PUNNETT SQUARES Page 19 PUNNETT SQUARES Punnett Square

PUNNETT SQUARES Page 19

PUNNETT SQUARES Punnett Square – model for predicting all possible genotypes resulting from a cross (mating) Used to calculate the probability of inheriting a particular trait § Probability – The chance that a given event will occur

MONOHYBRID CROSSES Monohybrid Crosses – involve only one trait The “normal” Punnett Square

MONOHYBRID CROSS Parent 1: Aa Parent 2: Aa Parent 1’s Gametes Parent 2’s Gametes Offspring

HOW TO COMPLETE A PUNNETT SQUARE a Aa aa

Genotypes: 1 AA : 2 Aa : 1 aa We can use the Punnett Square above to figure out the ratios of genotypes and phenotypes. Let’s say A = black and a = albino (no color). This means there is a 25% chance of inheriting the AA genotype, a 50% chance of inheriting the Aa genotype, and a 25% chance of inheriting the aa genotype.

Phenotypes: 3 Black : 1 Albino We can use the Punnett Square above to figure out the ratios of genotypes and phenotypes. Let’s say A = black and a = albino (no color). This means there is a 75% chance of inheriting the black phenotype, and a 25% chance of inheriting the white phenotype.

YOU TRY IT NOW! Give the genotype and phenotype ratios for the following cross: TT x tt (T = Tall and t = Short)

Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T t t T

Step Two: Complete the Punnett Square T T t Tt Tt

Step Three: Write the genotype and phenotype T T t Tt Tt Remember: Each box is 25% Genotype: 4 - Tt Phenotype: 100% Tall

YOU TRY IT NOW! Give the genotype and phenotype ratios for the following cross: Tt x tt

Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T t t t

Step Two: Complete the Punnett Square T t t Tt tt

Step Two: Complete the Punnett Square t t Tt tt Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short Remember: Each box is 25%

COMPLETE THE PUNNETT SQUARE Round seeds are dominant to wrinkled: § Cross Rr x Rr § What are the possible genotypes? § What are the possible phenotypes? § What’s the probability the offspring will be round? § What’s the probability the offspring will be wrinkled?

MONOHYBRID CROSS PRACTICE 1. Which one represents a dominant allele? G g 2. Which one represents a recessive allele? F f 3. Which one represents a homozygous dominant genotype? TT Tt tt

4. Which one represents a homozygous recessive genotype? RR Rr rr 5. Which one represents a heterozygous genotype? WW Ww ww 6. If purple eyes are dominant to pink eyes, which genotype(s) would result in purple eyes? PP Pp pp 7. If an organism is heterozygous and has brown fur, which trait is dominant? brown fur white fur