Chapter 3 Punnett Square vs Fork- line Method for Working Two Trait Genetic Problems

1. Chapter 3 Punnett Square vs Fork- line Method for Working Two Trait Genetic Problems

2. FOIL Method for Making Gametes(First, Outside, Inside, Last) Cross: For and individual with the genotype: PpYy This is how one would make gametes using the FOIL method: First: PY Outside: Py Inside: pY Last: py

3. Lunch Menu Method to Determine Gametes • Sandwich 2 Types: Peanut butter and jelly Turkey • Two Fruit Types: Apples Oranges • Two Dessert Types: Ice Cream Cake Making gametes is like having a lunch where you can only have one sandwich, one fruit and one dessert. In a two trait cross each allele of one of the genotypes represents the sandwiches and each allele of the other genotype represents the fruit.

4. If one were to self pollinate heterozygous purple flowered pea plants that were also heterozygous for the genes that produce yellow coated seeds, the genotype for the purple flower would be Pp and the genotype for the seed color would be Yy. The genetic cross would be: Pp Yy X Pp Yy. If one wanted to use the Punnett square method to show this cross then gametes for each of the plants must be produced. • So for the genotype of PpYy the P’s represent the sandwiches and the Y’s represent the fruits: • The gamete combinations would be: PY Py pY py Why wouldn’t Pp be an appropriate gamete?

5. for example, the following dihybrid cross: List gametes: PpYy x PpYy • normally to solve this we would • Use Lunch Menu to determine possible gametes, then • assemble the Punnet square, then • count up the genotypic and phenotypic ratios.

6. for example, the following dihybrid cross: List gametes: PpYy x PpYy • normally to solve this we would • Use Lunch Menu to determine possible gametes, then • assemble the Punnet square, then • count up the genotypic and phenotypic ratios.

7. Fork Line Method: • This is just another way to be able to predict genotype and phenotype ratios in dihybrid problems • this way you don’t have to write the box: 16, 64, etc… • but it does require you to know the basic ratios that arise from monohybrids • based on the idea that: in a dihybrid, the two traits sort INDEPENDENTLY of one another • i.e. what happens with one trait is completely unrelated to what happens with the other trait

8. PpYy x PpYy so to solve this dihybrid by fork line , separate the two traits (since they sort independently): Pp x Pp will produce: ¼ PP 2/4 Pp ¼ pp

9. PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): similarly, Yy x Yy will give: ¼ YY Pp x Pp will give: 2/4 Yy ¼ yy ¼ PP 2/4 Pp ¼ pp

10. PpYy x PpYy so to solve this dihybrid, separate the two traits (since they sort independently): ¼ YY Yy x Yy will produce: Pp x Pp will produce: 2/4 Yy ¼ yy multiply fractions 1/16 PPYY ¼ YY 2/4 Yy 2/16 PPYy ¼ PP ¼ yy 1/16 PPyy 2/4 Pp ¼ pp

11. PpYy x PpYy So to solve this dihybrid, separate the two traits (since they sort independently): Yy X Yy will produce: Pp x Pp will produce: F1 Genotypic Ratio multiply fractions 1/16 PPYY ¼ YY 2/4 Yy 2/16 PPYy ¼ PP ¼ yy 1/16 PPyy 2/16 PpYY ¼ YY 2/4 Pp 2/4 Yy 4/16 PpYy ¼ yy 2/16 Ppyy 1/16 ppYY ¼ YY ¼ pp 2/4 Yy 2/16 ppYy ¼ yy 1/16 ppyy

12. Genotype ratios Phenotype ratio P_ Y_ P_ yy pp Y_ pp yy