In this experiment, we are investigating the strength of the ratios discovered by Gregor Mendel in both the monohybrid and dihybrid cross. The ability to test these ratios stems from the use of Mendel’s law of segregation which states that during meiosis allele pairs will separate in gametes so one of each allele is present in a gamete. (Garey, et al, pg 8-13) These single alleles are then combined with the other parental gamete forming a new somatic cell. Another important law is the law of independent assortment which means that different gene pairs will separate independently of each other allowing two traits to be monitored at once.
With these laws, ratios can be assigned to both the Monohybrid, a cross of only one gene, and a dihybrid cross, a cross of two independent genes, which will result in ratios for both phenotype and genotype. The phenotypic ratio for a standard F1 monohybrid cross is 1:2:1 while a dihybrid cross has a phenotypic ratio of 9:3:3:1. The genotypic ratio for a monohybrid cross though is 3:1 as the dominant allele will generally overpower the recessive allele. The only cases where this doesn’t happen are when the alleles show codominance and a blending effect takes place.
The use of Drosophila melanogaster is very useful when trying to observe many genetic properties, due to the ability to distinguish between each phenotype. This is due to the clear sex-linked genes that are easily expressed and don’t result in sterile or deceased offspring and the genes that are located on different chromosomes so that no crossing over occurs. Also, the time between generations is exceptionally short allowing for a generation of flies to be produced almost every week. These flies can be easily secured and taken care of in a lab environment making them perfect for genetic studies.
From this, we can hypothesize that the ratios set up by Mandel will be shown in the monohybrid cross, the dihybrid cross, and the sex-linked cross. To test the ratios set up by Mendel’s laws five different crosses of flies from the F1 generation will be taken. Each F1 generation will have had homozygous parents which should result in all offspring having the same heterozygous genotype. Each cross is predicted to have its own outcome:
Se+/Se X Se+/Se
Se+Se
Se+Se+/Se+Se+/Se
SeSe/Se+Se/Se
Table 1.
This table represents the Monohybrid cross between heterozygous parents with a recessive sepia eye allele(Se) and a wild-type red-eye allele (Se+). This cross is predicted to show the 3:1 phenotypic ratio as stated by Mendal.
Ap+/Ap X Ap+/Ap
Ap+Ap
Ap+Ap+/Ap+Ap+/Ap
ApAp+/ApAp/Ap
Table 2 is the predicted cross of the monohybrid cross between recessive apterous eye color (Ap) and wild-type red eye color (Ap+). This cross also shows the 3:1 phenotypic ratios from Mendelian genetics.
Ap+/Ap; Se+/Se X Ap+/Ap; Se+/Se
Ap+/Se+Ap+/SeAp/Se+Ap/Se
Ap+/Se+Ap+/Ap+; Se+/Se+Ap+/Ap+; Se+/SeAp+/Ap; Se+/Se+Ap+/Ap; Se+/Se Ap+/SeAp+/Ap+; Se+/SeAp+/Ap+; Se/SeAp+/Ap; Se+/Se+Ap+/Ap; Se+/Se+ Ap/Se+Ap+/Ap; Se+/Se+Ap+/Ap; Se+/Se+Ap/Ap; Se+/Se+Ap/Ap; Se+/Se
Table 3 represents the dihybrid cross of both the Apterous and Sepia alleles. With each parent having a recessive Apterous wing trait (Ap) and a dominant normal wing trait (Ap+). The other set of alleles is the recessive Sepia eye color (Se) and the dominant wild-type red eye color (Se+). This cross is predicted to follow the 9:3:3:1 Dihybrid ratios.
Vg+/Vg; Se+/Se X Vg+/Vg; Se+/Se
Vg+/Se+Vg+/SeVg/Se+Vg/Se
Vg+/Se+Vg+/Vg+; Se+/Se+Vg+/Vg+; Se+/SeVg+/Vg; Se+/Se+Vg+/Vg; Se+/Se Vg+/SeVg+/Vg+; Se+/SeVg+/Vg+; Se/SeVg+/Vg; Se+/SeVg+/Vg; Se/Se Vg/Se+Vg+/Vg; Se+/Se+Vg+/Vg; Se+/SeVg/Vg; Se+/Se+Vg/Vg; Se+/Se
Table 4 represents the dihybrid cross between two parents each with a recessive Vestigial wing allele (Vg) and a dominant Wild type normal wing (Vg). The other pair of alleles is the recessive Sepia eye color (Se) and the dominant wild-type red eye color (Se+). This dihybrid cross results in a 9:3:3:1 genotypic cross.
X(w)X(+) X X+Y
X(w)X+
X(w)X(w) X(w)X(w) X+
YX(w)YX+Y
Table 5 represents the monohybrid cross between the sex-linked genes of the D. Melanogaster’s eye color. The recessive allele for white eye color (w) and the dominant allele is the wild type red allele (+). The predicted ratio is 1:1:1:1 for genotype and a 3:1 phenotypic ratio.
To be sure that results are not due to random chance a statistical chi-square test is performed. Which will test the observed against the expected and be able to produce a percentage chance of the results being by random chance.
Materials and Methods
The first thing done before the 5 groups of flies could be placed into new vials, to prevent F1 generation flies to mate with the F2 generation, a fly food mixture was made. To make this mixture 3 scoops of dry fly food were placed into an aqueous solution until the mixture had a thick consistency. This food was then split between 10 vials each vial having approximately an inch of food. To increase the maximum yield of offspring 3-5 granules of yeast were placed on the top layer of the food so that the yeast could break down the sugars in the food into essential vitamins for the flies to consume. (Tatum, 193-197) A piece of plastic netting was then folded and placed into the vial to allow the flies a place to stay out of the food.
The flies were exposed to a fly napping agent that put the flies into a paralysis or napping stage allowing them to be sorted by sex. To increase the sample size of our crosses 10 males and 10 females were split between two vials so that each vial had 5 males and 5 females resulting in two vials of the same allele cross. These flies were then left to produce the F2 generation. This resulting F2 generation was the generation that was assessed to check for the proper mendelian ratios.
Citation
- E. L. Tatum, Vitamin B Requirements Of Drosophila melanogaster, Proceedings of the National Academy of Sciences of the United States of America, Vol. 27, No. 4 (Apr. 15, 1941), pp. 193-197
- Garey, James, et al. Genetics Laboratory Manual. Tampa, Fl: Pro-Copy, 2012. 5-32. Print.
- P. G. Georgiev, et al. “Study Of The Regulatory Region Of Gene White Of Drosophila Melanogaster.” Doklady Biochemistry & Biophysics 405.1-6 (2005): 383-387. Academic Search Premier. Web. 21 Sept. 2012.
- Pierce, Benjamin. Genetics. 4th ed. 7. New York: W. H. Freeman, 2012. Print.
