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Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
Directions: Correctly complete the following trihybrid crosses for bonus points.
How to do a trihybrid cross (a/k/a The Comet Method)
(Note: This same method should be used to complete ALL crosses, but trihybrids need it, especially.)
Step 1 – List the known information given in the problem.
Step 2 – Determine the gametes which will be used in the possible breedings.
Step 3 – Complete the cross itself.
Step 4 – Determine the genotypes of each possible offspring and list as a ratio.
Step 5 – Determine the genotype, if asked.
So, let’s assume the following…
In pea plants, plant height, seed color, and pod thickness are all determined by separate
genes. We can, therefore, assume and assign the following: T = tall plant, t = short plant,
G = green seed, g = yellow seed and I = inflated pod, and i = flat pod.
Determine the possible genotypes is a heterozygous tall, heterozygous green
heterozygous inflated plant is bred with a heterozygous tall, heterozygous green
heterozygous inflated plant.
Step 1 – List the known information given in the problem.
“heterozygous tall, heterozygous green heterozygous inflated plant is bred with a
heterozygous tall, heterozygous green heterozygous inflated plant”
This means breed as follows: T t G g I i X T t G g I i
Step 2 – Determine the gametes which will be used in the possible breedings.
Most easily done with a tree diagram as follows (see the back)
Step 3 – Complete the cross itself.
See the cross, attached, on the next page
Step 4 – Determine the genotypes of each possible offspring and list as a ratio.
See the genotypic ratio, attached, on the next page
Step 5 – Determine the genotypes of each possible offspring and list as a ratio.
This is rarely asked for (at least from me)
Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
Determining the gametes with a tree diagram…
Therefore, the resulting possible phenotypes are:
Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
Cross: Breed - T t G g I i X T t G g I i
X
TGI
TGi
TgI
Tgi
tGI
tGi
tgI
tgi
TGI
TGi
TgI
Tgi
tGI
tGi
tgI
tgi
Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
And now, the bonus(es)
Bonus 1:
The weight of the fruit of a variety of squash is determined by three pairs of genes. The
homozygous dominant condition, AABBCC, results in 6-pound squashes. and the
homozygous recessive condition, aabbcc, results in 3-pound squashes. Each dominant
gene adds ½ pound to a minimum 3 pound weight. When a plant with 6-pound squashes
is bred with a plant that produces 3 pound squashes, all of the resulting offspring
produced are 4 ½ pound hybrids in the F1 generation. What would be the weights of the
F2 fruit if two of these F1 plants were crossed?
Bonus 2:
In pea plants, tall plants (T) are dominant over short plants (t), yellow color (Y) is
dominant over green color (y), and Smooth seeds (S) are dominant over wrinkled
seeds(s). What would be the result of breeding TtYySs X ttyyss?
For EACH Bonus question, you MUST follow the five steps listed in the “Comet
Method” in the spaces provided. Good luck.
Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
BONUS 1
Step 1 – List the known information given in the problem.
Step 2 – Determine the gametes which will be used in the possible breedings. (No need to
show a tree diagram, just list them)
Step 3 – Complete the cross itself.
X
Step 4 – Determine the phenotypes of each possible offspring and list as a ratio.
Step 5 – Determine the genotype, if asked (and I am not asking)
Name______________________________ Date________________________________
Trihybrid Cross Bonus
Living Environment: Comet 2007-2008
BONUS 2
Step 1 – List the known information given in the problem.
Step 2 – Determine the gametes which will be used in the possible breedings. (No need to
show a tree diagram, just list them.)
Step 3 – Complete the cross itself.
X
Step 4 – Determine the phenotypes of each possible offspring and list as a ratio.
Step 5 – Determine the genotype, if asked (and I am not asking)