Question

In own words, explain Thomas Hunt Morgan's famous experiment? (red eyed x white eyed fruit flies)

Answer 1

Ans) The ability of two genes to remain together in the same chromosome is called as linkage.He proposed that distance between the linked genes determines the strength of linkage. (Higher the distance between the two genes,higher will be the crossing over,higher the recombination frequency and thus no linkage).Thus,in linkage, the two genes are present with less distance between them.

Morgan crossed red eyed (wild type) X white eyed fruit flies and obtained F1 generation(1 red eyed female and 1 red eyed male) F1 generation was further self-crossed to obtain F2 generation (2 :1 :1 ratio was observed i.e 2 red eyed female, 1 red eyed male and 1 white eyed male. As , you can see that the white eyed colour trait was only observed to be 25% (1/4)and that also occured only in male. No independent assortment was observed (because the genes were linked). Eye colour gene is present on the X-chromosome.In females,if both the X-chromosome has this eye colour trait then only it will be displayed.If only one X-chromosome has this trait then this trait will not be displayed.Wheras,in males there is only one X chromosome,thus they are morely likely to show sex linked trait.This is known as sex linked inheritance. To confirm the hypothesis that the white eyed trait is present on X-chromosome, Thomas Morgan crossed red eyed males X white eyed female, in the next generation, obviously the Y chromosome will come from father and X-chromosome from mother,thus 50% white eyed males and 50% red eyed females were observed after the cross. In males, the X-chromosome is only 1 and that too came from motherthus the trait is displayed.Whereas in females, one X chromosome is from father and another from mother.Since,there are 2 X-chromosome in females,wild type (red eyed) trait or dominant trait is expressed and the white eyed ( mutant or recessive trait) is suppressed due to dominant trait. (In males,there is only 1 X chromosome, thus there is no other X-chromosome to suppress the recessive or white eyed trait). This is how,Morgan carried many crosses to study genes that were sex linked.