Heredity and Genetics Part Two Dihybrid Crosses

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.

Can you determine the genotype of a white-eyed fly just by looking at it? –Yes- it’s rr Can you determine the genotype of a red-eyed fly just by looking at it? –No- it could be RR or Rr 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 Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth In this example, we will cross a heterozygous individual with another heterozygous individual. Their genotypes will be: BbRr x BbRr

Dihybrid Cross BbRr x BbRr First, you must find ALL possible gametes that can be made from each parent. Remember, each gamete must have one B and one R.

Dihybrid Cross BbRr x BbRr Possible gametes: BR Br bR br Next, 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…

Dihybrid Crosses: a cross that shows the possible offspring for two traits Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth BbRr x BbRr BR bR br bR Br BR br Br Then, find the possible genotypes of the offspring

Dihybrid Crosses: a cross that shows the possible offspring for two traits Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth BbRr x BbRr BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr BBRr

BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr 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

BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr 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? Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth Phenotypic Ratio 9:3:3:1

More Practice… On the back of your notes… 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 genotypic and phenotypic ratios.

YyRr X yyrr YR yR yr Yr YyRryyRryyrr YyRrYyrryyRryyrr YyRrYyrryyRryyrr YyRrYyrryyRryyrr Yyrr