AP FLYo 🪰 (Modified Punnet Square), FIGURE 2
Intro: Mendel’s basic rules of heredity discuss the inheritance of characteristics from generation to generation that takes place during sexual reproduction such as sex-linked, dominant-recessive, codominance, incomplete dominance, and multiple allele. Aim: Figure out the model of inheritance (dominant-recessive, incomplete dominance, codominance, multiple allele, or sex-linked) that best represents how traits in fruit flies are inherited, after simulating the experiment in a virtual lab.
The guiding question of this investigation is: “Which model of inheritance best explains how eye color and bristle type are inherited in fruit flies?”
Claim: When true-breeding wild-type females are crossed with either white-eyed males or forked-bristled males, the sex-linked recessive model of inheritance best describes how the white eye color and bristle type traits are passed down.
Evidence and Data Analysis: As shown in Figure 1 (Figure 1 has been simplified for demonstration purposes), when we crossed a white-eyed male fly with a red-eyed female fly, also known as “Wild-Type,” the f1 progeny were all red eyed: 596 red-eyed females and 607 red-eyed males. The result of crossing one red-eyed female and one red-eyed male ( both from the F1 generation), shown in the F2 generation of Figure 1, was that all 632 females had red eyes, 315 of the males (around half of the males) had red eyes, and another 282 males (around half of the males) had white eyes.
Employing Mendel’s genetic principles, we predicted outcomes for the experiment in a model, so that we could determine if the patterns that we observed in the experiment match what would occur if my hypothesis was accurate. Noticing that the white eye color trait was being inherited in the same pattern as the X chromosome, we hypothesized, H0, that the white eye color trait is sex-linked recessive. Alternate hypotheses include the white eye trait being sex-linked dominant, autosomal dominant, autosomal recessive, codominant, incompletely dominant, and multiple allele pattern. Using Punnett squares, shown in Figure 2 and Figure 3, we predicted the outcomes of both the F1 and the F2 generations for the eye color trait. In Figure 2, when a homozygous female with red eyes was crossed with a white-eyed male, the two female progeny turned out to be phenotypically red-eyed, but genotypically heterozygous carriers of the white trait; the males were both phenotypically and genotypically red-eyed. In Figure 3, when a heterozygous female was crossed with a red-eyed male, one female was heterozygous, one was homozygous red, one male had red eyes, and one male had white eyes. When the data, along with the expected values coming from the Punnett Square predictions, were plugged into the Chi Squared Significance Test (shown in Figure 4), the corresponding probability that the white eye trait is sex-linked recessive was 1.641, well below the accepted P-value of 5.99 at a significance level of 5%, so we fail to reject the null hypothesis. Because plugging in the other models as expected values for the Chi Squared Statistic exceeded the critical value in the table for a 0.05 probability level, as shown in the Appendix, we reject the alternate hypotheses of equal distributions.
The trait for Forked-Bristles follow similar patterns, in addition to being inherited in the same pattern as the X chromosome, so we hypothesized, H0, that the Forked-Bristle trait is sex-linked recessive. Alternate hypotheses include the Forked-Bristle trait being sex-linked dominant, autosomal dominant, autosomal recessive, codominant, incompletely dominant, and multiple allele pattern. As shown in Figure 4, when we crossed a Wild-type Female fly with a Forked-Bristle Male fly, the f1 progeny were all Wild-Type, 600 Wild-Type females and 580 Wild-Type Males. The result of crossing one Wild-Type female and one Wild-Type male ( both from the F1 generation), shown in the F2 generation of figure 4, was that all 599 females were Wild-Type, 306 of the males (around half of the males) were wild-type, and another 287 males (around half of the males) had Forked-Bristles. When the data, along with the expected values coming from the Punnett square predictions, were plugged into the Chi Squared significance test, the corresponding probability that the Forked-Bristle trait is sex-linked recessive is 0.6359, well below the accepted P-value of 5.99 at a significance level of 5%, so we fail to reject the null hypothesis. Because plugging in the other models as expected values for the Chi Squared Statistic exceeded the critical value in the table for a 0.05 probability level, we reject the alternate hypotheses of equal distributions.
Justification of evidence:
While our evidence backs up our claim, we would like to justify our evidence and explain, through scientific concepts and principles, why our evidence matters. Our hypotheses that the white eye trait and that the Forked-Bristle trait are sex-linked recessive are also backed by scientific principles. The alternate hypothesis of the traits following incomplete dominance can be rejected because the phenotypes of the progeny do not have an appearance that is between that of both parents. The hypothesis of codominance can be rejected because both alleles are not expressed at the same time. The hypothesis of the trait being multiple allele could also be rejected because the expression of the white eye and Forked-Bristle traits did not vary. The hypothesis that the traits were sex-linked dominant also can be rejected because no females descending from males with the trait have the trait. The hypothesis of the trait being autosomal recessive can be rejected because there would need to be 50% of males and females affected for this hypothesis to ring true, which did not occur. The trait cannot be autosomal-dominant because there would need to be no carriers or skipping generations (which did occur) and 50% of males and females would need to be affected, which did not occur. Lastly, our experiments follow the inheritance patterns of sex-linked genes, where a father will always transmit the sex-linked trait to his daughter. His son receives the Y, and does not inherit the trait. Only females can be carriers of sex-linked traits. Therefore, a carrier female who mates with a normal male transmits the mutant allele to half her sons and half her daughters.
Conclusions of inheritance patterns were based off of the flies’ phenotypic ratios, after applying genetic inheritance principles. As a result of our parent generations in the experiment being “true breeding,” meaning homozygous for the studied traits, we were able to determine the genotypes throughout the lineage. Through the Chi-Squared Significance Test, we were able to assess if the predictions matched the data from our experiment. Because chromosomes assort independently during meiosis to form gametes, equal numbers of allele pairs are expected to be produced by F1. Traits on the same chromosome are not assorted independently. In order to determine the outcome of crosses in Mendelian genetics (laws of segregation and independent assortment), we used Punnett Squares because they clearly display the possible combinations in chart form. Our sample is representative of the fly population because of the law of large numbers, which states that as the number of randomly generated variables increases, their sample average approaches their theoretical mean. So, even though we did not test every single fly, because we chose a random fly to examine, it was representative of the group. Additionally, even though we did not repeat the experiment many times, because out finding matched those of Morgan’s fruit fly experiments, our data is supported. Some assumptions that our data are based on in order to perform Chi Squared are that the sampling method for choosing flies was simple random sampling and that the variables studied were categorical. The eye color gene is inherited in different patterns by male and female flies. Male flies have an X and Y chromosome, while females have two X chromosomes. The eye color gene was being inherited in the same pattern as the x chromosome. Error analysis: While the replication of DNA is extremely regulated and precise, mutations can arise as a result of an error, leading to genetic variation. The environment that the organism lives in can also result in genetic mutation. Genetic variation can occur if new genetic combinations are generated after chromosomes cross-over during meiosis. Although we could have inputted incorrect calculations, random and systematic errors were reduced though, because of the use of the simulation.
// HAND DRAWN + DIGITAL COLLAGE, 2017
// Created for my AP Bio Class in High School
Rodeyo
