Download Practice Punnett Squares

Name:________________________
Date:___________
Period:________
HW: Solving Monohybrid Crosses in 5 Steps
Directions: Grab your paper and pencil. For each problem, be sure to show the work for each step.
Problem #1 – Being a taster of the chemical PTC is dominant over being a non-taster (recessive). A
man heterozygous for this trait marries a female who is homozygous recessive for this trait. Use
a Punnett square to predict the phenotypic and genotypic ratios of the offspring.
Step 1 – Choose a letter to represent the dominant trait and recessive trait.
T = Taster
&
t = Non-taster
Step 2 - Write down the genotypes of each parent. Refer to the problem above.
Mom tt
&
Dad
Tt
Step 3 – Write the genetic cross.
tt x Tt
Step 4 - Write the possible alleles of mom across the top and dad along the side. Fill in the Punnett square.
t
t
T
Tt
Tt
t
tt
tt
Step 5 : Determine the phenotypic and genotypic ratio of the offspring. Don’t forget to label the ratios.
Genotypic Ratio: 0 TT: 2 Tt: 2 tt
Phenotypic Ratio: 2 Taster: 2 Non-taster
Genotypic %: 0%TT: 50% Tt: 50% tt
Phenotypic %
50% Taster: 50% Non-taster
Assignment_Science7
Name:________________________
Date:___________
Period:________
Problem #2 - In pea plants, yellow peas are dominant over green peas. Use a Punnett square to
predict the phenotypic and genotypic outcome (offspring) of a cross between two plants
heterozygous for yellow peas. **REMEMBER to show your work for steps #1-5.
Step 1: Choose a letter
Yellow = Y
Green = y
Step 2: Write the genotypes for mom & dad
Mom = Yy
Dad = Yy
Step 3: Write the genetic cross
Yy x Yy
Step 4: Draw a Punnett Square
Y
y
Y
YY
Yy
Yy
yy
y
Step 5: Write the genotype and phenotype ratios
Genotypic Ratio
1 YY: 2 Yy: 1 yy
Phenotypic Ratio: 3 Yellow: 1 Green
25% YY: 50% Yy: 25% yy
75% Yellow: 25% Green
Assignment_Science7