Dihybrid Crosses

What is a dihybrid cross? (You don’t need to write this.)

• Monohybrid cross: using Punnet squares to find the probabilities for one trait.

• Dihybrid cross: using Punnet squares to find the probabilities for two traits.

Man-eating pea plant traits:T = poisonous thorns t = no thorns B = big leaves b = small leaves

If the mad scientist crosses two plants that are both heterozygous for poisonous thorns and heterozygous for big leaves, what phenotypes might she see in the offspring? What is the probability of each? Step 1: Find the genotypes of parents:

_____________________x _______________________

Step 1: Find the genotypes of parents: ________Tt Bb_________ x ________Tt Bb________ Step 2: Separate traits, and do a monohybrid cross for each trait by itself. Punnett Square for Thorns: Punnett Square for Leaves:

Possible Phenotype Probability (Fraction) Possible Phenotype Probability (Fraction) 1) ___________________________ ________ 1) ______________________________ ________ 2) ___________________________ ________ 2) ______________________________ ________

Step 3: List all possible combinations and multiply to find the probability for each combination.

2) According to the table above, what is the probability that an offspring will have no thorns and big leaves? 

3) What is the probability that an offspring will have no thorns and small leaves?

4) What is the probability that an offspring will have no thorns and big leaves?

Man-eating pea plant traits:T = poisonous thorns t = no thorns B = big leaves b = small leaves

Next the mad scientist crosses a plant that is homozygous dominant for poisonous thorns and heterozygous for big leaves with another plant that is heterozygous for poisonous thorns and homozygous recessive for small leaves. What kinds of phenotypes could the offspring have, and what is the probability of each one?

Step 1: Find the genotypes of parents: _______________________x ____________________

Two plants with the following genotypes are crossed: Tt Bb x Tt bb 5) What fraction (or percent) of the offspring will most likely have poisonous thorns? SHOW YOUR WORK by drawing a Punnet Square. No work, no credit. 6) What fraction (or percent) of the offspring will most likely have small leaves? SHOW YOUR WORK by drawing a Punnet Square. No work, no credit. 7) What fraction (or percent) of the offspring will most likely have poisonous thorns AND small leaves? SHOW YOUR WORK by multiplying. No work, no credit.

Exit TicketT = poisonous thorns t = no thornsB = big leaves b = small leaves

TT Bb x Tt bb 1)What fraction will most likely have poisonous thorns? SHOW YOUR WORK with a Punnet Square.2) What fraction will most likely have small leaves? SHOW YOUR WORK with a Punnet Square. 3) What fraction will most likely have poisonous thorns AND small leaves? SHOW YOUR WORK by multiplying.