gene was identical to that of the X chromosome of Drosophila. This prompted

Morgan postulate that the gene for eye color is located on the X chromosome. This was the first demonstration of an association between a specific gene and a specific chromosome. His work gave direct evidence in support of the chromosomal theory of inheritance.

The white-eyed male fly was crossed with the red-eyed female, all the F1 offsprings were red-eyed.

This gives an indication that the white eye is controlled by a recessive allele. The predictions match the F1 phenotypes, but this set of phenotypes could also be explained by a gene that is not on the X chromosome since all the flies were red-eyed (regardless of sex). So, the real test comes when the F1 is mated to make the F2 generation. Something strange happened in F2: all of the female F2 flies were red-eyed, while about half of the male F2 flies were white-eyed. Clearly, the male and female flies were inheriting the trait in different patterns. In fact, they were inheriting it in the same pattern as a particular chromosome, the X.

The female flies have an XX genotype and male flies have an XY genotype. If we stick the eye color gene on the X chromosome (XR for red and Xr for white), we can use a Punnett square to show Morgan's first cross



The above cross produced a normal 3:1 F2 ratio with the exception that the recessive white-eyed
flies are always male. Morgan did lots of other experiments to confirm that the location of the eye
color gene is on the X chromosome. He was careful to rule out any alternative possibilities. He
also studied the progenies of reciprocal crossing.
By mating the F2 files from the cross above, Morgan was able to obtain white-eyed females,
which he then used for the reciprocal cross. He crossed white-eyed females with red-eyed males, All
the F1 female offspring were red-eyed and all the males were white-eyed. In the F2 generation, one-half of the males and females are white-eyed and the other half red-eyed. This type of inheritance
is similar to the transmission of the X chromosome.


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