Genetic Crosses
Level I
Objective: To become proficient in completing and analyzing Punnett Squares and
predicting outcomes.
Directions: Complete each Punnett Square and explain your reasoning for your answers
to the questions. Include in your explanation the type of genetic cross, i.e.,
dominant/recessive, incomplete dominance, codominance, or sex-linked.
1. In guinea pigs, the allele for rough coat (R) is dominant over the allele for smooth coat (r).
Complete the Punnett Square showing the cross between a homozygous recessive parent and a
heterozygous dominant parent.
A. What is the ratio of offspring with rough coat to
smooth coat? Explain.
r
r
R
r
B. What is the probability of getting an offspring
with a smooth coat? Explain.
___________________________________________________________________________
2. The Japanese four-o’clocks are a type of flower that shows incomplete dominance.
Complete the Punnett Square showing the cross between a white flower (W) and a red flower
(R).
A. What is the genotype of the offspring?
W
R
B. What is the probability of getting a red flower
from this cross?
R
C. What is the probability of getting a white flower
from this cross?
D. What is the phenotype of the offspring?
W
3. In humans, the gene for blood type A is dominant over gene for blood type O.
The gene for blood type B is dominant over gene for blood type O.
The gene for blood type A is NOT dominant over gene for blood type B.
The gene for blood type B is NOT dominant over gene for blood type A.
The gene for blood type O is recessive to genes for blood types A and B.
Complete the Punnett Square for the cross between a heterozygous A type blood and a
heterozygous B type blood.
A. What are the possible blood types of the offspring?
B. What is the probability of the offspring having
A
O
type A blood? Explain.
B
C. What is the probability of the offspring having
type AB blood? Explain.
O
D. What is the probability of the offspring having
type B blood? Explain.
E. What is the probability of the offspring having
type O blood? Explain.
___________________________________________________________________________
4. Hemophilia is a sex-linked disorder. Complete the Punnett Square for a woman carrier
for hemophilia who marries a normal man. To determine percentages, remember to consider
only the males or only the females depending on the question asked.
A. What percentage of daughters is normal?
B. What percentage of daughters has hemophilia?
C. What percentage of daughters are carriers?
X
X
D. What percentage of sons has hemophilia?
Xh
E. What percentage of sons is normal?
Y
Genetic Crosses
Level II
Objective: To become proficient in performing genetic crosses and predicting outcomes.
Directions: For each word problem draw and complete a Punnett Square. Explain your
reasoning for your answers to the questions. Include in your explanation the type of
genetic cross, i.e., dominant/recessive, incomplete dominance, codominance, or sexlinked.
1. In tomatoes, red fruit color is dominant to yellow. If a tomato plant heterozygous for
red is crossed with one that is yellow, what is the probability of getting offspring with
yellow tomatoes? What is the probability of getting red tomatoes?
________________________________________________________________________
2. In cattle, red coat is incompletely dominant over white coat. Heterozygous
individuals for this trait have roan-colored coats. Draw a Punnett Square showing the
cross between a red cattle, and a roan cattle. What is the probability of having white
offspring? What is the probability of getting red offspring? What is the probability of
getting roan-colored offspring?
3. A woman who has type O blood marries a man with type AB blood. What is the
probability of them having a baby with type AB blood? What is the probability of them
having a baby with type O blood? What is the probability of them having a baby with type
A blood? What is the probability of them having a baby with type B blood?
___________________________________________________________________________
4. Red-green colorblindness is a sex-linked recessive disorder. A colorblind male marries a
female carrier. What is the probability of the daughters having colorblindness? What is the
probability of the daughters being carriers? What is the probability of the daughters being
normal? What is the probability of the sons having colorblindness? What is the probability of
the sons being normal?
Genetic Crosses
Level III
Objective: To become proficient in performing monohybrid and dihybrid genetic crosses
and predicting outcomes.
Directions: For each word problem draw and complete a Punnett Square. Explain your
reasoning for your answers to the questions. Include in your explanation the type of
genetic cross, i.e., dominant/recessive, incomplete dominance, codominance, or sexlinked.
1. You purchase a package of sunflower seeds. The package states that the sunflowers
will all grow to be at least six feet in height. It further states that the seeds are hybrid for
tall. You plant the seeds and indeed, the plants grow to at least six feet in height. You
decide to harvest some of the seeds produced by the cross pollination to plant next season
instead of purchasing new ones. Much to your disappointment, some of the sunflowers
grow only two to three feet in height. By no means do all of them grow to six feet in
height.
Explain the genetics operating in a situation like this. (Assume that the genetics operating is
similar to that of the pea plant.)
__________________________________________________________________________
2. Let’s assume that you are familiar with a family in which the father has type A blood and
the mother has type O blood. The five children of the family all have type A blood.
(Theoretically, any future children of the family would also have type A blood.)
Explain this situation in genetic terms. Why will all the children have type A blood?
3. In a dihybrid cross, when two traits are considered, the number of possible combinations in
the offspring increases. Suppose that black hair (B) is dominant over blonde hair (b) and
brown eyes (E) are dominant over blue eyes (e).
A. What percentage of offspring could be expected to have blonde hair and blue eyes if both
parents have black hair (heterozygous) and brown eyes (heterozygous)? _______________
Genotype of father- ____________
Genotype of mother- ___________
Complete the Punnett Square below.
What is the phenotype ratio of individuals with brown hair and brown eyes, brown hair and
blue eyes, blonde hair and brown eyes, and blonde hair and blue eyes?
Genetic Crosses
Level I – Answers
1.
