Drosophila Cross Experiment

David Townsend

Genetics 214 Lab: Wednesday

Introduction In order to determine the allelic relationships between three different traits (body type, eye color, and bright eyes) two parental drosophila, both mutants, were breed to the second generation, where the different phenotypes and sexes were recorded and statistically analyzed.

Literature Review

Eye coloration in the eyes of drosophila flies is cause by proteins called pteridines. Seven of these proteins can create multiple different fly mutants depending if they are present in the eye or not (Hammersmith et al 211). Pteridines can be tested for by running a paper chromatography which allows for the proteins to be spaced out over a piece of paper by using the proteins different chemical properties. These proteins then can be seen when the paper is exposed to ultraviolet light. When test are run on wild-type flies the results show four of the seven proteins. When white eyed flies are run through the paper chromatography the result is that they have no eye pigments (Hammersmith et al 211). With this information mutant flies can be analyzed. Many of the mutants actually have an epistatic relationship with wild-type flies. The mutant genes alter the pathway that produces pteridines and this can create new pathways to new pteridines are can stop the production at certain points that result in precursor proteins showing up as the eye color in flies.

Materials and Methods For this experiment two stocks of lies were used. The first stock consisted of female flies that had a dark body and white-eye phenotype. The second stock consisted of male flies that had a wild-type body and wild-type eye phenotype. In order to cross the parental stocks males and females were isolated into male, female pairs in test tubes with a foam stopper. The flies were provided with a food source of banana medium that also served as the site for egg laying and larval development. When the first filial generation emerged the flies were observed within the test tube. The flies were given 7 to 10 days in order to mate and lay the second filial generation eggs. The first filial flies were then transferred to an etherizer. To do so the test tube was tapped lightly on a table or palm to knock the flies down from the top of the test tube. The test tube was the inverted and put into the top of the etherizer. The test tube was then tapped again to knock the flies down into the etherizing chamber where the flies were etherized for 20 to 30 seconds. When the flies ceased to move they were removed from the etherizer and placed on a white 3x5 note card and placed under a dissecting microscope. With the use of the dissecting microscope and a probe the flies were grouped by sex, phenotypic characteristics (wild/dark body type and wild/white eyes), and then counted. The

counted flies were discarded into the morgue jar. The test tubes were stopped again with a foam stopper and then left alone to let the second filial generation emerge. The second filial generation emerged 7 to 10 days later. The same procedure was followed in order to etherize the flies as stated above. The flies were grouped based on sex, phenotypic characteristics (wild/dark body and wild/white/bright eyes), and then counted. The counted flies were discarded into the morgue jar and the test tubes were disposed of as well.

Experimental Data The experiment started with the mating of female fly with the phenotype of a dark body and white eyes with a male fly with the phenotype of wild-type body and wild type eyes. The flies were introduced to one another and mated on February 27th 2012.

The first filial generation appeared March 12th 2012. The females in this generation had the phenotype of wild-type body and wild-type eyes. The males in this generation had the phenotypes of wild-type bodies and white eyes.

The second filial generation appeared on March 27th 2012. The second filial generation yielded 3265 female and 3253 male flies. The phenotypes and count of the second filial generation female flies were 1077 wild-type body with wild-type eyes, 1130 wild-type body with white eyes, 191 wild-type body with bright eyes, 175 dark body with wild-type eyes, 443 dark body with white eyes, and 240 dark body with bright eyes. The phenotypes and count of the second filial generation male flies were 1101 wild-type body with wild-type eyes, 1212 wild-type body with white eyes, 151 wild-type body with bright eyes, 115 dark body with wild-type eyes, 411 dark body with white eyes, and 263 dark body with bright eyes.

Discussion The cross and subsequent crosses of drosophila in this experiment will show that the genes that control three traits in the flies (eye color, bright eyes, and body color) sort independently of one another along with normal segregation for sex determining chromosomes in the XX, XY format. Theoretically, the first cross to be looked at will be the sex determining chromosomes. As Figure 1 shows, the sexes assort 1:1.

Figure 1. Sex determining chromosomes

This cross also involves sex related traits that influence eye color to be wither colored or white. A Figure 1 illustrates, white eyes (recessive w) and colored eyes (dominant W) segregate 1:1.

