PUNNETT SQUARE PRACTICE
1.
Explain the terms genotype and phenotype. Give an example of each.
Genotype: letters that stand for the 2 alleles which code for a trait: GG, gg
Phenotype: outward appearance of a trait: brown hair, tall plant, blue eyes
2.
How are genes related to chromosomes? How are alleles related to
genes?
Genes are found on chromosomes: alleles are alternate possibilities of a
gene, example: gene = blood type; alleles = o allele, a allele, b allele
3.
What is a Punnett square used to do?
Used to predict the probability of the offspring of a cross having a certain trait
4.
Explain how two organisms can have the same phenotypes, but
different genotypes.
Homozygous dominant and heterozygous genotypes both show the dominant
phenotype
5.
An individual has the genotype Dd. What alleles could each
gamete contain?
Half of the gametes will have a D allele and half of the gametes will have a d
allele
6.
A brown-eyed couple, both heterozygous for eye color (Bb), have three
brown-eyed children and are expecting a fourth child. The mother
insists that the child she is carrying will have blue eyes. Draw a
Punnett square for the cross and explain why you agree or disagree
with the mother. What are the chances of the fourth child having blue
eyes?
B
b
B
BB
Bb
b
Bb
bb
Disagree; each time they have a
have a child, there is a new
Punnett square. Previous children
do not have an effect on this child.
25% chance of blue eyes
7.
A gardener has two tall pea plants. How can the gardener determine
whether the two plants are homozygous or heterozygous for the gene
determining tallness? Show the two Punnett squares as evidence for
your conclusion. What is this type of cross called?
Cross unknown tall plants with homozygous recessives and compare their
Offspring with two possible Punnett squares to determine genotypes.
t
t
T
Tt
Tt
T
Tt
Tt
8.
t
T
Tt
Tt
t
tt
tt
In rabbits the allele for black coat color (B) is dominant over the allele
for brown coat color (b). Could a heterozygous black rabbit and a
homozygous black rabbit have a brown offspring? Support your
answer with a Punnett square. What is the chance of this cross having
a heterozygous black rabbit?
B
b
B
BB
Bb
B
BB
Bb
9.
t
No; 50%
In guinea pigs the allele for rough coat (R) is dominant over the allele
for smooth coat (r). In order for 50% of the offspring to be smooth
coated and 50% to be rough coated, what should the phenotype and
genotype of the male and female parents be? Show the Punnett
square to support your answer.
R_
rr
R_
rr
Rr x rr; rough x smooth
10.
Cystic fibrosis is a genetically transmitted condition in which the body
produces excess mucus that clogs lungs and intestines. Persons who
are heterozygous (Ff) for cystic fibrosis are carriers. Persons who are
homozygous recessive (ff) have the disease. If a person who does not
have the disease and is not a carrier marries a carrier for the disease,
what is the chance they will have a child with the disease? What is the
chance they will have a child who is a carrier? Show the Punnett
square to support your answer.
F
F
F
FF
FF
f
Ff
Ff
11.
O%; 50% chance of a carrier
In mice a new gene was identified for fur color. One allele produces
yellow fur color, another allele produces brown fur color. In five
crosses of brown-furred mice with yellow-furred mice, the F1
generation is always brown-furred.
a. What could be concluded about the dominance of
the fur color alleles to each other?
Brown is dominant to yellow fur
b. What could be concluded about the genotypes of
the P1 generation?
BB x bb
c. What could be concluded about the genotypes of
the F1 generation?
Bb
d. When two F1 mice are crossed they produce 5
brown-furred mice and 2 yellow-furred mice. What
are the genotypes and phenotypes of the F2 generation?
1BB; 2Bb; 1bb (approximately)
brown fur and yellow fur