NOTES: Genotypic and Phenotypic ratios
Geneticists use genotypic and phenotypic ratios to express the proportions of genotypes and phenotypes
generated in the offspring from a given cross.
Monohybrid crosses:
When using Punnett Squares to study the inheritance of a SINGLE TRAIT:
The GENOTYPIC RATIO indicates the ratio of genotypes observed in the four offspring written according to
the convention homozygous dominant : heterozygous : homozygous recessive.
The PHENOTYPIC RATIO indicates the ratio of phenotypes observed in the four offspring written according
to the convention dominant phenotype : recessive phenotype.
Please note: In such ratios the numbers are NOT reduced or simplified; the total of all numbers in the ratio
should add up to 4, since that is the total number of squares/offspring represented in the Punnett Square.
Consider, for example, pea plants, for which green color (G) is dominant to yellow (g).
In a cross between a heterozygous green pea plant (genotype Gg) and a yellow pea
plant (genotype gg) shown to the right:
The Genotypic ratio is 0 GG : 2 Gg : 2 gg or 0:2:2
The Phenotypic ratio is 2 green : 2 yellow or 2:2
G
g
g
Gg gg
g
Gg gg
Dihybrid crosses:
When using Punnett Squares to study the inheritance of TWO TRAITS:
The GENOTYPIC RATIO gets exceedingly complicated, to the point that I find it to be of little use. So don’t
worry about it. 
The PHENOTYPIC RATIO indicates the ratio of phenotypes observed in the sixteen offspring written
according to the convention shown below:
DOM Trait 1, DOM Trait 2 : DOM Trait 1, REC Trait 2 : REC Trait 1, DOM Trait 2 : REC Trait 1, REC Trait 2
Please note: In such ratios the numbers are NOT reduced or simplified; the total of all numbers in the ratio
should add up to 16, since that is the total number of squares/offspring represented in the Punnett Square.
Consider, for example, pea plants, for which green color (G) is dominant to yellow color (g) and tall plant
height (T) is dominant to short plant height (t).
Thus, in a cross between 2 plants that are both heterozygous green, heterozygous tall (genotype GgTt)
shown below:
GT
Gt
gT
gt
The Phenotypic ratio is
9 green, tall : 3 green, short : 3 yellow, tall: 1 yellow, short
or
9:3:3:1
GT GGTT GGTt
GgTT GgTt
Gt GGTt GGtt
GgTt Ggtt
gT GgTT GgTt
ggTT ggTt
gt
ggTt
GgTt Ggtt
ggtt