The next cross to be looked at will involve body type. Theoretically there should be a ratio of 3 dominant genotypes (EE, Ee) resulting in a wild-type body to 1 recessive genotype (ee) resulting in a dark body. This can be seen in the cross in Figure 2 and is a 3:1 ratio.

Figure 2. Body type cross

The gene for bright eyes operates the same way as body type does. Theoretically it assorts into the 3:1 ratio. This can be seen in 3 dominant genotypes (CC, Cc) and the 1 recessive genotype (cc). This cross is illustrated in Figure 3.

Figure 3. Bright eyes cross

When all bright eyes and body type are combined the theoretical result is a

9:3:3:1 ratio. The ratio accounts for the base value involving 2 genes being 16 (4n, n=2). This is illustrated by Figure 4.

Figure 4. Bright eye and body color cross.

The analysis of the data collected yields similar data to the theoretical data in some cases. See Table 1.1 for more information. The data involving the first cross, sex, the data shows a female to male ratio of 50:49.9. This is very close to a 1:1 ratio and this is proven with a chi-squared value of 6.91E-4 for both sexes, which gives a 99% confidence that the results are from chance alone. The next cross, white eyes and colored eyes, was included in the sex-determining cross. White eyes and colored eyes also should have sorted out into a 1:1 ratio and was found to be in a 50.9:49.1 ratio. These values are very close but when these values were run through a chi-squared test the results were 1.087 and 1.051 respectively. These values fall between 50 and 30 percent confidence that the results are due to chance alone. These values pass the chi-squared test and show that the genes followed Mendelian independent assortment. Body type was the next cross to be analyzed. Body types sorted out into a 74.6:25.3 ratio, which is close to the 3:1 ratio seen in the theoretical testing. The chi-squared analysis resulted in the value of .0819 for wild-type body, which passes the chi-squared test at between 80 and 70 percent random chance, and .2458 for dark body, which passes the chi-squared test at between 70 and 50 percent random chance. The genes in this cross follow Mendelian assortment. Eye color, bright and wild, was the next analysis. This cross sorted out to a 74.4:25.5 ratio. The ratio is very close to the theoretical cross that had a 3:1 ratio as the result. A chi-squared analysis for wild-type eyes resulted in a .116, which passes the chi-squared test at just under 90 percent chance. Bright eyes resulted in a .349, which passes the chi-squared test at between 90 and 80 percent random chance. This cross follows Mendelian independent assortment. The last cross performed involved two autosomal genes. Based on the theoretical test this cross should arrive at a 9:3:3:1 ratio. When the numbers are analyzed the cross does not reach that ratio. Instead the ratio is a 68.1:5.35:9.07:15.7. This is indicative that the genes are not independently assorting which means another mechanism must be influencing the genes. One explination is gene linkage between body color and bright eyes. To look at gene linkage the flies

with bright and wild eyes only should be analyzed and for gene linkage to occur there cannot be a 1:1:1:1 ratio. When the data is analyzed chi-squared values fall below 5% random chance and fail the 1:1:1:1 test (chi-squared values can be found in Appendix B).

Summary and Conclusions In order to determine the mechanism for inheritance in Drosophila crosses were performed ultimately resulting in a second filial generation that exhibited all of the traits that were desired. The second filial generation was counted and grouped into phenotypic groups, which were then analyzed to see if the experimental numbers fit theoretical numbers for the same crosses. If the crosses fit the theoretical values then the cross fit the Mendelian model of independent assortment or sex determination. When a cross didn’t fit more tests were run to see the relationship of that gene with others that were involved in the whole cross.

1. Sex follows normal sex determination at a 1:1 ratio and is a XX, XY system 2. White eyes follows the sex determining system at a 1:1 ratio with wild-type

eyes 3. White eyes are recessive to wild-type eyes 4. Wild-type eyes are in an epistatic relationship with bright eyes 5. Body color alone follows standard independent assortment at a 3:1 ratio 6. Bright eyes alone follows standard independent assortment at a 3:1 ratio 7. Bright eyes and body color are linked genes 8. Females have normal recombination of gametes while males have no

recombination of gametes

Literature Cited

Mertens, Thomas Robert, and Robert L. Hammersmith. "Investigation 19." Genetics: Laboratory Investigations. Thirteenth ed. Upper Saddle River, NJ: Prentice Hall, 1998. 211-17. Print.

Appendix A

Appendix B

Appendix C