Lab 10

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Dec 6, 2023

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Lab 10: Mendelian Genetics Group Number: Section: Student Names (First and Last) Student Panther ID #s James Cordova 6415838 Emilys Perez 6439296 Malek Barakat Melissa Trevol OBJECTIVES: ● Understand Mendel’s laws of segregation and independent assortment. ● Differentiate between an organism’s genotype and phenotype. ● Recognize different patterns of inheritance. ● Perform monohybrid and dihybrid crosses. ● Use pedigree analysis to identify inheritance patterns. INTRODUCTION: Through his studies of the inheritance patterns of the garden pea, Pisum sativum, Gregor Mendel changed our understanding of heredity. Mendel studied characters/traits that differed between plants and designed cross-fertilization experiments to understand how these characters transmit to the next generation. The results of Mendel’s work refuted the prevailing hypothesis of blending inheritance and provided a new framework for understanding genetics. Ultimately, Mendel postulated two laws to explain heredity: (1) the law of segregation and (2) the law of independent assortment . Monohybrid crosses and the law of segregation The law of segregation states that during gamete formation the alternate forms of a gene (i.e. alleles ) on a pair of chromosomes segregate randomly so that each allele in the pair is received by a different gamete. For example, if you were to examine the gene responsible for petal color, you may discover that the gene can be expressed as either yellow or white flowers. In this scenario, the gene is petal color, while the alleles are yellow and white. Depending on which allele is expressed, petal color will vary (Fig. 1). Figure 1. Schematic of Mendel’s law of segregation 1

In diploid organisms, all alleles exist in pairs; identical alleles within a pair are homozygou s, while different alleles are heterozygous . Allele forms are represented by a single letter that explains whether a particular trait is dominant or recessive . Dominant alleles are assigned an uppercase letter (E), while recessive alleles are lowercase (e). In general, a dominant trait is expressed when at least one of the alleles present in the resulting allelic pair is dominant (EE or Ee). In contrast, for a recessive trait to be expressed, both alleles within the pair must be recessive (ee). For example, when considering ear lobe shape, two forms (attached and unattached) are apparent (Fig. 2). This trait is regulated by a single gene where unattached ear lobes are dominant (E) while attached ear lobes (e) are recessive. Figure 2. (a) Unattached (EE or Ee) vs. (b) attached earlobes (ee) An organism’s genotype (EE, Ee, ee) is the combination of alleles present whereas the phenotyp e is the physical expression of the genotype. In the earlobe shape example above, an individual can have a genotype of EE, Ee or ee. People with EE or Ee genotypes have the unattached earlobe phenotype (Fig 2a), while those with an ee genotype express the attached earlobe form (Fig 2b). Note that in general, dominant traits can be either homozygous (EE) or heterozygous (Ee) while recessive traits are always homozygous (ee). Question : Given that the allele for brown eyes (B) is dominant and the allele for blue eyes (b) is recessive, which of the following genotypes would result in individuals with brown eyes? Which genotype(s) is/are homozygous and which is/are heterozygous? BB: Homozygous—Brown eyes Bb: Heterozygous—Brown eyes bb: Homozygous—blue eyes TASK 1 – Patterns of Inheritance I: Simple Dominance Simple dominance describes a common outcome of allelic combinations, where one allele, if present, will dominate over the other and will be expressed. Information about alleles present in a parental population can be used to determine the probability of different genotypic and phenotypic ratios for a variety of traits in the offspring. In instances when only 1 or 2 traits are being considered the Punnett square (Fig. 3) approach is used to predict the possible outcomes of the parental cross. When only one trait is being considered the cross is monohybrid , while a dihybrid cross involves 2 traits. 2

