Imagine you are a farmer and you perform a monohybrid cross between two plant species. One plant is homozygous recessive for yellow leaves and the other is heterozygous with green leaves. After you complete the cross you produce the following outcome in the offspring:

122 Yellow leaves

78 Green Leaves

In order to perform a Chi-Squared analysis, you first need to determine the expected outcome. What outcome do you expect to get based on this mating?

Question

Imagine you are a farmer and you perform a monohybrid cross between two plant species. One plant is homozygous recessive for yellow leaves and the other is heterozygous with green leaves. After you complete the cross you produce the following outcome in the offspring:

122     Yellow leaves

78       Green Leaves

In order to perform a Chi-Squared analysis, you first need to determine the expected outcome. What outcome do you expect to get based on this mating?

BlackTom AI · Accepted Answer

The question describes a monohybrid cross between two plants with different leaf color genotypes. Here’s how to reason through it.
First, identify the parental genotypes and what the colors indicate: one parent is homozygous recessive for yellow leaves, which we can denote as yy. The other parent is heterozygous with green leaves, which we denote as Yy. The key is to know which allele is dominant for color.
Next, construct a simple Punnett square for the cross yy × Yy. The possible gametes from the yy parent are y, and from the Yy parent are Y or y. Filling in the square gives: offspring genotypes Yy and yy in a 1:1 ratio, specifically 50% Yy (green phenotype if Y is dominant) and 50% yy (yellow phenotype if y is recessive).
From this, deduce the expected phenotypic ratio: if green is dominant over yellow, you expect 50% green and 50% yellow among the offspring.
Now translate the ratio into actual counts given in the problem. With a total of 200 offspring (122 yellow + 78 green observed, but the question asks for the expected outcome), the expected numbers are 200 × 0.5 = 100 for green and 200 × 0.5 = 100 for yellow. So the expected outcome is 100 Green leaves and 100 Yellow leaves.
Note a common point of confusion: if someone misreads which color is dominant, they might flip which phenotype corresponds to the 50% genotype, but the core calculation is the 1:1 expectation from a yy × Yy cross when green is the dominant phenotype.
Since the problem’s data show 122 Yellow and 78 Green, you would later compare observed vs. expected using a Chi-Squared test to assess goodness-of-fit to the 1:1 expectation, but the immediate expected outcome is 100 Green and 100 Yellow leaves.

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