Heredity and Genetics Part Two Dihybrid Crosses Review
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: Next, arrange all possible BR gametes for one parent along the Br top of your Punnett Square, and all possible gametes for the other b. R parent down the side of your Punnett Square… br
Dihybrid Crosses: a cross that shows the possible offspring for two traits Bb. Rr x Bb. Rr BR Fur Color: B: Black b: White BR Coat Texture: R: Rough r: Smooth Br Then, find the possible genotypes of the offspring b. R br Br b. R br
Dihybrid Crosses: a cross that shows the possible offspring for two traits BR Br b. R br BR BBRr Bb. RR Bb. Rr Br BBRr BBrr Bb. Rr Bbrr b. R Bb. Rr bb. RR bb. Rr br Bb. Rr Bbrr bb. Rr bbrr Bb. Rr x Bb. Rr 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 b. R br BR BBRr Bb. RR Bb. Rr Br BBRr BBrr Bb. Rr Bbrr b. R Bb. Rr bb. RR bb. Rr br Bb. Rr Bbrr bb. Rr bbrr Fur Color: Coat Texture: B: Black R: Rough b: White 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? Phenotypic Ratio 9: 3: 3: 1 BR Br b. R br BR BBRr Bb. RR Bb. Rr Br BBRr BBrr Bb. Rr Bbrr b. R Bb. Rr bb. RR bb. Rr br Bb. Rr Bbrr bb. Rr bbrr Fur Color: Coat Texture: B: Black R: Rough b: White r: Smooth
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.
Yy. Rr X yyrr YR Yr y. R yr yr Yy. Rr Yyrr yy. Rr yyrr
- Slides: 13