Dihybrid Crosses Incomplete Dominance Codominance Multiple Alleles In a nutshell!

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

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 PP Pp PHENOTYPES: Pp pp - purple (3); white (1) - ratio 3:1

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.

r r R r Rr Rr R RR Rr R R Rr Rr r Rr rr 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 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

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

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.

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

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

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…

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

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

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

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? BR bR br Br BBRR BbRR BbRr BBRr BBrr Bbrr bbRR bbRr bbrr Phenotypic Ratio 9:3:3:1 Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth

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.

YyRr X yyrr YR yR yr Yr YyRr yyRr yyrr Yyrr

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

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

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

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 NS SS PHENOTYPES: - carrier (2); sick (2) - ratio 2:2:0

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)

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

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 AO BO PHENOTYPES: - type A (2); type B (2) - ratio 2:2:0:0

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 AB OO BO AO PHENOTYPES: type AB (1); type B (1) type A (1); type O (1) - ratio 1:1:1:1