Download Trait Determination Practice

Name_____________________________________________ Date______________ Hour_______ Table #____
Punnett Squares
A Punnett square is a chart which shows/predicts all possible gene combinations in a cross of parents
(whose genes are known). Punnett squares are named for an English geneticist, Reginald Punnett. He
discovered some basic principles of genetics. He worked with the feather color traits of chickens in order to
quickly separate male and female chickens.
In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using
Punnett squares, you can predict the genotypes and phenotypes of the offspring of a cross between
homozygous (pure) tall pea plant and a homozygous (pure) short pea plant.
Step 1: Designate letters which will represent the gene/traits. Capital letters represent dominant traits, and
lowercase letters represent recessive traits. T = tall t= short
Step 2: Write down the genotypes (genes) of each parent. These are often given to you or are easy to
determine. TT x tt or (tall) (short)—both homozygous pure
Step 3: List the genes that each parent can contribute
Parent 1
TT
T
Parent 2
tt
t
Step 4: Draw a Punnett square--- 4 small squares in the shape of a window. Write the possible gene(s) of one
parent across the top and the gene(s) of the other parent along the side of the Punnett square.
Step 5: Fill in each box of the Punnett square by transferring the letter
above and at the side of each box into the appropriate box. As a general
rule, the capital letter goes first and a lowercase letter follows.
Genotype Phenotype
TT
Tall
Tt
Tall
tt
short
Step 6: List the possible genotypes and phenotypes of the offspring for this cross.
Since the capital letter indicates a dominant gene, T (tall) is dominant over t (short).
Therefore, each of the offspring in this case will be tall.
Name_____________________________________________ Date______________ Hour_______ Table #____
Use Punnett squares to determine the answers to the following samples.
1. One cat carries heterozygous, long-haired traits (Ss), and its mate carries homozygous short-haired traits
(ss). Use the Punnett square to determine the probability of one of their offspring having long hair.
A. 100%
S
s
B. 25%
C. 75%
s
D. 50%
s
2. One flower is heterozygous red (Rr) and it is crossed with a homozygous white (rr) plant. Use the Punnett
square to determine the probability of one of their offspring having a red color.
A. 75%
B. 25%
C. 50 %
D. 100%
3. In certain species of plant, the color purple (P) is dominant to the color white (p). Use the Punnett square
to determine the probability of the offspring being purple if a pure purple plant is crossed with a pure white
plant.
A. 25%
B. 0 %
C. 100%
D. 75%