A105 Mendel Activity

A105 Human Origins and Prehistory
A105 Mendelian Genetics Activity

Imagine an experiment in which true breeding lines of smooth green and wrinkled yellow pea plants were mated.

1. What would the genotypes of each line be for these characteristics, assuming that smooth shape and yellow seed color are both dominant?

Let S be the allele for smooth, s the allele for wrinkled, C the allele for yellow, and c the allele for green. Then:

Smooth green = SScc
Wrinkled yellow = ssCC

2. What will the F1 generation (produced by mating these two plants) look like phenotypically?

Each plant will give the next generation one allele for color and one for shape. Since these plants are homozygous for both traits, each will produce only one type of sex cell -- the smooth green plants will pass on one S allele and one c allele, to be combined with the one s allele and one C allele from the wrinkled yellow plant. Thus, the genotype will be SsCc for the F1 generation, which indicates that the phenotype will be smooth yellow.

3. Imagine the F2 generation (offspring of individuals from the F1 generation). What are the possible phenotypes, and how many can we expect of each?

This is where the Punnett square comes in. One parent's possible sex cells go across the top, and the other parent's possible sex cells go down the left side. Then, simply add each maternal sex cell to each paternal sex cell to get offspring genotypes, and convert to phenotypes.

First, the sex cells. Since all F1 individuals are genetically the same, we only have to do this once. Possible gametes from SsCc are: SC, Sc, sC, sc.

Since we know that S and C are dominant, we can figure out the phenotypes and how many of each should appear by noting the phenotype for each of the 16 offspring above. We obtain:

Smooth yellow: 9
Smooth green: 3
Wrinkled yellow: 3
Wrinkled green: 1

This illustrates Mendel's Law of Independent Assortment: Traits are inherited independently of one another.

This page was last updated 27 January 2002

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