Section 1:
Grandma Erna was trying to grow red and white flowers. When she accidentally put pollen from
plants with red flowers onto plants with white flowers, all of their offspring plants were white. When
the white plants crossed with each other, however, some of them were red.
Our job was to figure out how this could have happened. We needed to find out if the gene
causing white flowers is dominant or recessive and if the first plants with white flowers was homozygous
or heterozygous for the white gene.
Section 2:
We wanted to look at our data strategically and logically, so we used a pedigree and Punnett
Squares. A pedigree is a graphic representation that allows us to see which traits are present in every
generation. By using our data to make a pedigree, instead of just the data table, it was easier to figure
out what was going on.
A Punnett Square is used to tell what gene combinations are possible if two organisms cross
with each other. We never knew for sure what genes the plants had, so we made different Punnett
Squares to see which crosses would match the actual outcomes of the flowers.
Section 3:
After examining our pedigree and Punnett Squares we are able to determine that the white
gene is dominant and the first plants with white flowers were homozygous for the white gene.
As you can see in our pedigree, Figure 1, all of the plants in the second generation had white
flowers. This means that, even if a plant got the red gene, the flowers weren’t red. The red gene must
have been “overtaken” by the white gene, so the plants turned out white. The gene that shows up even
if another gene is present is called the dominant gene. The white was always showing up, so its gene is
dominant.
If the white gene is dominant, the red gene is recessive. This means that the only way a flower
can be red is if it has two genes for red flowers (and no white genes). When we were making Punnett
Squares, we always gave the plants with red flowers ww because we knew they were homozygous
recessive.
A plant could be white if it had Ww genes or ww genes. In order to figure out if the first plants
with white flowers were homozygous or heterozygous, we made two different Punnett Squares with
both combinations. These Punnett Squares are in Figure 2. As you can see, Ww crossed with ww would
let half of their offspring be red. All of their offspring were white, so we know WW must have crossed
with ww. This would have made all of their offspring white, with Ww genes.
Just to double check, we crossed Ww with Ww. Just like in the third generation of Grandma
Erna’s plants, about ¼ or 25% of the plants would have red flowers. We can confidently say that our
evidence supports the claim that the gene causing white flowers is dominant and the first plants with
white flowers were homozygous.