Pea Plant genetics
Monohybrid crosses
Gregor Mendel discovered genetics through the use of pea plants. One of the more obvious characteristics of pea plants
is their flowers. Mendel didn’t know this at first, but purple flowers are the dominant expression(allele) and white
flowers are from a recessive allele.
P = purple allele
p = white allele
Parental generation – To create the (P)arental generation, Mendel first spent a great deal of time and effort creating
two pure breeding strains of pea plants. One only produced purple flowers and the other only produced white flowers
with the respective genotypes of PP and pp. Make a Punnett square for the cross of the parental strains. Also indicate
the genotypic and phenotypic ratios.
F1 generation – Every offspring produced by crossing Mendel’s parental generation resulted in plants that had purple
flowers. White flowers seemed to have disappeared!!! The genotype for all of these plants was Pp. The white flowers
were simply masked by the dominant purple allele. When the F1 generation plants are crossed the white flowers come
back. Make a Punnett square for the cross of two F1 plants. Include genotypic and phenotypic ratios as well as
phenotypic frequencies.
F2 generation- The F2 generation had every possible combination of alleles possible for flower color: PP, Pp, and pp.
If we were to allow the F2 generation to breed at will with each other, what would be the phenotypic frequencies of their
offspring. To get the numbers right, you need to take into account every possible mating situation for each of the F2
genotypes. Some F2 genotypes are more frequent than others. This disparity must be taken into account when
calculating overall frequencies. The Punnett square forms on the back of this sheet can help with sorting this out.
Dihybrid crosses
In his experiments, Mendel discovered a number of traits in pea plants that followed the dominant/recessive pattern that
he had found in the flowers. The pods of his pea plants displayed the independent assortment of two of these types of
traits: pod shape S-smooth s-constricted and pod color C-yellow c-green. Independent assortment is when one gene
does not affect the expression of another gene, even though they may be expressed on the same specific part of the
plant/organism(ie: pods). These two traits could be expressed in separate monohybrid Punnett squares of their own,
however, since they independently assort, and they both affect the pod, a more comprehensive view of pea plant pods is
demonstrated by using a dihybrid Punnett square.
Parental generation – In the same vein as the flowers, we would start with two Parent groups of plants that were
homozygous dominant for both genes SSCC and another parent group that was homozygous recessive sscc. Make a
dihybrid Punnett square for the mating of these two parental strains. Include genotypic and phenotypic ratios.
F1 generation – Make a dihybrid square for the crossing of two F1 offspring. Once again, indicate the genotypic ratios,
phenotypic ratios, and phenotypic frequencies.
F2 generation – There are many different F2 crosses that can be made. Choose two F2 offspring and make a dihybrid
square showing the resulting offspring from this mating.
Comprehensive Pea Plant genetics
As we have learned, the pea plant has many traits that are all expressed at once. For this problem we will be looking at
the assortment of these traits into all the frequencies of different offspring that result from a cross between two specific
parent individuals. The traits: P-purple flowers /p-white flowers R-round peas /r-wrinkled peas
Y-yellow peas
/y-green peas S-smooth pod/s-constricted pod C-yellow pod/c-green pod T-tall plant/t-short plant V-vertical
flowers/v-lateral flowers
PPRrYyssCcTtvv
X
PprrYySSCcTtVv
It is impossible to use a single Punnett square to address this problem . Instead, monohybrid squares or acute
observation coupled with the rule of counting will allow you to determine all the possibilities and their frequencies.
Show all your work, including monohybrid squares, on a separate sheet of paper. In the end, you do not need to list every
possible phenotypic frequency…just the matrix that demonstrates how to calculate each frequency.
Parental
F1
Genotypic ratio(s) –
Phenotypic ratio(s) –
F2(x2)
F2(x2)
Genotypic ratio(s) –
Phenotypic ratio(s) –
Phenotypic frequencies:
F2(x1)
F2(x1)
Genotypic ratio(s) -
Genotypic ratio(s) –
Genotypic ratio(s) -
Genotypic ratio(s) -
Phenotypic ratio(s) –
Phenotypic ratio(s) -
Phenotypic ratio(s) -
Phenotypic frequencies:
Phenotypic frequencies:
Phenotypic frequencies:
Phenotypic ratio(s) Phenotypic frequencies:
Combined phenotypic frequency:
P
F1
Genotypic ratio(s) –
Genotypic ratio(s)-
Phenotypic ratio(s) –
Phenotypic ratio(s)Phenotypic freq. –
F2(choice)
Comprehensive Workspace
Genotypic ratio(s) –
Phenotypic ratio(s) –
Phenotypic freq. –