A Horns Activity: Group Discussion: From the head to the mouth
GenScope Materials: Horns Experiment Data
In the Horns Experiment, you mated two dragons and gave them 40 children. You wanted to see what the children would look like. Both the parents had horns, but no wings. The mother had four legs, the father had none. The question was: will the kids look like their parents?
It is important for the students to reflect
upon, and articulate, what they have learned from the data gathering and
prediction activities they have done with the Horns Experiment file. The
questions on this handout are intended to remind them of their findings,
and to point out the differences between them. Your job will be to lead
them in a discussion of the genetic differences that explain the differences
in the phenotypic distributions for the various traits.
When you looked at horns, wings, and
legs, each of you produced a slightly different pedigree, but everyone
saw something like this:
Most of the children had horns.
Only a few of the children had wings.
All of the children had two legs.
So, to sum up:
1. In the case of horns and wings, most of the children looked like their parents, but a few (about a quarter of them) didn't.
2. In the case of legs, none of the children looked like their parents.
What's going on here???
Horns are dominant over no horns, wings are recessive to no wings, four legs are incompletely dominant to zero and two legs. Both parents were heterozygous for horns and wings, resulting in an approximately 3:1 split in favor of the parental phenotype for these traits. One parent was homozygous for four legs and the other parent was homozygous for zero legs, so all the offspring were heterozygous for legs and displayed the two-legged phenotype.
The discussion can be started by going over the questions and having the kids answer based on their own data. It might be useful to point out that wings, unlike horns, are recessive. The students usually don't have any trouble sorting out the legs inheritance. They seem to accept the idea of three traits for two alleles.
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