In the study of the genetics and in particular the study of inheritance the chosen organism experiments are based upon the fruit/ vinegar fly, Drosophila melanogaster. It is widely used due to its ease of cultivation within the laboratory as well as single pair being able to provide several hundred offspring within a space of two/three weeks. These offspring can also be observed at any stages of the life cycle and are inexpensive and readily available for genetic studies such as this.
Drosophila has been used widely in genetics for many years and has aided its development greatly. The U.S geneticist Thomas Hunt Morgan used it in am attempt to detect induced mutations, when he discovered sex linkage using white eyed flies in the 190. He then went on to find further sex linked mutants and in 1911 he put forward a theory in recombination. It is his work that forms the basis of modern genetics, however Gregor Mendel formulated the idea of inheritance in 1865. He suggested that the units of inheritance, “genes” would be passed on from generation to generation. Mendel observed his results through experimentation with pea plants in order to formulate his laws of inheritance.
In this study I wish to prove the hypothesis that the phenotypic characteristics of wing length and eye colour of the Drosophila are in fact sex linked. The term sex linkage by biological definition is “the tendency for certain inherited characteristics to occur more frequently in one sex than the other.” This implies that the alleles for these characteristics are expressed in the males while the second X chromosome in the females masks them. This occurs as the males have a region of the X chromosome, which is non-homologous to the y hence always expressed. Crossing certain genotypes of flies and observing the phenotypic characteristics expressed in the F1 and F2 generations will test the hypothesis. This will then be followed by a statistical test of the results to determine if they support or reject the hypothesis in question.
In order to test my hypothesis I intend to cross pure breeding white eyed, normal winged females with red eye, vestigial winged males. The F1 generation will then be bred with each other and the 3:1 ratio of dominant to recessive characteristics is expected with the male showing the recessive characteristic more frequently than the female. This will show that wing length is sex-linked, while the reciprocal cross will be set up with the F1 generation being interbred in order to determine that eye colour is also sex linked due to the presence of another 3:1 ratio with the male again showing the recessive characteristic more frequently than the females.
PLAN
During his study of Drosophila, Thomas Hunt Morgan concluded that the genes for eye, colour and wing length have the allomorphs, red and white-eye and normal and vestigial wing length. He also concluded that red eye colour was dominant over white-eye and that the inheritance of eye colour in Drosophila was related to the sex of the parent flies. He concluded this by crossing a white- eyed male with a red-eyed female and producing equal numbers of F1 red eyed males and females. However, in breeding of these F1 flies produced red-eyed females, red-eyed males and white-eyed males but no white-eyed females. By the fact that the males showed the recessive characteristic more frequently than the females it suggested that the recessive white-eye allele was present on the X chromosome, hence being masked in the females yet expressed in the males due to sex linkage. [Biological Sciences 2, 1990]
After reading through and being fascinated by Morgan’s work, I wish to repeat his investigation into the inheritance of eye colour and confirm his results of eye colour being sex linked. I also wish to take Morgan’s work further and investigate the inheritance of wing length and using a similar method to Morgan’s, determine that wing length is also sex linked. Therefore in this genetic linked study, I wish to investigate the hypothesis that:
In the fruit fly Drosophila Melaragoster the inheritance of eye colour and wing length is sex-linked. If this is the case then 3:1 dominant to recessive characteristic ratio will be produced in the F2 phenotypes, with the males showing the recessive characteristic more often than females.
This will be shown by initially crossing a vestigial winged, red-eyed male with a normal winged, white-eyed female. From this cross I would expect normal winged, red- eyed females and normal winged, white-eyed males. Two F1 generation Drososphila will then be crossed in order to produce an F2 generation. At the same time a reciprocal cross will be set up with red-eye, vestigial winged females being crossed with white-eye normal winged males. Two of the F1 generation flies will then be crossed again in order to produce an F2 generation. If the characteristics of wing length and eye colour are in fact sex-linked a 3:1 ratio of dominant to recessive characteristics will be present in both crosses. The crosses should also show that the males show the recessive characteristic more often than the females which suggests that the alleles for wing length and eye colour are in fact on the X chromosome and so masked in the females, yet always expressed in the males due to the region of the X chromosome being non homologous to the Y. The reasoning behind this is clearly laid out in the written crosses that show the 3:1 ratio in the F2 generation.
However, there is a possibility that the characteristics of wing length and eye colour are not sex linked but inherited normally, hence the cross showing the outcome if this was the case is also shown. If normal inheritance occurred than a Mendelion 9:3:3:1 ratio would be expected as is shown. Hence the phenotypic ratio’s obtained after an F2 generation would distinguish whether the characteristics were sex linked, shown by a 9:3:3:1 ratio. [Bio Sciences 2, 1990]
The recessive characteristic will be shown more in the males than the females in my opinion due to the recessive characteristic (if present on the X chromosome as thought) being masked out in the females by the alleles present on its second X chromosome ,as the female genotype is XX. However, males have one X chromosome and one Y chromosome, hence due to this and the fact that the X chromosome has region which is non homologous to the Y, the alleles on the X chromosome are always expressed and so recessive characteristics are not masked out by dominant ones but expressed. Due to this the recessive characteristics is expressed more in males than females due to sex linkage that occurs.
Therefore, if 100 flies were produced in the F2 phenotypic generation, 75 flies would show the dominant characteristic and 25 would show the recessive characteristic in order to give a 3:1 ratio. Also, of the 25, the majority of the flies would be males in order to satisfactorily conclude that sex linkage has occurred.
Cross showing expected results if characteristics are sex linked:
Let: - N represent normal wing - dominant Φ= female = XX
n represent vestigial winged - recessive Δ = male = XY
R represent red eye - dominant
r represent white eye - recessive
(This cross concentrates upon investigating if inheritance of wing is sex linked)
Parental phenotypes: White eye, normal wing X Red eye, vestigial wing
Parental genotypes: XNr XNr XnRY
Meiosis:
Gametes:
Random fertilisation:
F1 Genotypes XNr XnR XNrY XNrXnR XNrY
F1 Phenotypes Normal wing Normal wing Normal wing Normal wing
Red eye white eye red eye white eye
F1 generation cross: Normal wing, white eye X Normal wing, red eye
F1 phenotypes:
F1 genotypes: XNrY XNrXnR
Meiosis:
xxxxxxxxxxxx
cross if the characterisitics are not linked
parental phenotypes: vestigial wing, red eye X normal wing, white eye
parental genotypes: RRnn rrNN
meiosis:
gametes: Rn rN
random fertilization
F1 genotypes RrNn
F2 phenotypes all heterozygous normal winged, red eyed
F1 generation cross: normal wing, red eye X normal wing, red eye
F1 genotypes: RrNn RrNn
Meiosis