General instructions on how to perform a cross using the Punnett square approach: 1. Write down the genotypes of the parents 2. Note the gametes that each parent can contribute 3. Draw a Punnett Square 4. Across the top write the gametes that one parent contributes and along the side write the gametes contributed by the other parent 5. Perform the cross 6. Determine the genotypic and phenotypic ratios In the example above (Fig. 3), the genotypic ratio is 1:2:1 (1: CC, 2: Cc, 1: cc) while the phenotypic ratio is 3:1. Since C = curly hair and c = straight hair, ¾ of the possible offspring will have curly hair while only ¼ will have straight hair. Procedure: 1. You will now simulate a cross between two heterozygous individuals, Tt and Tt. Each group should obtain two coins from your TA. You will flip the coins simultaneously to represent the potential outcomes of a cross between two Tt individuals. A head represents the dominant tall allele (T) while a tail symbolizes the recessive dwarf allele (t). Before you begin flipping the coins, perform the Tt x Tt cross in the Punnett square below to estimate the expected genotypic and phenotypic ratios. Figure 3. Example of a Monohybrid cross Parent 1 T t Parent 2 T TT Tt Based on this cross, what do you anticipate the genotypic and phenotypic ratios to be? Write your hypotheses (H o and H a ) in Table 1 t Tt tt Table 1: Genotype Phenotype Expected Ratio: 1:2:1 3:1 H o : Have a dominant gene does correlate to your child having a dominant gene Having brown eyes will correlate to your child having brown eyes. H a : Having a dominant gene does not correlate to having a dominant gene Having brown eyes will not correlate to your child having brown eyes. 1. Begin flipping the two coins simultaneously for a total of 16 times. Record your results in Table 2. Table 2: Genotype Number TT 3 Tt 7 3

tt 6 Questions : a. What ratio of allele combinations did you observe? 3:7:6 b. What genotypes and phenotypes result from these crosses? Genotypes: TT,Tt,and tt Phenotypes : Tails and heads c. What are the genotypic and phenotypic ratios? Genotypic ratio: 3:7:6 Phenotypic ratio: 5:3 d. How did your results compare to your expectations? Do your results support or reject your null hypothesis? They aligned for the most part with my expectations—therefore supporting my nul hypothesis e. Do you think your results would have been closer if you flipped the coins 1600 times instead of just 16? Why or why not? I think so—due to the law of large numbers(statistics) the more you do something—the closer it will be to the mean. 2. Albinism, a recessively inherited trait, results in organisms that lack pigment in their skin, hair or eyes. Anna is a female with normal pigmentation, but her mother, Sara, was albino. Anna’s husband, John, is albino. Anna and John have one child. Using the information you have learned so far complete Table 3. Table 3: Genotype of Anna Aa Genotype of John aa Allele(s) possible in Anna’s gametes Aa, AA Allele(s) possible in John’s gametes aa Possible genotype and phenotype of the child aA or aa—either albino or not albino Genotypic ratio of children 1:2:1 Phenotypic ratio of children 3:1 4

TASK 2 - Patterns of inheritance II: Incomplete vs. Complete Dominance & Codominance Inheritance of traits can occur in multiple forms. So far you have considered complete dominance , where a homozygous dominant or a heterozygous individual expresses the dominant phenotype, while an individual that is homozygous recessive expresses the recessive phenotype. However, in certain cases a cross between two different allele forms results in a phenotypic expression that combines the two allelic traits - incomplete dominance . For example, if an offspring resulting from a cross between a red (RR) and a white (rr) snapdragon plant receives the dominant allele for red flower color (R) from one parent and the allele for white flower color (r) from the other, the resulting genotype will be Rr. The heterozygous form (Rr) of the plant will bear pink flowers since neither allele is completely dominant over the other (Fig. 4). Figure 4. Pink snapdragons are an example of incomplete dominance 1. Determine the possible phenotypes of the F1 offspring when two pink snapdragons are crossed. Calculate the probability (percent chance) of the offspring having each phenotype. Parent 1 R r Possible phenotypes: Red or white Parent 2 R RR Rr Phenotypic ratio/ probability: 3:1 r Rr rr 2. What would be the resulting genotypes of a cross between a pink and a white snapdragon? Calculate the probability (percent chance) of the offspring having each genotype. Parent 1 R R Possible genotypes: RR and rr Parent 2 r Rr Rr Genotypic ratio/ probability: 100% probable that the offspring will have a dominant gene and a recessive gene carrier. r Rr Rr 5

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