Dihybrid Crosses Incomplete Dominance Codominance Multiple Alleles

1. Dihybrid CrossesIncomplete DominanceCodominanceMultiple Alleles In a nutshell!

2. PP = purple Pp = purple pp = white Review: Dominant/Recessive • One allele is dominant over the other (capable of masking the recessive allele) • Example: Purple flowers are dominant over white

3. PP Pp Pp pp Review Problem: Dominant/Recessive • In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants. P p GENOTYPES: - PP (1); Pp (2); pp (1) - ratio 1:2:1 P p PHENOTYPES: - purple (3); white (1) - ratio 3:1

4. Review of Monohybrid Crosses • Remember, monohybrid crosses involve only ONE trait • Practice… In fruit flies, red eyes are dominant over white eyes. • Cross a white-eyed fly with a homozygous dominant red-eyed fly. • Cross two heterozygous red-eyed flies. Draw a Punnett square for each cross, and determine the genotypic and phenotypic ratios.

5. r r R r Rr Rr R RR Rr R • Can you determine the genotype of a white-eyed fly just by looking at it? Why? • Yes- it’s rr • There is only one genotype for white eyes • Can you determine the genotype of a red-eyed fly just by looking at it? Why? • No- it could be RR or Rr • There are two genotypes for red eyes • How could you determine the genotype of the red-eyed fly? • Perform a test cross R Rr Rr r Rr rr

6. Review of Monohybrid Crosses • What two organisms would you cross in the test cross? • The unknown red-eyed fly with a white eyed fly • If the red-eyed fly was RR, what would the offspring of the test cross look like? • All of the offspring would have red eyes • If the red-eyed fly was Rr, what would the offspring of the test cross look like? • Half of the offspring would have red eyes, half would have white eyes

7. Dihybrid Cross:a cross that shows the possible offspring for two traits Coat Texture: R: Rough r: Smooth Fur Color: B: Black b: White In this example, we will cross a heterozygous individual with another heterozygous individual.

8. Step 1: Determine the parents’ genotypes Step 2: You must find ALL possible gametes that can be made from each parent. Remember, each gamete must have one B and one R. Parent 1: BbRr Parent 2: BbRr

9. Parent 1 Possible gametes: BR Br bR br Parent 2 Possible gametes: BR Br bR br Parent 1: BbRr Parent 2: BbRr

10. Step 3: arrange all possible gametes for one parent along the top of your Punnett Square, and all possible gametes for the other parent down the side of your Punnett Square…

11. BR Br bR br BR Br bR br BbRr x BbRr Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth Step 4: Find the possible genotypes of the offspring

12. BR Br bR br BR BBRR BBRr BbRR BbRr Br BBRr BBrr BbRr Bbrr bR BbRR BbRr bbRR bbRr br BbRr Bbrr bbRr bbrr BbRr x BbRr Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth

13. BR Br bR br BR BBRR BBRr BbRR BbRr Br BBRr BBrr BbRr Bbrr bR BbRR BbRr bbRR bbRr br BbRr Bbrr bbRr bbrr How many of the offspring would have a black, rough coat? How many of the offspring would have a black, smooth coat? How many of the offspring would have a white, rough coat? How many of the offspring would have a white, smooth coat? Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth

14. BR Br bR br BR BBRR BBRr BbRR BbRr Br BBRr BBrr BbRr Bbrr bR BbRR BbRr bbRR bbRr br BbRr Bbrr bbRr bbrr How many of the offspring would have black, rough coat? How many of the offspring would have a black, smooth coat? How many of the offspring would have a white, rough coat? How many of the offspring would have a white, smooth coat? Phenotypic Ratio 9:3:3:1 Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth

15. On a blank sheet of paper… In pea plants, yellow seeds (Y) are dominant over green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r). Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the phenotypic ratio.

16. YR Yr yR yr yr YyRr Yyrr yyRr yyrr yr YyRr Yyrr yyRr yyrr yr YyRr Yyrr yyRr yyrr yr YyRr Yyrr yyRr yyrr YyRr X yyrr

17. RR = red rr = white Rr = pink Incomplete Dominance • A third (new) phenotype appears in the heterozygous condition. • Flower Color in 4 O’clocks

18. R r r r Rr Rr rr rr Problem: Incomplete Dominance • Show the cross between a pink and a white flower. GENOTYPES: - Rr (2); rr (2); RR (0) - ratio 2:2:0 PHENOTYPES: - pink (2); white (2) Red (0) - ratio 2:2:0

19. NS = some of each SS = sickle cells NN = normal cells Codominance • The heterozygous condition, both alleles are expressed equally • Sickle Cell Anemia in Humans sick

20. NS SS SS NS Problem: Codominance • Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S GENOTYPES: - NS (2) SS (2) - ratio 2:2:0 S S PHENOTYPES: - carrier (2); sick (2) - ratio 2:2:0

21. Multiple Alleles • There are more than two alleles for a trait • Blood type in humans • Blood Types? • Type A, Type B, Type AB, Type O • Blood Alleles? • A, B, O (in book – IA, IB, I)

22. Rules for Blood Type • A and B are codominant • AA = Type A • BB = Type B • AB = Type AB • A and B are dominant over O • AO = type A • BO = type B • OO = type O

23. AO AO BO BO Problem: Multiple Alleles • Show the cross between a mother who has type O blood and a father who has type AB blood. O O GENOTYPES: - AO (2) BO (2) A B PHENOTYPES: - type A (2); type B (2) - ratio 2:2:0:0

24. BO AB AO OO Problem: Multiple Alleles • Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood. GENOTYPES: A O • AB (1); BO (1); • AO (1); OO (1) • - ratio 1:1:1:1 B O PHENOTYPES: • type AB (1); type B (1) • type A (1); type O (1) • - ratio 1:1:1:1