The following four ears of corn were taken from separate plants. Each kernel represents an offspring. The kernels on the four ears were counted for the color trait (purple or white) and/or sweetness trait (starchy or sweet). The data gave these phenotypic ratios for the four ears:

Ear #1 – 3 purple : 1 white
Ear #2 – 3 starchy : 1 sweet
Ear #3 – 9 purple, starchy : 3 purple, sweet : 3 white, starchy : 1 white, sweet
Ear #4 – 1 purple : 1 white

Which traits are recessive?

The phenotypic ratio provides information about how many genes are involved in the cross, which is the dominant trait and which is the recessive trait. In the exposed example, white and sweet are the recessive traits.

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By looking at the phenotypic ratio of the progeny, you can tell which trait is the dominant one and which is recessive.

Let us assume two diallelic genes, each coding for one trait.

Let us also say that these genes assort independently -they are not linked-, and they do not interact with each other.

When we are only considering one single gene,

the phenotypic ratio after crossing two heter0zyg0us individuals is 3:1. In these situations, we expect to see 3/4 of individuals expressing the dominant trait, and 1/4 expressing the recessive trait.

the phenotypic ratio after crossing a heter0zyg0us individual with an h0m0zyg0us recessive one is 1:1. We expect to see 2/4 of the progeny expressing the dominant trait and 2/4 the recessive trait.

When we are only considering two genes simultaneously, we expect to find the following phenotypic ratio 9:3:3:1

9/16 individuals expressing both dominant traits

3/16 individuals expressing one dominant trait and one recessive trait

1/16 individuals expressing both recessive traits.

Now let us analyze each Ear,

Ear #1 – 3 purple : 1 white

Only the color trait is considered.
The phenotypic ratio is 3:1
Two heter0zyg0us individuals must be involved in the cross

3/4 are purple and 1/4 is white
Dominant trait: purple
Recessive trait: white

Ear #2 – 3 starchy : 1 sweet

Only the sweetness trait is considered.
The phenotypic ratio is 3:1
Two heter0zyg0us individuals must be involved in the cross
3/4 are starchy and 1/4 is sweet
Dominant trait: starchy
Recessive trait: sweet

Ear #3 – 9 purple, starchy : 3 purple, sweet : 3 white, starchy : 1 white, sweet

Both traits are considered.
The phenotypic ratio is 9:3:3:1
Two dihybrid individuals must be involved in the cross
9/16 are purple and starchy, 3/16 are purple and sweet, 3/16 are white and starchy, and 1/16 is white and sweet
Dominant trait: purple and starchy
Recessive trait: white and sweet

Ear #4 – 1 purple : 1 white

Only the color trait is considered.
The phenotypic ratio is 1:1
One heter0zyg0us individual and one h0m0zygous recessive individual must be involved in the cross
2/4 are purple and 2/4 are white
Dominant trait: Purple
Recessive trait: White

The recessive traits are white color and sweet taste.

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