| Murder case | Thomas Jefferson / Sally Hemings case |
Murder case
A woman has been brutally stabbed to death outside of her home. Two suspects have been arrested – 1) her ex-husband, whom the deceased woman claimed had been stalking her in the two months prior to her death, and 2) an acquaintance of her ex-husband who had been living in the ex-husband’s house for about six months and who could not provide an alibi for the time of the murder. Blood samples are taken from the crime seen – one spot found near the victim’s body and one taken from a glove found near the crime scene.
DNA is isolated from these blood spots, as well as from blood samples taken from the victim and the two suspects. Each DNA sample is subjected to PCR analysis, amplifying a polymorphic region of chromosome 1. (NOTE: Use the PCR function of the Data Screen, rather than 96-well PCR.) Digesting this amplified DNA with HindIII will yield distinctive banding patterns that should help identify the source of the blood spots from the crime scene.
[Note: There are three versions of this scenario, Case A, B, and C, each with a different outcome.]
- blood spot 1 (from sidewalk)
- blood spot 2 (from glove)
- victim’s blood
- suspect 1 (ex-husband)
- suspect 2 (acquaintance)
- What conclusions can you draw from these results?
- Do you think these data are sufficient to convict someone?
- What additional issues are raised by this type of testing?
Thomas Jefferson / Sally Hemings case
Background: There has long been controversy regarding whether Thomas Jefferson fathered any children with Sally Hemings, one of his slaves. Jefferson was accused of fathering two of Hemings’ sons: Thomas Woodson, who was born in 1802 shortly after Jefferson and Hemings returned from an extended stay in France, and Eston Hemings Jefferson (born 1808), who bore a striking resemblance to Jefferson and took his name as an adult. No known documentation either directly supports or refutes these claims. Recently, researchers in the United Kingdom have attempted to address these questions scientifically by analyzing DNA from the Y chromosome of male descendants of Jefferson’s uncle, Jefferson’s sister, and Hemings. Thomas Jefferson himself had no undisputed, surviving sons.
Most of the Y chromosome passes unchanged from father to son, except for occasional mutations. Several Y chromosome genetic markers, some of which are genes while others are non-coding, can be used for this analysis since they can be inherited in one of two allelic forms (called bi-allelic). The alleles are detected by dot blot analysis using probes that will bind to one allele or the other. It is possible to determine whether two individuals are closely related by comparing how frequently their alleles match. Another type of marker that can be used in this analysis is microsatellite short tandem repeats (microsatellite STRs). These are regions where short (2-3 base pairs) sequences are repeated. The number of repeats is inherited like an allele, and can be determined by the size of the band detected on a gel after PCR amplification of that region.
The Case: DNA was isolated from the following individuals (numbers correspond to reference numbers used in the study):
- H21 – Eston Hemings Jefferson’s great-great grandson
- W55, W70 – Thomas Woodson’s great-great grandson
- J41, J47, J49 – Descendants of Field Jefferson, Thomas Jefferson’s uncle
- C27, C31 – Descendants of John Carr, Thomas Jefferson’s nephew
Procedure: To determine which allele each individual has at each of the Y chromosome bi-allelic markers, use the appropriate primers to amplify the DNA by PCR (use the PCR function on the Data Screen rather than 96-well PCR) and then perform a dot blot using the probes for that marker. Load the probes into the spots and the amplified DNA into the corresponding wells. To detect microsatellite STRs, use the appropriate primers to amplify the DNA and then run the PCR products on a gel to determine the relative sizes of the fragments.
| Bi-allelic markers: | Microsatellie STRs: |
| YAP | 19 |
| sY81 | 389A |
| 92R7 | 389D |
| SRY | 392 |
- What can you conclude from the dot blot and gel results? Which individuals appear to be the most closely related?
- Are these results consistent with Thomas Jefferson having fathered either Easton Hemings Jefferson or Thomas Woodson?
- Are there other ways to interpret these results?