A. What is the ratio of offspring with rough coat to
smooth coat? Explain.
r
Rough to smooth is 1:1, because two of four
offspring will have rough coat (Rr) and two
will have smooth coat (rr). Rough coat is
dominant over smooth.
B. What is the probability of getting an offspring
with a smooth coat? Explain.
50% because 2 out of four equals 50%.
2. A.
r
R
Rr
Rr
r
rr
rr
What is the genotype of the offspring?
RW
W
W
R
RW
RW
R
RW
RW
B. What is the probability of getting a red flower
from this cross?
0%
because none are RR
C. What is the probability of getting a white flower
from this cross?
0% because none are WW
D. What is the phenotype of the offspring?
Pink all are RW. This is an example of incomplete dominance.
3. A.
What are the possible blood types of the offspring?
AB, AO, BO, OO
B. What is the probability of the offspring having
type A blood? Explain.
25% because one out of four has AO blood
and A is dominant.
C. What is the probability of the offspring having
type AB blood? Explain.
25% because one out of four has AB blood.
A
B
O
O
AB
BO
AO
OO
D. What is the probability of the offspring having
type B blood? Explain.
25% because one out of four has BO blood and B is dominant.
E. What is the probability of the offspring having
type O blood? Explain.
25% because one out of four has OO blood and O is recessive.
4.
A. What percentage of daughters is normal?
50% because one out of two daughters are XX
B. What percentage of daughters has hemophilia?
0% because none are XhXh
C. What percentage of daughters are carriers?
50% because one out of two areXhX. This is
A carrier because Xh is recessive.
D. What percentage of sons has hemophilia?
50% because one out of two are XhY
E. What percentage of sons is normal?
50% because one out of two are XY
X
h
X
X
Y
XX
XY
XhX
XhY
Genetic Crosses
Level II – Answers
1.
R
r
r
Rr
rr
r
Rr
rr
R = red
r = yellow
50% are red because two out of four are Rr and R is dominant over r.
50% are yellow because two out of four are rr and r is recessive.
2.
RR = red cattle
RW= roan cattle
WW = white cattle
R
R
R
RR
RR
W
RW
RW
0% is white because none of the offspring are WW.
50% are red because two out of four are RR.
50% are roan because two out of four are RW which shows incomplete dominance.
3.
A
B
O
AO
BO
O
AO
BO
0% will have type AB blood because none are AB.
0% will have type O blood because none are OO.
50% will have type A blood because two out of four are AO and A is dominant over O.
50% will have type B blood because two out of four have BO. And B is dominant over
O.
Xc
4.
Y
X
XcX
XY
Xc
XcXc
XcY
50% of the daughters will be colorblind because one out of two is XcXc.
50% of the daughters will be carriers for colorblindness because one out of two is XcX.
0% of the daughters will be normal because none are XX.
50% of the sons will be colorblind because one out of two is XcY and the recessive gene
is not part of the Y chromosome, so it is expressed here.
50% of the sons will be normal because one out of two isXY.
Genetic Crosses
Level III – Answers
1.
T
t
T
t
TT
Tt
Tt
Tt
When two hybrids are crossed the recessive trait can be produced 25% of the time. The
dominant trait will be produced 75% of the time (25% pure TT, 50% hybrid).
2.
A
A
O
AO
AO
O
AO
AO
When the father is homozygous for type A blood (AA) and the mother is homozygous
for type O blood (OO), this represents a cross between a homozygous dominant and a
homozygous recessive. In this case all of the offspring will be heterozygous (AO).
Therefore all the offspring will have type A blood.
3. Both parents have black hair (heterozygous) and brown eyes (heterozygous).
Genotype of father BbEe
Genotype of mother BbEe
Complete the Punnett Square
BE
Be
bE
be
BE
BBEE
BBEe
BbEE
BbEe
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEE
bbEe
be
BbEe
Bbee
bbEe
bbee
One out of sixteen
1/ 16 or 6.25% will have blonde hair and blue eyes.
The phenotype ratio of individuals with brown hair and brown eyes, brown hair and
blue eyes, blonde hair and brown eyes, and blonde hair and blue eyes is:
9:3:3:1
because 9 out of 16 will have brown hair and brown eyes, 3 out of 16 will have brown
hair and blue eyes, 3 out of 16 will have blonde hair and brown eyes, and 1 out of 16
will have blonde hair and blue eyes.
In this review lesson, the students’ final outcome is to recognize certain patterns in genetic
crosses, perform Punnett Squares, analyze the crosses for possible outcomes and explain the
genetics at work. In level one, the student should become comfortable recognizing how to set
up a Punnett Square, the difference between heterozygous and homozygous, dominant and
recessive and calculating probability. In level two the student will be able to determine the
genotypes to be used in the Punnett Square, analyze the data, explain the genetics at work
and predict the possible outcomes. In level three, the students have to be able to recognize
the genetics at work from the outcomes provided and set up a Punnett Square to support the
outcomes, as well as, explain their reasoning. Also, the students must be able to perform a
simple dihybrid cross which involves the same concepts from level two, except two at a time.
Studying genetics can be very challenging and intimidating to some students. Using the
graduated difficulty strategy as their review will allow each student to set his/her own pace
with which he/she will be comfortable and not feel frustrated. It will allow the student to
move up into the more challenging level when he/she feels ready. To make the move the
students have to reflect on what they just did and how they did it. They need that
understanding to go to the next level. This gives the student the sense that he/she is in
control of his/her own learning. It eliminates the feeling of being pushed and dragged when
he/she is confused, lost or uncertain. It allows each student the time and pace he/she needs to
truly understand the concepts and feel confident about it.