Determining Inheritance Patterns of Purple Eyes, Lobe Eyes, and White Eyes in Drosophila Flies. Introduction

Transcription

1 1 Determining Inheritance Patterns of Purple Eyes, Lobe Eyes, and White Eyes in Drosophila Flies (PI) Wenhui Jessie Zhao, Xinyi Lucy Chen, Maggie Connellan May 2, 2014 Due: May 12, 2014 Introduction This experiment focused on indicating if certain traits are dominant or recessive or X-linked in Drosophila melanogaster by crossing flies of two different alleles to the F2 generation. The three traits that were tested in this experiment were purple eyes, lobe-shaped eyes and white eyes. Determining whether the three traits are dominant or recessive and whether these traits are X-linked is important, because Drosophila melanogaster is widely used as a model organism in scientific researches on genetics and life evolution. Besides, it is essential to know the inheritance patterns of the traits of Drosophila melanogaster when it is used in investigations (1). Moreover, since the inheritance patterns in humans indicate whether an individual will have a chance inheriting certain genetic diseases, like hemophilia and red-green color blindness (2), specifying whether X-linked traits are dominant or recessive becomes vital because it can help people predict if a child will inherit certain disease from parents or not. During the experiment, the level of significance should be kept larger than 0.05 to ensure that human error does not have too much influence on the experiment s result, thus indicating that the conclusion that is drawn from the observed data is logical. As is predicted by Gregor Mendel, the factors (gene pairs) of traits are expressed and inherited independently from each other (2). On the basis of Mendel s predictions, Thomas Morgan claimed that genes of some traits which are called X-linked trait are only carried by X chromosomes;; these X-linked traits can be either dominant or recessive (3).

2 2 According to Mendel, if a homozygous dominant allele pairs with a homozygous recessive allele, all the F1 offspring will have heterozygous alleles and thus have the dominant trait. As a result, 100% of the F1 offspring will have dominant traits (Figure 1). When two heterozygous individuals mate with each other, about 25% of the F2 offsprings will have homozygous dominant alleles, 50% of the F2 offsprings will have heterozygous alleles and another 25% of the F2 offsprings will have homozygous recessive alleles (Figure 2). As a result, about 75% of the F2 offsprings will express the dominant trait and about 25% of the F2 offsprings will have the recessive trait. C C c Cc Cc c Cc Cc Figure 1: The capitalized C represents dominant allele while the lowercase c represents recessive allele. The Punnett square indicates the inheritance pattern in F1 generation. C c C CC Cc c Cc cc Figure 2: The offspring in F1 generation self-fertilizes and the result is determined in the Punnett square shown above, which indicates the inheritance pattern in F2 generation. If a male that carries the X-linked trait mates with a homozygous female that does not carry the trait, all females in F1 generation will carry the trait but none of them will express the trait and none of

3 3 the F1 males will carry the gene of the trait (Figure 3 and 4). However, in the F2 generation, half of the females and half of the males will carry the gene. X X X W X W X X W X Y XY XY Figure 3: This Punnett square describes the inheritance pattern F1 generation of an X-linked trait. X represents the X chromosome that doesn t carry the gene of trait W;; X W represents the X chromosome that carries the gene of trait W;; Y represents the Y chromosome. X W X X X W X XX Y X W Y XY Figure 4: This Punnett square describes the inheritance pattern F2 generation of an X-linked trait. X represents the X chromosome that doesn t carry the gene of trait W;; X W represents the X chromosome that carries the gene of trait W;; Y represents the Y chromosome. It is hypothesized that if a homozygous dominant genotype crosses with a homozygous recessive genotype, 100% of the offspring in the F1 generation and 75% of the offspring in the F2 generation will express the dominant trait;; if the trait is X-linked and recessive, when a male who carries the trait mates with a female that does not carry this trait, then none of the F1 offspring will express the trait while 50% of the male in F2 generation will express the trait.

