Many people are interested in genealogy and their family history. In order to try to figure out the rules of inheritance, dominant or recessive, it is instructive to simulate a pedigree. The following exercise, though seemingly simple, accurately simulates how we inherit single-gene traits. You may find that trying to figure out whether something is dominant or recessive is not that simple. Remember this only applies to traits that are clearly dominant or recessive. Many traits of interest are not so clear cut.
We first start out with a genotype chart, a simple version of which is shown in figure B.3.1.A. In this simulation, as in real life, each parent contributes one of the two copies of the gene to each offspring. The choice of which version is transmitted is completely random. Thus to produce offspring in this simulation, flip a coin. Decide which copy of the gene corresponds to heads and which to tails, for example, heads = A and tails = a. Then determine the genotype of a select offspring. In this example, toss a coin once for the father. This determines which copy of the gene the father contributes to his offspring. You need not toss a coin
Figure B.3.1 Examples of Pedigree Charts. A. Genotype chart with just the genotype of parents shown. B. Genotype chart with the parents and potential offspring shown. C. Phenotype chart for the genotype chart shown in B for a dominant trait. D. Phenotype chart for the genotype chart shown in B for a recessive trait.
for any homozygous parent since both copies of his or her gene is the same. In the example given here, you need not toss the coin for the mother since she can only contribute an a.
Continue for each blank individual on the genotype pedigree chart. Faithfully note the result of each coin toss. Even if the expected ratio of offspring genotypes is 1 Aa : 1 aa, as in this example, if each time you toss the coin you came up with Aa, do not fudge the data, but write "Aa" for each offspring. By tossing a coin we are simulating nature; each offspring is a result of a random choice from the two copies of the gene in each parent.
After completing the genotype pedigree chart, fill in the phenotype pedigree charts appropriate for the trait that you are assigned. For example, let's say that we got the genotypes shown in figure B.3.1.B for the above chart.
If the trait were dominant, your phenotype chart would look like figure B.3.1.C
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