Blackett Family DNA Activity 2

Paternity Testing with STR Data

In this activity, you will assume the role of a Human Geneticist in a DNA Paternity Testing Laboratory. You have just obtained the DNA Profiles for Bob, Anne, David and Katie. You also have information about Bob's parents, Fred and Norma. In your role as a Human Geneticist, it is not essential that you know all of the laboratory techniques used to obtain the Blackett family genotypes. Your work is based on understanding the principles of Mendelian Genetics as applied to STR loci.

Here are your options:

  • Go immediately to the questions below and interpret the data you have already collected.
  • Review the principles of genetics needed for this activity
  • Use the data that we have collected for you or, if you prefer, use the data that you have already collected.
  • Download a worksheet for this activity in PDF format with the data that we have collected for you.

Choose from among the following questions to test your understanding of human genetics.

1. Who are the parents of David and Katie?
Do all of the data you have collected on the genotypes of Bob, Anne, Katie, and David support the conclusion that Bob and Anne are the biological parents of David and Katie? You should justify your answer by reference to the specific genotypes for the STR loci.
2. What is the genetic legacy of Fred and Norma?
The alleles that Bob passes on to his children have in turn been inherited from Bob's parents, Fred and Norma. Identify the alleles among the 13 CODIS STR loci in the genotypes of Katie and David that have been unambigously inherited from each of their paternal grandparents. Now identify any additional alleles that might have been inherited from their paternal grandparents.
3. Genetic Diversity and Sexual Reproduction
Human geneticists are often asked why children have not inherited a particular trait from their parents. As a human geneticist, you know that one mechanism to insure genetic diversity is the independent assortment of alleles of different loci during gamete (egg and sperm) production, i.e. Mendel's Second Law of Genetics. To illustrate this important genetic principle, calculate how many genotypes would be possible among the children of Bob and Anne for the combined DNA profile from the D3S1358, vWA, and FGA. If you feel really ambitious, now calculate the possible genotypes of the children of Bob and Anne for all 13 CODIS STR loci.
4. How many genotypes are possible in a population for a three locus DNA Profile?
If there are two alleles, A and B, at a genetic locus in a population, there are three possible genotypes, namely AA, BB, and AB. If there are three alleles, A or B or C, there are six possible genotypes, namely AA, BB, CC, AB, AC, and BC. For N different alleles, the total possible genotypes is given by the following expression:
If we assume that the allele reference ladders from our data collection exercise represent all possible alleles (a conservative estimate), how many genotypes are possible in a population for the combined STR loci of D3S1358, vWA, and FGA?
5. How many genotypes are possible in a population for the combined CODIS 13 STR loci?
If you feel really ambitious, you may wish to calculate the number of possible genotypes considering all 13 CODIS STR markers. The table below shows the number of alleles for each locus. Beware, the number will be very large.
Locus D3S1358 vWA FGA D8S1179 D21S11 D18S51 D5S818
Alleles 8 11 14 12 22 21 10

Locus D13S317 D7S820 D16S539 THO1 TPOX CSF1PO AMEL
Alleles 8 10 9 7 8 10 X Y

Introduction : Overview | STR P | CODIS | Analysis | Inheritance | Frequency Calc.

Activities : Pedigree | Collect data | Paternity testing | Missing person | RCMP freq. calc.

The Biology Project
University of Arizona
October 27, 2000
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