OBJECTIVE:
1. Identify the genotypic and phenotypic ratios of offspring in the standard
monohybrid
DIRECTION:
Solve the monohybrid cross following the step-by-step procedure:
Sample Problem Word Problem
1. Read the problem carefully.
Given that tall stem length (T) is dominant
over dwarf stem length (t).
A garden pea plant with heterozygous gene
pair for tall is crossed with a garden pea
plant with homozygous gene pair for tall. Is
there a possibility that a dwarf garden pea
plant is produced from the cross?
Given that round seed (R) is dominant over
wrinkled seed (r).
A garden pea plant with homozygous gene
pair for round seed is crossed with a garden
pea plant with heterozygous gene pair for
round seed. Is there a possibility that a
garden pea plant with wrinkled seed is
produced from the cross?
2. Identify the parents from the problem then write the GENOTYPE of the parents.
Remember: For a particular character, an individual contains a PAIR.
Genotype can be a HOMOZYGOUS (wherein the pair is BOTH small or BOTH big)
or HETEROZYGOUS (the pair contains one big and one small letter)
PARENT 1: Garden pea plant with
heterozygous gene pair for tall
(GENOTYPE: Tt)
PARENT 2: Garden pea plant with
homozygous gene pair for tall
(GENOTYPE: TT)
(Since it is homozygous used BOTH CAPITAL
letter since it was stated that “ homozygous
gene pair for tall” and tall is dominant and
have the symbol T)
PARENT 1:
_____________________________________
(GENOTYPE:___________)
PARENT 2:
_____________________________________
(GENOTYPE:___________)
3. Now, according to the “PRINCIPLE OF SEGREGATION” during gametes
formation, the pair separate. Simply, put the separated pair of the parents to
the TOP of the first and second column and SIDE of the first and second rows of
the Punnett square. Combine the gametes from the top and side for each box
inside the Punnett square.
PARENT 1: T t (top)
PARENT 2: TT (side)
T t
T
T
TT
__________ __________
______
______
_____________ ____________
_____________ ____________
4. Identify the phenotype for each
box.
If the genotype is homozygous follows the
symbol for each trait. But if the pair is
heterozygous, follow the “PRINCIPLE OF
DOMINANCE which states that for a
heterozygous gene pair the dominant
trait will mask the recessive trait.
T t
T
T
TT
Tall
Tt
Tall
TT
Tall
Tt
Tall
5. Identify the genotypic and phenotypic ratio of the possible offspring of the
cross.
GENOTYPIC RATIO: 2:2 or 1:1
Since there are TT-2 and Tt-2
PHENOTYPIC RATIO: 4:0 or 1:0
Since there are 4 Tall in the Punnett square
GENOTYPIC RATIO: _________________
PHENOTYPIC RATIO: ________________
6. Answer the problem.
No. There is no possibility for a dwarf garden
pea plant to be produced from the cross since
no dwarf is in the PHENOTYPIC RATIO; all of
the possible offspring of the cross is tall.
___________________________________________
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Answer:
K
Explanation: