How did Morgan discover X-linkage in Drosophila?

The discovery of X-linkage was one of the most important milestones in classical genetics. This concept was first established by Thomas Hunt Morgan in 1910 during his genetic experiments on the fruit fly, Drosophila melanogaster. Morgan's work not only confirmed Mendel's laws but also extended them by explaining how certain traits are inherited through the sex chromosomes, especially the X chromosome.

How did Morgan discover X-linkage in Drosophila?

Thomas Hunt Morgan discovered X-linkage in Drosophila melanogaster through careful breeding experiments that studied the inheritance of eye color. This discovery was very important because it showed that genes are located on chromosomes, supporting the chromosome theory of inheritance.

Morgan started his research with fruit flies that normally had red eyes. One day, he found a male fly with white eyes, which was a rare mutation. To understand how this trait was inherited, he crossed this white-eyed male with normal red-eyed females. The first generation (F1) offspring all had red eyes, which showed that red eye color is dominant over white eye color.

Next, Morgan crossed the F1 flies with each other to produce the second generation (F2). Here he noticed something interesting. Among the F2 offspring, all the white-eyed flies were males, while females had only red eyes. This was unusual because, according to Mendel's laws, a recessive trait like white eyes should have appeared in both males and females equally if the gene was on an autosome (non-sex chromosome). The fact that only males showed the white-eyed trait suggested a different pattern of inheritance. After this discovery, Morgan proposed that the gene for eye color was located on the X chromosome, which is one of the sex chromosomes.

In fruit flies, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). Because males have only one X chromosome, if they inherit the recessive white-eye allele on their single X chromosome, they will show white eyes. Females, on the other hand, need two copies of the recessive allele (one on each X chromosome) to have white eyes. Since the white-eyed female was rare and the mutation was recessive, most females were red-eyed because they usually carried at least one dominant red allele.

Importance of Morgan’s Discovery

Morgan's discovery of this inheritance pattern provided the first clear evidence of sex-linked inheritance, which is when genes are located on sex chromosomes and affect males and females differently. This work was crucial in proving that chromosomes carry genes and that the position of a gene on a chromosome influences how it is inherited.

This discovery also explained why some traits, like certain types of color blindness and hemophilia in humans, are more common in males, because these traits are controlled by genes on the X chromosome. Morgan's work opened a new chapter in genetics by linking specific genes to specific chromosomes, helping to explain the inheritance of many sex-linked traits.






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