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Simple Punnett Square Warm-up

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1 Simple Punnett Square Warm-up
In cats, long hair is recessive to short hair. A true-breeding (homozygous) short-haired male is mated to a long-haired female. What is the genotypic and phenotypic ratios for their offspring?

2 Simple Punnett Square Warm-Up
Mr. and Mrs. Miller both have widow’s peaks (dominant). Their first child also has a widow’s peak, but their second child doesn’t. Mr. Miller accuses Mrs. Miller of being unfaithful to him. Is he necessarily justified? Why or why not? Use a Punnett square to defend your response.

3 Independent Assortment

4 Two-Factor Crosses: F1 Mendel crossed true-breeding plants that produced only round yellow peas (genotype RRYY) with plants that produced wrinkled green peas (genotype rryy) All of the F1 offspring produced round yellow peas. What does this show? This shows that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas.

5 Two Factor Crosses: F2 Mendel knew the genotypes of the F1 generation were RrYy. All heterozygous for seed shape and seed color genes. When he crossed these F1 plants he found that in the F2 generation, there were plants that had combinations of phenotypes (combos of alleles) not found in either parent. Therefore, the alleles had segregated independently of each other. We call these tests two-factor or dihybrid crosses.

6 Independent Assortment

7 Independent Assortment
The Principle of Independent Assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, etc. Example: Blue eyes and brown hair; one dominant and one recessive trait are visible in phenotype. The dominance of brown hair had no bearing on the expression of blue eyes.

8 Setting Up a Dihybrid Cross
In cats long hair is dominant to short hair and short tail is dominant to long tail. A long-tailed male cat that is heterozygous for hair length is mated with a female cat who is heterozygous for both traits. What are the genotypic and phenotypic ratios for their kittens? First step: Write out genotypes of both parents.

9 Setting Up a Dihybrid Cross
Parents genotypes – (H for hair-length; T for tail-length) Male: Hhtt Female: HhTt Second Step: Figure out all possible combinations (Independent Assortment). Use FOIL method to determine combinations. FOIL (First, Outside, Inside, Last)

10 Setting Up a Dihybrid Cross
Possible combinations (should be four for each) Male: Ht, Ht, ht, ht Female: HT, Ht, hT, ht Third Step: Take the four combinations for each parent and put them on the outside of your dihybrid Punnett square. Male Gametes Female Gametes

11 Fourth Step: Fill in Punnett square
Long-hair, short-tail

12 Setting Up a Dihybrid Cross
Fifth step: Tally up all the different genotypes and phenotypes from the cross. Determine the probability of each genotype/phenotype and right it down as a ratio (ex. 9:3:3:1).

13 Setting Up a Dihybrid Cross
Genotypes HHTt: 2 HHtt: 2 HhTt: 4 Hhtt: 4 hhTt: 2 hhtt: 2 Phenotypes Long hair, Short tail: 6 Long hair, Long tail: 6 Short hair, Short tail: 2 Short hair, Long tail: 2 Phenotypic Ratio: 6:6:2:2 or 3:3:1:1

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