Exploration of Human Species: A computer simulation of the classic introductory high school activity in human genetics.
GenScope Files: Human
To begin, start GenScope with the Human species file.
1. Look at some of the traits. Click on the magnifying glass at the ORGANISM level. Hold your cursor over the spots on the human figures. These spots are "hot spots" - which means that when you place the magnifying glass on top of one of the circles, you will get a picture of what the trait looks like. Examples: an enlarged picture of a cheek, a thumb, and earlobe, a chin or a hairline will appear.
2. Your job is to figure out how each trait is inherited. You can use the chromosome tool if you like to explore. Fill out the table below with the alleles for each trait.
|Trait||Homozygous Dominant (TT)||Heterozygous (Tt)||Homozygous recessive (tt)|
|Polydactyly (extra fingers)|
3. Ear lobes:
4. Cleft Chin:
5. Hitchhiker's Thumb:
6. Polydactyly (Extra digit):
How do you know?
The student, by this time has had lots of practice in going from phenotype to genotype, and determining which allele is dominant or recessive. The hard part of this exercise to derive the rule, that is, to generalize from the particular. Each of the traits will have its own rule which they must derive and write in the appropriate place.
Having dimples, polydactyly, a widow's peak and unattached ear lobes are dominant traits. Having a cleft chin, or a hitchhiker's thumb are recessive traits. Most beginning genetics students think that having a trait means that the allele for that trait is dominant. They have trouble with this exercise because the reality goes against their intuituon.
The question about sex-linkage might upset them: Discussions about sex-linkage have concentrated on figuring out how it works, not how you know the trait is x-linked. The first thing the student will do to answer this question will be to look at the chromosomes with the chromosome tool in order to determine if the gene is, indeed, on the X chromosome. But this is not how you "know" if it is x-linked in the real world. S/he should be encouraged to try crossing two individuals with the trait to check on the distribution of the allele in the population. A discussion of probability might be appropriate here, also.
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