4 4 Results In the cross between wild type and purple eyes, the F1 generation was composed of 100% wild type flies;; the F2 generation was composed of 941 wild type flies and 320 purple-eyed flies, χ2 (1, 1261) = , p = (Figure 5). The observed percentage of wild type eyes is 74.6% and the observed percentage of purple eyes is 25.4%. Traits of the F2 Generation from Crossing Wild Type with Purple Eyes Observed Number Expected Percentage Observed Percentage Wild Type Eyes Purple Eyes Total Number Figure 5: The observed percentage is really close to the expected percentage. The difference between expected and observed percentage in wild type flies is 0.4% and the difference in purple-eyed flies is 0.4%, as well. In the cross between wild type and lobe eyes, the F1 generation was made up of 100% lobe-eyed flies;; the F2 generation was composed of 310 wild type flies and 906 lobe-eyed flies, χ2 (1, 1216) = , p = (Figure 6). The observed percentage of the wild eyes is 25.5% and the observed percentage of lobe eyes is 74.5%. Traits of the F2 Generation from Crossing Wild Type with Lobe Eyes Observed Number Expected Percentage Observed Percentage

5 5 Wild Type Lobe Eye Total Number Figure 6: There is a slight difference between expected percentage and observed percentage. The observed percentage of wild type type flies is 0.5% higher than the expected percentage while the the observed percentage is 0.5% lower than the expected one in lobe eyed flies. Lastly, in the cross between wild type and white eyes, the F1 generation included 100% of heterozygous wild type flies;; the F2 generation was composed of 893 wild type flies, including 604 wild type female flies and 289 wild type male flies as well as 320 white-eyed male flies and no white-eyed female flies. Within this generation, χ2 (2, 1213) = , p = (Figure 7). The observed percentage of wild eyes is 73.6% and the observed percentage of white eyes is 26.4%, with 0% of those being female and 100% male. Traits of the F2 Generation from Crossing Wild Type Observed Number Expected Percentage Observed Percentage Wild Type (Male) Wild Type (Female) White Eyes (Male) White Eyes (Female) Total Number Figure 7: The percentage of expected value and observed value is not exactly the same. The observed percentage is 1.2% lower than expected percentage in wild type males. There is a 0.2% difference

6 6 between expected percentage and observed percentage in wild type females. Notice that there is a difference of 1.4% in white-eyed males. In contrast, there are no white-eyed females. Discussion Three Distrophila traits that were tested in this experiment to determine the inheritance patterns included purple eyes, lobe eyes, and white eyes. The hypothesis was that 100% dominant traits would be expressed in the F1 generation and only 75% of the offspring would have the dominant traits shown in the F2 generation. The percentage of wild eyes Drosophila flies in F2 generation is (941/1261) x 100% = 74.6% and the percentage of purple-eyed flies is (320/1261) x 100% = 25.4%. Also, since p = , which is larger than 0.05, there is no significant difference between expected and observed data and also substantiating that the predicted inheritance pattern is supported by collected results. In conclusion, purple eyes are recessive traits in the cross of wild type and purple-eyed Drosophila flies. The percentage of the wild type eye flies is (906/1216) x 100% = 74.5% and the percentage of the lobe-eyed flies is (310/1216) x 100% = 25.5%. As is mentioned, since p = is larger than 0.05, the conclusion that is addressed is logical and trustworthy. Therefore, lobe eyes are dominant traits in the cross of wild type and lobe-eyed Drosophila flies. White eyes is a recessive X-linked trait, meaning that male flies can express the trait when it is inherited on only one of his X chromosomes, but females need it on both of their X chromosomes, because about 50% of the males in F2 generation have white eyes and no female in F2 generation has white eyes. It is concluded that the white-eye trait is X-linked, because when the X chromosome of a male doesn t carry the gene for white eyes, the male doesn t have white-eye traits. The white-eye trait is

7 7 recessive because all of the females in the F1 generation carried the trait, but none of them expressed it;; the wild type eyes dominate over the white eyes when there are 2 inherited X chromosomes. However, if there is one X W chromosome and one Y chromosome, the white eye trait expressed because there is no other X chromosome dominating over the X W chromosome. Based on this analysis, the logical conclusion can be drawn and it supports the stated hypotheses - when the wild type flies breed with purple-eyed flies, purple eyes are recessive traits;; When the wild type flies breed with lobe-eyed flies, lobe eyes are dominant traits;; and when the wild type flies breed with white-eyed flies, white eyes are X-linked recessive traits. References 1. "Drosophila Melanogaster: Genetic Portrait of the Fruit Fly." N.p.: n.p., n.d Print. < 2. BSCS Biology A Molecular Approach. (9 ed.) (2006). Chicago, Illinois: The McGraw-Hill Companies 3. "X-linked Recessive: Red-Green Color Blindness, Hemophilia A - Morgan Stanley Children's Hospital of NewYork Presbyterian." X-linked Recessive: Red-Green Color Blindness, Hemophilia A - Morgan Stanley Children's Hospital of NewYork Presbyterian. N.p., n.d. Web. 03 May <