Outer. Last. Possible gamete combinations for parent 1: RY RY ry ry F (first) O (outer) I (inner) L (last)

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1 Dihybrid Crosses Explained: Mendel s Law of Independent Assortment says that genes for different traits can segregate independently during the formation of gametes. What does that mean? It means that the inheritance of one gene does not influence the inheritance of another gene. For example, just because you got your dad s tall gene doesn t mean you will also necessarily get his brown eye gene the two aren t connected! What if we want to know the chances of getting both brown eyes and tallness? Or what if we wanted to know the chances of having brown eyes and being short? A monohybrid cross will not work. We need to do a dihybrid cross. A dihybrid cross is a cross that examines the inheritance of two different traits. The first step to completing a dihybrid cross is to determine the four possible combinations of alleles that each parent can contribute to their gametes. To figure this out, write each parent genotype and FOIL it. EXAMPLE The shape and color of peas are determined by two different traits. Pea Shape: R = round Pea color: Y = yellow r = wrinkled y = green Let s cross a plant whose genotype is RrYY (parent 1) with a plant whose genotype is Rryy (parent 2). First determine the gametes for parent 1 whose genotype is RrYY (FOIL it!): Outer STEP ONE First Parent 1 genotype: R r Y Y Inner Last Possible gamete combinations for parent 1: RY RY ry ry F (first) O (outer) I (inner) L (last) Next determine the gametes for parent 2 whose genotype is Rryy: STEP TWO Parent 2 genotype: R r y y Possible gamete combinations for parent 2: Ry Ry ry ry F (first) O (outer) I (inner) L (last)

2 Now we put these gamete combinations in the gametes on the top and left side of the Punnett square shown below. Remember that any time you do a Punnett square, you are really crossing two gametes to get an offspring. Combine the two gametes the same way you do with a monohybrid cross to complete the large Punnett square. Parent 1 RY RY ry ry STEP THREE Ry Ry Parent 2 ry ry The last step is to determine the genotypic and phenotypic ratios of the offspring the same way in which we did for monohybrid crosses. STEP FOUR Genotypes: 4 Genotypic ratio: 1 : 2 : Phenotypes: 12 Phenotypic ratio: 3 : % : 25%

3 KEY to Dihybrid Cross Practice 1 In mice, big ears (E) is dominant over small ears (e) and black fur (B) is dominant over white fur (b). Cross two mice that are heterozygous for both traits. Parent 1 phenotype: Parent 1 genotype: EB, Eb, eb, eb Parent 2 phenotype: Parent 2 genotype: EB, Eb, eb, eb EB Eb eb eb EB EEBB EEBb EeBB Eb EEBb EEbb big ears, white fur Eebb big ears, white fur eb EeBB eebb small ears, black fur eebb small ears, black fur eb Eebb big ears, white fur eebb small ears, black fur eebb small ears, white fur 1. What fraction of the offspring are heterozygous for both traits? 1/4 2. What fraction of the offspring are homozygous recessive for both traits? 1/16 3. What fraction of the offspring are BOTH black and have big ears? 9/16 4. What fraction of the offspring are BOTH black with small ears? 3/16 5. What fraction of the offspring are BOTH white with big ears? 3/16 6. What fraction of the offspring are BOTH white and have small ears? 1/16

4 KEY to Dihybrid Cross Practice 2 Now we will follow the traits of height and flower color in pea plants. Tall (T) is dominant to short (t) and purple (P) is dominant to white (p). Cross a plant whose genotype is with a plant whose genotype is tp. Parent 1 phenotype: Parent 1 genotype: TP, Tp, tp, Parent 2 phenotype: Parent 2 genotype: tp all TP Tp tp Tp tp Tp tp Tp tp Tp tp 1. List all of the possible genotypes of the offspring. _, Tp,, tp 2. What is the genotypic ratio of the offspring? 1 : 1 Tp: 1 : 1 tp 3. List all of the possible phenotypes of the offspring. ; ; ; 4. What is the phenotypic ratio of the offspring? 1 : 1 : 1 : 1 short, white 5. What are the percentages of the different phenotypes of the offspring? 25% ; 25% ; 25% ; 25%

5 Research Questions I want to answer: Dihybrid Crosses STEP FOUR STEP THREE STEP TWO STEP ONE

6 Dihybrid Cross Practice 1 In mice, big ears (E) is dominant over small ears (e) and black fur (B) is dominant over white fur (b). Cross two mice that are heterozygous for both traits. Parent 1 phenotype: Parent 1 genotype: Parent 2 phenotype: Parent 2 genotype: 1. What fraction of the offspring are heterozygous for both traits? 2. What fraction of the offspring are homozygous recessive for both traits? 3. What fraction of the offspring are BOTH black and have big ears? 4. What fraction of the offspring are BOTH black with small ears? 5. What fraction of the offspring are BOTH white with big ears? 6. What fraction of the offspring are BOTH white and have small ears?

7 Dihybrid Cross Practice 2 Now we will follow the traits of height and flower color in pea plants. Tall (T) is dominant to short (t) and purple (P) is dominant to white (p). Cross a plant whose genotype is with a plant whose genotype is tp. Parent 1 phenotype: Parent 1 genotype: Parent 2 phenotype: Parent 2 genotype: 1. List all of the possible genotypes of the offspring. 2. What is the genotypic ratio of the offspring? 3. List all of the possible phenotypes of the offspring. 4. What is the phenotypic ratio of the offspring? 5. What are the percentages of the different phenotypes of the offspring?