Mystery Trait - Scales and Plates Activity: Discovering the inheritance patterns of hidden genes through inductive reasoning.
GenScope File: ScaledDragon.gs, PlatedDragon.gs
(These files were created using the
species builder application. The gene does not show up on the chromosomes
for either trait. The dragon, Shaiyda, is homozygous dominant for scales.
It is an autosomal trait carried on the first chromosome. Carl has little
plates, Pp.The trait, plates, is incompletely dominant and x-linked. Big
plates, PP and no plates, pp are the other two possibilities.)
Open the Scaled Dragon file and you
will see a dragon that you may not have ever seen before. It has scales
all over its body! What's going on here??? Here are some questions for
you to work on as you think about this problem.
1. Is this new trait genetic? In other words, did the scaled dragon inherit scales from its parents? Circle one answer and then explain why you think so.
The answer here is yes.
Since ony the female dragon is given,
the student will have to create a male dragon, which will be normal, no
scales. When mated both phenotypes are found among the offspring. If the
trait wasnít inherited, then none of the dragon babies would have
The trait is a dominant condition, like horns.
When Shaiyda is mated with a male without scales, the resulting baby dragons are mixed-about 1/2 and 1/2 with and without scales. Shaiyda must be Ss and the scaleless male ss if having the trait is dominant. If Shaiyda were SS, then all of the kids would be scaled. If the trait were recessive, then mating two dragons with scales would result in all scaled babies. What actually results is a mixture of scaled and scaleless in an approximate ratio of 1 scaleless for every 3 scaled. If the trait were incompletely dominant, then three different traits would appear-possible scaled, unscaled and partially scaled. This does not occur.
How can you tell?
The gene is autosomal on chromosome number 1.
If the gene were x-linked, then all of the male offspring would be scaled and none of the females would have scales. The reason for this is, of course, that the females get their X chromosome from their father, who does not have scales. On the other hand, the males get one X chromosome from each of their parents, and since having scales is dominant, all of the males would have scales.
4. Is this new trait genetic? In other words, did the plated dragon inherit plates from its parents?
How can you tell?
The trait is genetic because some of the offspring inherit the trait.
If the trait were not genetic, then
none of the children would inherit the trait. More to the point, and harder
for a student to recognize, females can never have small plates! Because
small plates is Pp, with an allele on both X chromosomes, only males can
have that trait. Only males have two X chromosomes in dragons. Females
have one X chromosome and one Y chromosome (indicated by a "-" , which
indicates that there arenít any alleles on it.) and so can only have one
allele for plates, thus only a P or a p. Because of this females either
have big plates or no plates.