Heredity and Genetics Part Two Dihybrid Crosses

Dihybrid CrossesBiologyEssential QuestionHow can we determine the genotypic and phenotypic ratios of possible offspring using two traits?

Objective: We will predict the outcomes of dihybrid crosses.Review of Monohybrid CrossesRemember, monohybrid crosses involve only ONE traitPractice 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.

Review of Monohybrid CrossesCan you determine the genotype of a white-eyed fly just by looking at it?Yes- its 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 CrossesWhat two organisms would you cross in the test cross?The unknown red-eyed fly with a white eyed flyIf the red-eyed fly was RR, what would the offspring of the test cross look like?All of the offspring would have red eyesIf 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 traitsFur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothIn this example, we will cross a heterozygous individual with another heterozygous individual. Their genotypes will be:BbRr x BbRr

Dihybrid CrossBbRr 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 CrossBbRr x BbRr

Possible gametes:BRBrbRbr

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 SquareNotice:We can do this because of the Law of Independent Assortment!Dihybrid Crosses:a cross that shows the possible offspring for two traitsFur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothBbRr x BbRrBRbRbrbRBrBRbrBrThen, find the possible genotypes of the offspringDihybrid Crosses:a cross that shows the possible offspring for two traitsFur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothBbRr x BbRrBRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRrBRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRrHow 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: WhiteCoat Texture:R: Roughr: SmoothBRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRrHow 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: WhiteCoat Texture:R: Roughr: SmoothPhenotypic Ratio9:3:3:1RememberThese are Cornell notes, so3 questions in the left hand columnHonors- 2 must be Costas level 2 or 3Regulars- 1 must be Costas level 2 or 3Summary at the bottom of the page