Labrador Colors: Writing Rules for a polygenic trait
GenScope File: LabradorColors.gs
Start GenScope and open the Labrador species file. (Note: You may start GenScope and then use the File: New Species menu selection to open the Labrador species file, OR simply drag and drop the Labrador file onto the GenScope application program).
Produce a new organism. In this case,
it will be a Labrador retriever dog. Alter the alleles, using the chromosome
tool, and watch the changes in the dog. Then fill out the second column
of the chart below.
|Ee BB||Black =|
|EE bb||Chocolate =|
|ee BB||Yellow =|
The student will fill out the chart by writing the color of the dog next to the various combinations of alleles. S/he will use the chromosome tool to change the combinations on the chromosomes and observe what happens to the color of the dog in the organism window.
EEBB, EeBB, EEBb and EeBb all give the black color.
EEbb and Eebb give chocolate.
eeBB, eeBb and eebb give the yellow color.
This first exercise involves nothing more than manipulating the alleles of the dog and observing the changes that occur as a result. Of course, the student is working at two levels when s/he does this, the chromosomes level and the organism level. If s/he is at all alert s/he will be thinking about why the color changes when the allele combinations are changed.
Since the color in the dogs is controlled by the combination of two genes, a comparison can be made with color inheritance in dragons, which is also polygenic. (However, there is no lethal in dogs, nor is color x-linked.)
By examining your data very carefully, try to derive general rules for color inheritance in Labradors and write them in the "General Rule" column above.
To derive a general rule, use this instance from dragons as an example: Horns H _, where H_ can stand for HH or Hh. Similarly, dragon color can be written as:
A_ BB is topaz, where A_BB is AABB or AaBB.
A_Bb is bronze, where A_Bb is AbBb or AABb.
aaBB is amethyst.
aaBb is azure.
Now, using the data you collected from the Labrador Retrievers, derive the rules for color in the same way that you did for dragon color.
The answers that the student should give are:
E_B_ is black.
E_bb is brown
eeB_ and eebb is yellow.
Students seem to have a lot of trouble deriving rules from data, even data they have collected themselves. Here the task is fairly simple, once the data is put into tabular form. They can see by inspection that the common form for black is to have at least one E and one B. Two recessive b's give brown, as long as the dog has at least one E. And two ee's always give yellow, no matter what alleles the dog has for the other gene.
It might be useful to put together a bunch of problems with data like this and have them practice finding the rules of inheritance for various species. For example, you could make up a species like "Alien" which has ears (Long E_ and short ee), antennae (AA round, Aa pointed, aa droopy), eyes (Purple P_ and yellow pp), etc. Give them the genotypes of 3 or 4 different members of the alien spaceship and have the kids make a chart like the Labrador's chart and derive the rules for that species.
The important thing is to help them
learn to look for patterns in data and make sense of these patterns -that
is , make generalizations or rules based on the data. This is very difficult
to do. Many adults have trouble with this type of mental acrobatics. Stress
to the students that this is not a Mickey Mouse activity. This is the way
real scientists make sense of the data they collect in an experiment.
[ Table of Contents ] [ Previous Activity: Bronze and Gold ] [ Next Activity: Sickle Cell ]