Damaris Brooks Genetics Lab Report PDF

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Mendelian Genetics using Corn Genotypes and Phenotypes Predicting the Parentage and Potential Offsprings...

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Mendelian Genetics using Corn Genotypes and Phenotypes Predicting the Parentage and Potential Offsprings Biology 1311-002 April 5, 2012

Abstract In this experiment the main goal was to ascertain the lineage of two corn samples and determine how similar they were. By looking at two ears of corn, the outcome for each phenotype was counted: purple smooth, purple wrinkled, yellow smooth and yellow wrinkled. The outcomes were a 9:3:3:1 ratio. The ratio means that there were 9/16 of the purple smooth, 3/16 of the purple wrinkled, 3/16 yellow smooth, and 1/16 of the yellow wrinkled. The ratios were put into decimal form and a Chi-square test was performed to ascertain whether or not there was a significant difference between the two corn samples. Introduction The study of genetics began many years ago and was studied by many, but it was Gregor Mendel who studied several strains of the garden pea; not only studying the hybridization between two different varieties of species, but Mendel focused his attention on the “easily classified traits with contrasting characteristics (Morris 16-3).” Genetics are based on the parents’ genetic traits that are transferred to the offspring. “Expressions of the trait in the female and male parents are interchanged, are known as reciprocal crosses (Morris 16-5).” Genes are the molecular units of heredity in living organism. An allele is one of two or more versions of a gene that codes for a trait (L. Biesecker). When two alleles are the same the alleles are homozygous and if the two are different this makes the alleles heterozygous. Phenotypes are the observable traits such as eye color, blood type and height, while the genotype is the combination of alleles present in an individual. A trait that expresses a dominant trait the result of a heterozygous cross; while a recessive trait expressed is

the result of homozygous cross. An individual only needs a single strain of the dominant trait to express it. This Mendelian Genetics experiment was designed to examine the manner in which genes are passed down from parent to offspring. It also produced a better understanding of which traits are chosen to be passed down to the offspring. Based on the visible phenotypes of the corn, the samples expressed both dominant and recessive alleles for each trait which lead to the hypothesis. It was hypothesized that the parent lineage for both corn samples would be heterozygous. Materials & Methods In this experiment you will use two varieties of corn to determine the parents’ phenotypes and to determine whether the phenotypes are homozygous or heterozygous. There are four phenotypes that will be observed. It is necessary to note how many are in each phenotype, which will be: purple smooth, purple wrinkled, yellow smooth, and yellow wrinkled. Corn samples that expressed the dominant trait for color were purple (P), while the recessive trait for color was yellow (p). The dominant trait for texture was smooth (S), while the recessive trait was wrinkled (s). Start counting each phenotype separately to ensure accuracy, count each row, until all rows have been counted. Using the numbers from each genotype on the sample of corn, make a punnett square to demonstrate the sixteen possible genotypes. The phenotype ratio is produced by the number of genotypes in comparison to the other genotypes. Take the ratios and put them into decimal form and multiply the numbers by the total outcomes, which are obtained by adding all the observed outcomes together. This will produce the numbers for the expected outcomes.

Use the numbers for the observed and expected and use them to perform a Chi-square chart. Once the outcomes are calculated the total number is used as the degrees of freedom which will help find the p-value. Doing this determined whether there is a significant difference between the two corn samples. Results By observing two varieties of corn, it was determined that both corn samples one and two were produced by two heterozygous parents based on this the genotypes were predicted using a punnett square (Table 1). This punnett square gave the phenotype ratio of 9:3:3:1 for purple smooth, purple wrinkled, yellow smooth and yellow wrinkled. The lineage of corn sample I was predicted by hypothesizing the possible parental lineage and performing a Chi-square test from the corn samples. After performing a Chi-square test to determine the difference between the observed and the expected, there was no significant difference for corn sample II because the pvalue was greater than 0.05, which shows that the predicted parentage was correct. The difference between the observed and the expected for corn sample I showed a large difference producing a p-value of less than 0.001 which indicates that the parentage we hypothesized was incorrect. A different parental lineage had to be produced using different genotypes and performing another Chi-square test. Crossing PpSs x ppss in a punnett square gave a phenotype ratio of 1:4 that was the outcome for all possible phenotypes that were crossed (Table 2 and 3). The Chi-square calculations produced a Chi-square total of 6.15, which made the degrees of freedom equal three determining the p-value to be greater than 0.1(Table 4). This p-value did not show a significant difference in the parental lineage from corn samples I and II. The results also indicate that the parental genotypes are a good match for the actual observed.

The p-value indicates that there is not a significant difference in the data. The Chi-square value produced a total p-value that was greater than the critical value of greater than 0.05, making it not significant. It shows that the genotypes that were hypothesized were actually correct.

PS

Ps

pS

ps

PS PPSS PPSs PpSS PpSs Ps PPSs PPss PpSs Ppss pS PpSS PpSs ppSS ppSs ps PpSs Ppss ppSs ppss Table 1. Punnett square a for heterozygous PpSs x PpSs cross

PS Ps pS

ps PpSs Ppss ppSs

ps PpSs Ppss ppSs

ps PpSs Ppss ppSs

ps PpSs Ppss ppSs

ps ppss ppss ppss ppss Table 2. Punnett square for PpSs x ppss

Purple and Smooth: ¼ Purple and Wrinkled: ¼ Yellow and Smooth: ¼ Yellow and ¼ Wrinkled: Table 3. Phenotypes from Table 2.

Observe d (O)

Expecte d (E)

Purple and Smooth:

537

555

Purple and Wrinkled:

519

555

Yellow and Smooth:

569

555

Yellow and Wrinkled:

595

555

2

(O−E) E

(537−555)2 555 2 (519−555) 555 (569−555)2 555 2 (595−555) 555

Chisquar e 0.584 2.34 0.353 2.88

Table 4. Chi-square calculation comparing the observed outcomes to the expected outcomes for PpSs x ppss

Discussion It was hypothesized that the parent lineage for both corn samples would be heterozygous. These results do not demonstrate support for this hypothesis, knowing that the parental lineage for corn sample II was the result of a heterozygous and homozygous cross for each trait (Table 2). However, this does support the rules of genetics and the manner in which traits are passed down from parent to offspring. Although the two samples appeared extremely similar, because of the visible similarities in the phenotypes, the parent lineages for the two samples were different. Genetics are passed down from parent to offspring, and the parents carry traits that were passed down from their parents, and it continues until the lineage reaches the beginning. Studies have shown that it is common for traits to skip generations but appear in the next. This is the result of recessive traits. “For a recessive trait that is significantly rare, virtually all affected individual have unaffected parents( Morris 16-15).” This occurs with recessive traits because recessive genes are passed on from one generation to the next without establishing a recessive phenotype. Another cause for traits skipping generation could be the result of alleles with

incomplete dominance, where the phenotype of a heterozygous genotype is between a homozygous genotype. Genetics are often studied in terms of probability. Performing a pedigree, a diagram of family history, shows patterns of inherited traits. A genetic test is an excellent method for determining the genotype of an individual. Because it poses as an effective source for tracing diseases, risk factors for health problems, helps to determine the cause and possible treatments and allows carriers to take the necessary precautions....