Heredity and Genetics Part Two Dihybrid Crosses Two
Heredity and Genetics Part Two Dihybrid Crosses
Two Trait Crosses: The Dihybrid Cross Essential Questions: How are dihybrid crosses different from monohybrid crosses? What Mendelian Conclusion explains the possibility of Dihybrid crosses? How do you set up a Dihybrid cross?
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
What is a Dihybrid Cross? Determines the probability of genetic outcomes for TWO traits instead of ONE. Single Trait cross are called MONOHYBRID crosses! What 2 traits are being considered here? Coat Colour Tail Length
How are Dihybrid Crosses Possible? Mendel Concluded that due to the Law of Independent Assortment, different traits are not linked and have equal probability of showing up in offspring. Example: Tall Purple plants, Tall White plant, short purple plants and short white plants. Keep in mind, dominance will skew the ratios, but all of these outcomes are POSSIBLE!
Mendel’s Laws of Heredity Law of independent assortment: each pair of alleles segregates into gametes independently 4 sets of gametes are produced in equal probability YR, Yr, y. R, yr Only true for genes on separate chromosomes! How come?
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 Br Br b. R 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 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.
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? Remember that you get ONE gamete from each parent for each trait Parent 1 Yy. Rr Parent 2 rryy YR Yr y. R yr ry
Yy. Rr X yyrr YR Yr y. R yr yr Yy. Rr Yyrr yy. Rr yyrr Phenotypic ratio: 4: 4
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