Karen Jacques and Audrey Puleio Mrs. Lajoie Honors Biology April 30, 2012 Determining the Inheritance Patterns of Purple Eye, Lobe Eye, and Yellow Body Traits of Drosophilia Flies Introduction This experiment focuses on the different inheritance patterns for various traits of the Drosophilia flies: dominant versus recessive and X-linked traits. Inheritance is the passing down of traits through generations. Genes code for the instructions of these traits (1). Past studies have found a way to predict the likelihood of inheriting certain traits. These studies were done by an English geneticist named Reginald Punnett. He created a simple graphical way to discover all of the potential combinations of genotypes (2). These Punnett squares help make predictions on which genes will be inherited from two parent generations. Since genotypes are the combination of two alleles, each offspring will receive one allele from their mother and one allele from their father. The Punnett square can be set up by placing all the alleles one parent carries at the top and all the alleles the other parent can carry on the side. The rest of the table is then filled in like a multiplication problem (3). This can be useful in predicting the chances of an offspring having a certain phenotype. The Punnett square, though, came after the findings of a monk named Gregor Mendel. Gregor Mendel investigated the crossing of pea plants. He first observed plants that had self fertilized themselves to make sure characteristics remained constant. He then focused on the height, pod shape, seed shape, and pea color. He then cross pollinated them and what he
discovered was that some traits seemed to transfer from one generation and miss the next. What he later concluded was the idea of factors, which later became known as traits (4). Mendel also discovered the idea of dominant and recessive traits. He realized that some traits always expressed if they were inherited. These traits were called dominant traits and the traits that only expressed if they were homozygous were the recessive traits (5). If a genotype contains a dominant allele with a recessive allele, then the dominant alleles characteristic will be the one that expresses. Past studies have shown that the purple eye trait is recessive to the wild type trait in the flies (6). Using this knowledge, a Punnett square can be completed (figure 1). The Punnett square shows the chances of the possible genotypes. It can then be hypothesized that in the F2 generation of crossing wild type flies with purple eyed flies, 25% of the offspring will have purple eyes and 75% of the offspring will be wild type. F2 W w W WW Ww w Ww ww Figure 1: Notice that the dominant trait is represented with an uppercase letter. Also notice that there is only ¼ chance of a homozygous pair meaning that there is only a 25% chance for the recessive allele to show. Past studies have also shown that the lobe eye trait is dominant over the wild type (7). This can also be placed into a Punnett square to help predict the chances of possible genotypes (figure 2). Using this it can be hypothesized that in the F2 generation of crossing wild type flies with lobe eyed flies, 75% of the offspring will be lobe eyed and 25% of the offspring will be wild type.
F2 L l L LL Ll l Ll ll Figure 2: Notice that the dominant trait is represented by an uppercase letter. Also notice that because there is an uppercase letter in ¾ squares 75% of the time the dominant trait will express. There is also another type of inheritance known as X-linked traits. Thomas Hunt Morgan discovered X-linked traits when studying fruit flies. He realized that some of his males had white eyes. He began to breed his white eyed flies with regular flies expecting to get the normal 75% dominant and 25% recessive in the F2 generation. Instead he discovered that none of his females had white eyes but 25% of his males did along with 25% of his males being normal. With this he discovered that some traits are dependent on a chromosome like the X chromosome. He realized males received the trait because they only have one X chromosome where females have two X chromosome making it nearly impossible for the females to receive two defective X chromosomes (8). While a female may carry the chromosome it can be expected that no female expresses it. Past studies have shown that the yellow bodied trait is an X-linked trait (9). This knowledge along with an X-linked based Punnett square can help predict what genotypes will happen (figure 3). It can then be hypothesized that 0% of the female offspring will be yellow bodied, 50% of the offspring will be wild type female, 25% of the offspring will be wild type male and 25% of the offspring will be yellow bodied. It can also be hypothesized that while none of the females will express the yellow body trait in the F2 generation, some may be carriers.
