Heredity and Genetics Part Two Dihybrid Crosses

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.

Review of Monohybrid Crosses • 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: Bb. Rr x Bb. Rr

Dihybrid Cross Bb. Rr x Bb. Rr 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 Bb. Rr x Bb. Rr Possible gametes for BOTH parents: BR Br b. R 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… BR Br b. R br

Dihybrid Crosses: a cross that shows the possible offspring for two traits Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth. Bb. Rr x Bb. Rr B R b rb RBr. B R b r. 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. Bb. Rr x Bb. Rr B R b rb RBr. B R b r. Br BBR R Bb. Rr BBRr BBrr Bb. Rr Bbrr Bb. RR Bb. Rr bb. RR bb. Rr Bbrr bb. Rr bbrr. BBRr

BR b. R b rb. RBr. BR b r. Br BBR R Bb. Rr BBrr Bb. Rr Bbrr Bb. R R Bb. Rr bb. R R bb. Rr Bbrr bb. Rr 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

BR b. R b rb. RBr. BR b r. Br BBR R Bb. Rr BBrr Bb. Rr Bbrr Bb. R R Bb. Rr bb. R R bb. Rr Bbrr bb. Rr 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 ? Fur Color: B: Black b: White Coat Texture: R: Rough r: Smooth. Phenotypic Ratio 9: 3: 3:

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.

What Are the Dihybrid Genotypes of the Parents? • Parent 1: Heterozygous for both traits – Yy. Rr Phenotypes: Yellow & Round • Parent 2: Homozygous for both traits – yyrr Phenotypes: Green & Wrinkled Parent 1 X Parent 2 • Yy. Rr X yyrr

What are all the possible gamete combinations? Parent 1 Yy. Rr YR Yr y. R yr ry. Parent 2 rryy. Remember that you get ONE gamete from each parent for each trait

Yy. Rr X yyrr YR y. R yryryr yr yr. Yr Yy. Rr yyrr Yy. Rr Yyrr yy. Rr yyrr. Yyrr Phenotypic ratio: 4: