Biology TEACHER
EQ: Do I carry deadly genes?
Monohybrid
Genetics
Problems
Targeted Skills
analysis, probability
Target
1. Review classical Mendelian Genetics
Enduring Understanding
Nucleic acids transfer genetic information from generation to
generation.
Broad Brush Knowledge
Mendelian genetics, probability
Concepts Important to Know and Understand
Heredity
Core Objectives
7. Interpret the role of genetics in determining heredity and as it
applies to biotechnology.
TEKS
(6D) compare genetic variations observed in plants and animals.
PURPOSE: To predict and compare the genetic variations that will result from a genetic cross
TEACHER MANAGEMENT
Estimated Time:
45 minutes
Materials:
student handout, transparency
PRE-LAB
Students should understand the terms including dominant, recessive, homozygous, heterozygous,
monohybrid cross, phenotype, and genotype.
Students should also understand the concept of probability.
KEY
PUNNETT SQUARES
Label the Punnett square with the following terms: gametes A, gametes B, offspring
Gametes A
Gametes B
Offspring
SETTING UP CROSSES:

What is used to represent the genes of an organism? Alphabet letters

How is the letter used determined? Usually the first letter of the dominant trait

What kind of letter is used for dominant traits? Upper case

What kind of letter is used for recessive traits? same letter lower case
Monohybrid Genetics Problems- Biology TEACHER (Revised June 26 2008)
(printed 2/12/2016) p. 1
PRACTICE:
1. In a certain plant, the gene for red flowers (R) is dominant to the gene for white flowers (r).
a) What is the genotype of a heterozygous plant? Rr
b) What is the genotype of a homozygous red flowered plant? RR
c) What is the genotype of a white flowered plant? rr
d) What is the phenotype of a plant with two dominant genes? Red flowers
e) What is the phenotype of a plant with 1 dominant and 1 recessive gene? Red flowers
2. Homozygous Red x Homozygous White
RR x rr
a) Genotypic ratio: 0:4:0
b) Phenotypic ratio: 4:0
R
R
r
Rr
Rr
r
Rr
Rr
c) What is the probability of having heterozygous offspring? 4/4 = 1 = 1.0 = 100%
d) What is the probability of having offspring expressing the dominant trait? 4/4 = 1 = 1.0 = 100%
3. Heterozygous x Hybrid
Rr x Rr
a) Genotypic ratio: 1:2:1
b) Phenotypic ratio: 3:1
R
r
R
RR
Rr
r
Rr
rr
c) What is the probability of having heterozygous offspring? 2/4 = ½ = .5 = 50%
d) What is the probability of having dominant looking offspring? ¾ = .75 = 75%
e) What is the probability of having hybrid offspring? 2/4 = ½ = .5 = 50%
4. Dragons have either smooth or wrinkled noses. Wrinkled is dominant over smooth. Cross a smoothnosed female dragon with a heterozygous wrinkle-nosed male.
w
w
ww x W w
a) Genotypic ratio: 0:2:2
W
Ww
Ww
b) Phenotypic ratio: 2:2
w
ww
ww
c) What is the probability of having a smooth-nosed
offspring? 2/4 = ½ = .50 = 50%
d) What is the probability of having two smooth-nosed offspring in a row? ½ x ½ = ¼ = .25 = 25%
Monohybrid Genetics Problems- Biology TEACHER (Revised June 26 2008)
(printed 2/12/2016) p. 2
5. Mendel found that the allele for tallness in plants (T) is dominant to its allele for shortness (t).
T
t
Cross a heterozygous plant that self fertilizes
a) Genotypic ratio: 1:2:1
T
TT
Tt
b) Phenotypic ratio: 3:1
c) Probability of having a homozygous tall plant: ¼ = .25 = 25%
t
Tt
tt
d) there is genetic variation when a plant self fertilizes
Homozygous tall x heterozygous
a) Genotypic ratio: 2:2:0
b) Phenotypic ratio: 4:0
T
T
t
T
TT
TT
Tt
Tt
c) Probability of having a heterozygous plant: 2/4 = ½ = .50 = 50%
T
t
Heterozygous and homozygous short
a) Genotypic ratio: 0:2:2
t
Tt
tt
b) Phenotypic ratio: 2:2
t
Tt
tt
c)Probability of having a homozygous short plant: 2/4 = ½ = .50 = 50%
T
Homozygous tall and homozygous short
T
a) Genotypic ratio: 0:4:0
t
Tt
Tt
b) Phenotypic ratio: 4:0
t
Tt
Tt
c) Probability of having two homozygous short plants in a row: 0%
6. Two fruit flies with normal wings are crossed. Among their offspring are 89 with normal wings and 32
with dumpy wings.
a. What were the genotypes of the parents? Ww (hybrid or heterozygous)
b. Which trait is dominant? normal Which trait is recessive? dumpy Explain how you would
know. 89 is close to 90, and 32 is close to 30, giving a 90:30 or 3:1 ratio.
Therefore, we know that both parents are heterozygous
7. Sniffles, a male mouse with a colored coat, was mated with Esmeralda, an albino mouse. The resulting
litter of six young all had colored fur. The next time around, Esmeralda was mated to Whiskers, a male
the same color as Sniffles. Some of Esmeralda’s next
litter was white. What are the probable genotypes of
a. Sniffles? Homozygous Dominant (CC)
color x albino
color x albino
all color
some white
b. Whiskers? Heterozygous (Cc)
c. Esmeralda? Homozygous Recessive (cc)
Monohybrid Genetics Problems- Biology TEACHER (Revised June 26 2008)
(printed 2/12/2016) p. 3
8. A number of plant species have a recessive allele for albinism; making homozygous albino
individuals unable to synthesize chlorophyll. If a heterozygous tobacco plant is allowed to self
pollinate and 500 of its seeds germinate…
a. How many of these offspring will be expected to have the same genotype as the parent plant?
Since the probability of heterozygous is 2/4 = ½ = .50 = 50%, 250 seeds should have
the same genotype as the parent plant.
b. How many seedlings will be expected to be white? Since the probability of homozygous
recessives is ¼ = .25 = 25%, 125 seeds should be white (albino).
A dominant phenotype could have two possible genotypes: Homozygous dominant (DD) or Heterozygous
(Dd). To determine the genotype, a test cross should be performed where the unknown dominant
phenotype is crossed with a known recessive phenotype.
Pretend you own a plaid tribble. Plaid (P) is dominant to striped (p)in tribbles.
Test cross #1: Homozygous plaid x striped
Genotypic ratio: 0:4:0
Phenotypic ratio: 4:0
Probability of heterozygous offspring: 4/4 = 1 = 100%
P
P
p
Pp
Pp
p
Pp
Pp
Probability of striped offspring: 0/4 = 0 = 0%
Test cross #2: Hybrid plaid x striped
Genotypic ratio: 0:2:2
P
p
Phenotypic ratio: 2:2
p
Pp
pp
Probability of heterozygous offspring: 2/4 = ½ = .50 = 50%
p
Pp
pp
Probability of striped offspring: 2/4 = ½ = .50 = 50%
Conclusion:
If the recessive trait does show in the offspring of a test cross, the unknown parent’s genotype is
hybrid/heterozygous.
Monohybrid Genetics Problems- Biology TEACHER (Revised June 26 2008)
(printed 2/12/2016) p. 4