1. Dihybrid Crosses Biology

2. What is a dihybrid cross? • Work 2 genes at once • Find the expected offspring for both traits • They are a little more work than the monohybrid cross, but… • So much fun!! 

3. Steps to solving a dihybrid problem • 1. Read the problem and find the two separate traits. • Sample problem: • In Springfield, red hair (H) is dominant to blue hair (h) and having four fingers (F) on your hand is dominant to having five fingers (f). If Side Show Bob and Mrs. Van Houten get married and decide to have a little brother for Millhouse, what is the chance that the newborn will have red hair and five fingers if Side Show is heterozygous for red hair and heterozygous for four fingers and Mrs. Van Houten has blue hair and is homozygous for four fingers?

4. 2. Find the two parents and write down their genotypes. • HhFf x hhFF

5. 3. Read the question and see what you have to look for. Do they want you to find a ratio, percent, fraction, or do they want you to list all possibilities?

6. 4. Once you have found all the information, you can start to set up the problem by finding the gametes for each parent by using FOIL. • FOIL stands for First, Outside, Inside, Last

7. Here is an example of how to use FOIL: • (a + b) * (c + d) • First = (a + b) * (c + d) = ac • Outside = (a + b) * (c + d) = ad • Inside = (a + b) * (c + d) = bc • Last = (a + b) * (c + d) = bd • So our gamete pairs are: ac, ad, bc, bd

8. Another example of FOIL: • (a + b) * (c + d) • Gametes from this allele pair: AC, BC, AD, BD

9. Now try it with the parents alleles • Side Show = HhFf • So, HF, Hf, hF, hf • Mrs. Van Houten = hhFF • So, hF, hF, hF, hF

10. 5. Once you have the gametes, you have to line them up in a punnett square. • You are going to need a bigger square! • Put the gametes of one parent across the top of the square and the other down the side of the square

11. HF Hf hF hf hF hF hF hF

12. 6. Now you have to do the actual cross. • This is a single box example. • Remember to always put the two like alleles back together, • The same letters go together, and • The capital letter should go first • Keep the same sequence of alleles (letters) HF HhFF hF

13. Fill in the rest of the box!

14. HF Hf hF hf hF hF hF hF

15. 7. Find the genotypic ratio • HhFF = 4/16 (25%) • HhFf = 4/16 (25%) • hhFF = 4/16 (25%) • hhFf = 4/16 (25%)

16. 8. Find the phenotypic ratio • Red hair, four fingers = 8/16 (50%) • Blue hair, four fingers = 8/16 (50%)

17. 9. Now, don’t forget to answer the question! • If Side Show Bob and Mrs. Van Houten get married and decide to have a little brother for Millhouse,what is the chance that the newborn will have red hair and five fingersif Side Show is heterozygous for red hair and heterozygous for four five fingers and Mrs. Van Houten has blue hair and is homozygous for four fingers? • 0%