Genetics notes 9 and 10 - Dihybrid crosses PDF

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GENETICS NOTES AND LEARNING GOALS Lecture 9: Dihybrid Crosses Lecture 10: Beyond the dihybrid In these lectures, we are applying the fundamentals of Mendelian genetics to more complex genetic scenarios. In particular, we are using what we’ve learned about sexual reproduction to predict how multiple genes behave simultaneously. These problems may seem intimidating at first, but it’s really nothing you haven’t seen before, and we will teach you multiple ways to tackle these questions. Dihybrid (double heterozygote) crosses: After performing his monohybrid experiments, the next question Mendel asked was whether different kinds of traits are inherited independently of one another? Or in other words, if a gamete receives a green allele, does that increase or decrease its likelihood of receiving a dwarf allele? Or are these unit factors completely independent? This may seem like an odd question to ask, but to Mendel this was the next logical step in testing his hypothesis that traits are controlled by “discrete” factors. [To jump ahead a little bit, many genes, if they lie close to one another on the same chromosome, are NOT inherited independently. But Mendel was fortunate that most of the genes he was studying were on different chromosomes. This will make more sense when we get to genetic linkage.] Dihybrid Cross part 1 (P to F1): As an example, let’s consider a P1 cross between two true-breeding plants: one that makes yellow round peas (GGWW; wild-type phenotype for both traits) and another that makes green wrinkled peas (ggww; recessive phenotype for both traits). The F1 offspring of this cross all produce yellow round peas. You can think of this as two monohybrid crosses happening simultaneously, in which case this result should not be surprising––both dominant phenotypes are observed in these F1 dihybrids. [Note that a slightly different cross, yellow wrinkled X green round, would give you the same results]. Dihybrid Cross part 2 (F1 to F2): But what happens in the F2 generation? When Mendel crossed the F1 dihybrids to one another, he observed four distinct classes of offspring in very reproducible ratios. The doublerecessive phenotype was the rarest. For each one of these double-recessive individuals he saw, he found about nine individuals with the double-wild-type phenotype. And for each of the double-recessive plants, he saw about three of the single-recessive phenotypes. Or in other words there was 9:3:3:1 phenotypic ratio for the four classes (9 dominant A, dominant B : 3 dominant A, recessive B : 3 recessive A, dominant B : 1 recessive A, recessive B). This is another Mendelian ratio you should be ready to recognize. Okay…that’s interesting…but what does it mean? What it means, is that the two traits are inherited completely independently of one another. [LEARNING GOAL: To visualize this, make a Punnett square of the above experiment. Assume the genes controlling pea color and shape are on different chromosomes and figure out what types of gametes the F1 plants can make. Your Punnett square should be 4x4, giving you 16 boxes. Notice that 9+3+3+1 equals…16! Figure out the genotypes and phenotypes for each of those 16 boxes. How many different classes are there? Count them and see what the ratios are.] Punnett squares are useful tools when you start working out these kinds of problems, but they are pretty cumbersome to construct, and once you start dealing with three or more genes, they become comically large. There is a simpler method for figuring out these crosses, and it takes advantage of the product law that you learned in math class. The probability of two independent events both occurring is simply the product of their two likelihoods. For example, the likelihood of 1) flipping a coin and getting tails and 2) rolling a dice and getting four is simply 1/2 multiplied by 1/6, or 1/12. So, when you are predicting the likelihood of getting a specific combination of phenotypes in from a dihybrid cross (or a trihybrid cross, or a tetrahybrid cross…), just figure out the likelihood of each individual phenotype (it doesn’t matter what order you do it in) and then multiply the fractions. For example, what is the likelihood of getting a plant that makes green wrinkled peas from a GgWw dihybrid cross? First, just focus on one gene (shape) and ignore the other (color); the probability of getting a plant that makes wrinkled peas is 1/4. Now focus on the other gene (color) and ignore the first

(shape). The probability of getting a getting a plant that makes green peas is 1/4. Multiply those together and you get 1/16. So, the probability of getting the double-recessive class is 1/16. This is the same result that you got from a Punnett square, but it’s much easier. [LEARNING GOAL: Use the product law and the forked-line method (Figure 3.9 in the textbook) to predict the ratios of phenotypes you get from a dihybrid cross. Do this for a couple different gene combinations until you are comfortable with the process. Now do it for the following trihybrid F1 selfcross: WwGgDd x WwGgDd. Hint: you should get 8 classes of offspring. What is the Mendelian ratio for a trihybrid cross?] [LEARNING GOAL: What is Mendel’s fourth postulate, and what is the molecular basis of this phenomenon? Hint: it happens very early in meiosis.] [LEARNING GOAL: How do you perform a testcross for two genes at the same time? For example, how would you determine the genotype of a G–W– pea plant?]...