F2 X y X X XX y XX Y X y Y XY Figure 3: Notice that the yellow body trait is represented with a superscripted y above the X. Also notice that females will only be carriers since they have one unaffected X. The main objective of this experiment is to discover the inheritance patterns of certain traits of the flies. The experiment focuses on the inheritance of the purple eyed traits, lobe eyed trait and yellow body trait. The importance of experimenting on the inheritance of these traits is to better understand dominant versus recessive traits and X-linked traits. This experiment will help to show that inheritance happens in consistent patterns that can be predicted through Punnett squares. Chi-square analysis will then determine if the expected amounts are close to the observed amounts. If the chi-square value is less than the 0.05 value it can be assumed that the hypothesis can be accepted. If the value is higher, the hypothesis can be rejected (10). Some questions this experiment may help answer are: Will dominant traits always appear in 75% of the F2 generation? Will female flies ever show an X-linked trait? Are assumed inheritance patterns accurate? Results In this experiment, the wild type bred with the purple eyes, it was seen that the wild type had more flies than the purple eye in the F2 generation. This can be seen because out of the total number of flies found, only 24.7% had the purple eye trait where 75.3% had the wild type trait. Notice that 892 of the flies in the F2 generation were wild type and that only 293 of the flies had purple eyes. Also, the expected percentage to be found of the wild type, 75%, is very close to the
observed percentage, 75.3%. This is similar to the expected percentage of purple eye fly and the observed percentage (Fig. 4). Traits of the F2 Generation From the Breeding of Wild Type with Purple Eyed Number Observed Expected Percentage Observed Percentage Wild Type 892 75% 75.3% Purple Eye 293 25% 24.7% Totals 1185 100% 100% Fig. 4: Notice that the expected percentage and observed percentage is not exact. There were more wild type flies observed than purple eyed. This experiment tested the trait of wild type and lobe eyes after they were bred together. It was found that 894 of the flies in the F2 generation had lobe eyes. The number of wild type flies found in the F2 generation was 303. Traits of the F2 Generation from Breeding Wild Type with Lobe Eyed Number Observed Expected Percentage Observed Percentage Wild Type 303 25% 25.3% Lobe Eye 894 75% 74.7% Totals 1197 100% 100% Fig. 5: Notice that the expected percentage and the observed percentage are not perfectly the same. There are more lobe eyed flies observed than wild type.
This study showed that 50% of the offspring found were female wild type while there were no female yellow bodied flies found. It also found that 25% of the flies were found to be male wild type and the remaining 25% was found to be male yellow body. Female wild type was the most common offspring with 612 and the yellow body male had slightly higher numbers, 314, than those of the wild type, 309. Traits of the F2 Generation from Breeding Wild Type with Yellow Bodied Number Observed Expected Percentage Observed Percentage Wild Type (Boy) 309 25% 25.0% Wild Type (Girl) 612 50% 49.6% Yellow Body (Boy) 314 25% 25.4% Yellow Body (Girl) 0 0% 0% Totals 1235 100% 100% Fig. 6: Notice that the expected percentage and the observed percentage are not identical. There are no yellow bodied females and about half of the flies were wild type females. The remaining flies were split between wild type males and yellow bodied males. This experiment shows the actual percentages of the F2 generation of wild type bred with purple eye flies. The purple eyed trait only makes up for 24.7% of the whole pie chart and the
wild type makes up 75.3% (Figure 7). Figure 7: Notice that the wild type is represented in blue and the purple eye represented in red. There is a fewer percentage of purple eyed flies than the percentage of wild type flies. This study found that 25.3% of the F2 generation of flies of wild type and lobe eyes bred together were wild type. 74.7% of the flies found in the F2 generation were had lobe eyes (Figure 8).
Figure 8: Notice that the wild type is represented by the blue and the lobe eye is represented by the red. There are This study found that 25.4% of the F2 Generation flies had yellow bodies and were male. 25% of the flies found in the same generation were wild type and were male. 0% of the female flies found in the F2 generation had yellow bodies and 49.6% of the flies found in the F2 Generation were female and wild type (Figure 9).
Figure 9: Notice that the wild type male is in blue, the wild type female is in red, and the yellow bodied male is in yellow. There are more wild type female than male and the male are split between wild type and yellow bodied. Discussion This experiment looked at the traits for specific types of flies with different characteristics to determine which trait is dominant or recessive. From the results and past research on this topic, it was found that the characteristics found to be dominant where the wild type over the purple eye, lobe eyed flies over the wild type, and with the cross breed between wild type and yellow body flies, it was seen that it was a sex-linked trait where the females inherited the wild type gene and the male flies were a mix between wild type and a yellow body.
In the first cross, female wild type flies and purple eyed males were bred together. From the F2 generation resulting in 75.3% of the flies being wild type and only 24.7% containing purple eyes, it can be concluded that the wild type is dominant over the purple eye trait. After the Chi-Square Test was performed on this cross it was seen that Χ 2 was equal to 0.0405 which allows the conclusion that the hypothesis that wild type would be dominant to be correct. The degree of freedom was equal to one and the level of significance was equal to 0.8404 (11). In the second cross between wild type male and female lobe eye, the lobe eye is seen to be dominant over the wild. Being that only 25.3% of the F2 generation was wild type and 74.7% of the F2 generation had lobed eyes, this support the hypothesis that the lobe eye would be dominant. In the Chi-Square Test, the calculation for X 2 was equal to 0.0713 which allows the conclusion to be further concluded that the hypothesis that lobe eye would be dominant to be correct. The level of significance was 0.7894 and the degree of freedom was one (11). In the last cross of yellow body male and wild type female it was found that it is a sex linked trait. The yellow body is recessive where the wild type is dominant. Calculations from the Chi-Square Test showed that 0.1385 would be the value for X 2. The degree of freedom was equal to two. It can be concluded from the results that the trait is X linked. There will never be a female fly that has a yellow body in this cross because they are not receiving that allele (11). This experiment was a good experiment in that it supported the hypotheses that were formed by using Punnett squares. The results that were found are enforcing beliefs about certain traits that were already previously established through other experiments done on the same flies. Although the lobe eye is dominant over the wild type, it does not mean that the lobe eye gene is more common.
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