Lab 11 Patterns of Inheritance

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Note: All your answers to questions must be in Red or other color (not including blue) for easier grading. Points will be deducted if you do not distinguish your answers. Lab 11. Patterns of Inheritance Objectives:  Perform a Monohybrid (one-trait) cross and explain how the results predict all the possible offspring  Relate Mendel’s law of segregation to the results of a Monohybrid (one-trait) cross  Explain and predict the results of a monohybrid cross in corn plants .  Perform a Dihybrid (two-trait) cross and explain how the results predict all the possible offspring  Explain and predict the results of X-Linked crosses in Drosophila Vocabulary:  Gene  Alleles  Homozygous dominant  Homozygous recessive  Heterozygous  Genotype  Phenotype  Carrier  Punnett square  Probability  Law of Segregation  Law of Independent Assortment  X-linked or sex-linked  Sex chromosomes  Autosomes Introduction: Gregor Mendel is known as the “father of genetics” due to his extensive research of inherited traits – specifically in the pea plant. Because of Mendel’s work, the contributions of many other scientists, and advances in technology we now know that diploid individuals have two copies of each gene, called alleles , which correspond to exhibited traits.

We also know that alleles are located on chromosomes. Since diploid organisms have two copies of each chromosome it is possible for them to be homozygous dominant (two dominant alleles, AA), homozygous recessive (two recessive alleles, aa), or heterozygous (one dominant and one recessive allele, Aa). These combinations of alleles are called the genotype . When we refer to the visible traits, or appearance, we are referring to the organism’s phenotype . If the phenotype associated with a given version of a gene is observed when an organism has only one copy, the allele is said to be dominant (denoted by uppercase letters, A). The phenotype will be seen whether the organism has one copy of the allele (heterozygous, Aa) or two copies of that allele (homozygous, AA). If the phenotype associated with a given version of a gene is observed only when an individual has two identical copies, the allele is said to be recessive (aa). The phenotype will be observed only when the individual is homozygous for the allele concerned. An individual with only one copy of the allele will not show the phenotype but will be able to pass the allele on to subsequent generations. As a result, an individual heterozygous for an autosomal recessive allele is known as a carrier . Mendelian Inheritance Patterns Scientists use a grid-like tool ( Punnett Square ) to make predictions about various genetic problems. The Punnett Square shows only the probability (the chance of something occurring) of what might occur and not the actual results. For example, if one wants to flip a coin 100 times, since there are 2 sides to the coin, they can expect 50 heads and 50 tails. However, if you actually flip the coin 100 times, you may actually get 60 heads and 40 tails. Punnett Squares only show the chances of what might occur each time the event is undertaken. They do not show the actual outcome. Recall that Mendel formulated the First Law of Inheritance which states that: Each organism contains two alleles for each trait (gene), and the alleles segregate (separate) during formation of gametes. Each gamete (egg or sperm) contains only one allele for each gene. Upon fertilization, the resulting offspring will have two alleles for each trait – one from each parent. Using a Punnett square , we can predict the possible genotypes and phenotypes of resulting offspring, when crossing two parents whose genotypes are known. For example: If we want to know the possible offspring genotypes from a cross between a homozygous dominant male and a homozygous recessive female, we can use a Punnett square to predict the possible outcomes.  Homozygous dominant Male = GG  Homozygous recessive female = gg

