Example: A red flowered plant is fertilized
with pollen from a plant of unknown
phenotype. 62 seeds are collected, planted
the next year and grow into the F1
offspring. 16 of these offspring have white
flowers and the rest have red flowers. What
was the phenotype and genotype of the
unknown parental plant?
?
?
?
?
?
 Flower color is controlled by a single gene.
 The dominant form of this gene is often
written R
 The recessive form of this gene, written r.
 Plants with the genotype RR or Rr have red
flowers.
 Plants with the genotype rr have white
flowers.

Phenotypes and Genotypes of parents (P
generation)
?
?

?
?
?
Genotypes and Phenotypes of Offspring
(F1 generation)

Phenotypes and Genotypes of parents (P generation)
Parental Phenotypes ... one red flowered, one unknown
 Parental Genotypes ... unknown, but the red flowered
parent must have contained at least one R gene, with a
genotype of RR or Rr.



Genotypes and Phenotypes of Offspring
(F1 generation)
The data given was for 62 F1 offspring.
 Phenotypes: 16 white flowers, 46 red flowers.
 Genotypes: all 16 white flowered plants = rr. All red
flowered plants = R?

 Heterozygous x Heterozygous
Aa x Aa, which gives a ratio of 3:1 among the
offspring.
 Heterozygous x Homozygous recessive
Aa x aa, which gives a ratio of 1:1 among the
offspring.
 Heterozygous x Heterozygous (two factors)
AaBb x AaBb, which gives a ratio of 9:3:3:1
among the offspring
What type of ratio is provided in the example we are
currently solving
?
?
?
?
?
?
?
? When compared to the known ratios above, the
cross is most likely _____________ ______________ or
? x ?.
In the example we have worked on:
 There are 62 plants in the F1 generation and 16
have white flowers.
 Therefore 46 plants have to have red flowers
(62-16).
 This gives us a ratio of 46 red flowers to 16 white
flowers.
 If we divide both sides by 16, this can be reduced
to 2.875 red flowers to 1 white flower or
approximately 3:1.
 When compared to the known ratios above, the
cross is most likely Heterozygous x Heterozygous
or Rr x Rr.
 Parents: R? x ??
(the genotype of the second parent, the male,
is the whole point of the problem)
 NOTE: In the Punnett Square a genotype of
rr has been entered for one quarter of the
offspring; why?
 The data said that 16 out of 62 F1 offspring had
white flowers.
 White flowered plants can only have the
genotype rr.
 The raw numbers were reduced to the ratio of 3:1
in step 3.
 So one quarter of the F1 offspring (one box in the
Punnett Square) must hold the genotype rr.
 Each of the other three offspring must have at
least one R gene in order to be red.
 This was also given in the data and deduced in
step 3.
 Now fill in the blanks.
 If it is not obvious from the Punnett Square,
work backwards. Since some of the offspring
have white flowers (genotype rr), the gametes
that formed them must have been r. So the
only arrangement that would give this result
is...
 Now most of the gametes are filled in along
the sides of the Punnett Square, some of the
missing genotypes can also be filled in...

This only leaves one gamete and one F1
genotype unresolved. There are only two
choices for the missing gamete; R or r.
Putting a recessive, r gamete in place of the
question mark does not give the right
answer…
 so the missing gamete must be R. So the
complete Punnett Square is...
 ANSWER: The genotype of the unknown male
plant must have been Rr (a fact deduced from the
two gametes this plant must have provided for
the Punnett Square to work). The phenotype of
this plant must have been - red flowers.
 Do Questions 1-4
In fruit flies, a single gene controls wing phenotype. The diagram below
shows the phenotypes for long wings and vestigial wings in fruit flies.
1.
Two fruit flies that have long wings are crossed. Of the 95 offspring produced, 73
have long wings. The other 22 have vestigial wings.
Which of the following conclusions about the inheritance of long wings and
vestigial wings is best supported by the results of this experiment?
A. The alleles for long wings and vestigial wings are sex-linked.
B. The alleles for long wings and vestigial wings are codominant.
C. The allele for long wings is dominant and the allele for vestigial wings is
recessive.
D. The allele for long wings is recessive and the allele for vestigial wings is
dominant.
2. In sheep, the allele for white wool (W) is dominant,
and the allele for black wool (w) is recessive. A
farmer has mated two Suffolk sheep for a few years.
These matings have resulted in six offspring, four
with white wool and two with black wool. One
parent has white wool and the other has black wool.
Which of the following could be the genotypes of the
parent sheep?
A. WW and Ww
B. WW and ww
C. Ww and Ww
D. Ww and ww