The possible gametes from these two parents are as follows: G or G, and g or g. Now we place the gametes on the top and sides of the Punnett square. Then fill in the spaces with one gamete being contributed by each parent. g g G G g G g G G g G g 1. What is the probability of having offspring with the following genotypes? 1. GG 0 :4 or 0% 2. Gg 4 :4 or 4 % 3. gg 0 :4 or 0 % Part 1: Monohybrid Crosses – Simulation of Mendel’s Work When a single pair of alleles is involved in one trait, such as green peas, Mendel found that crossing individuals with heterozygous genotypes (Gg x Gg) would result in both dominant and recessive phenotypes among the offspring. Crosses such as these, where ONLY one allele pair and its resulting phenotypes are investigated are called monohybrid crosses . Mendel studied seven different traits in pea plants and saw that each time he conducted monohybrid crosses he could expect a phenotypic ratio of 3:1. Three offspring having the dominant phenotype and 1 having the recessive. Data collected from counting 1000s of offspring produced by monohybrid crosses allowed Mendel to determine the law of segregation, which states that each trait should have at least two inheritable alleles, these alleles should segregate during gamete formation and at fertilization organisms again have two alleles one from each parent. In this experiment you will be simulating a monohybrid cross using any 2 two-sided (fair) coins that you have. Since each coin has 1-heads and 1-tails it will represent a heterozygous parent (Hh). You will use two coins at the same time, so your cross is Hh x Hh, a monohybrid cross. Question: When simulating a monohybrid cross does Mendel’s conclusion that Hh x Hh results in 3:1 phenotypic and 1:2:1 genotypic ratio holds true? Yes. Hypothesis: If Mendel’s conclusion that Hh x Hh is true, then my random coin toss should be closely related to his work.

Materials:  (2) Any two-sided fair coin (heads on one side, tails on the other)  Calculator

Procedure: 1. You will pick up 2 coins. Each side represents one allele of the same gene, Heads (H) and tails (h), respectively. Since each coin has 1- heads and 1-tails it will represent a heterozygous parent. Since you will use two coins at the same time your cross is Hh x Hh. 2. To simulate a monohybrid cross, you will toss TWO coins, SIMULTANEOUSLY, each coin represents one of the heterozygous parents (Hh x Hh). 3. Record the resulting genotype from the tossed coins, which side lands face up for each coin. The only possibilities that can be made from this toss are: HH (homozygous heads), Hh (heterozygous heads), or hh (homozygous tails). Mark the resulting genotype and phenotype in the data table. 4. Pick up your two coins and conduct the same process (steps 1-3) 14 more times (15 total trials). Record your data in Table 1. Results: Table 1: Monohybrid Cross Simulation – 2 two-sided coin toss Trial Offspring Genotype Offspring Phenotype 1 Hh Heterozygous heads 2 hh Homozygous tails 3 Hh Homozygous tails 4 Hh Heterozygous heads 5 Hh Heterozygous heads 6 HH Homozygous heads 7 HH Homozygous heads 8 Hh Heterozygous heads 9 Hh Heterozygous heads 10 Hh Heterozygous heads 11 HH Homozygous heads 12 Hh Homozygous tails 13 HH Homozygous heads 14 Hh Heterozygous heads 15 Hh Heterozygous heads Total number of offspring with: 2. Homozygous dominant genotype: 4 3. Heterozygous genotype: 8 4. Homozygous recessive genotype: 3

5. Calculate the genotypic ratio of your data: 4:8:3 Total number of offspring with: 6. Dominant Phenotype: 12 7. Recessive Phenotype: 3 8. Calculate the phenotypic ratio of your data: 12:3 Questions: 9. What is the dominant trait and how do you know it is dominant? The dominant trait/allele is identified by a capital letter, example: (A) 10. What is the recessive trait? The recessive trait/allele is identified by a lower-case letter, example: (a); which tends to be masked by other inherited traits. 11. What are the genotypes of the parents? HH, Hh 12. What are the phenotypes of parents? hh 13. Fill Punnett Square on the right using the parents given in the procedure. Male Hh x Female Hh 14. Looking at your Punnett square, what is the genotypic ratio? 1:2:1 15. Does your genotypic ratio from the coin toss match the ratio of your Punnett square? (You calculated this in the results section) Why or why not? The genotypic ration from the coin toss does not match the Punnett Square. The reason is because the disparity between the results of a coin toss and the predictions from the Punnett Squares is that a coin toss is entirely random vs the genetic probabilities of inheritance of traits through alleles.

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