Question

13.(2) A real life case in humans that stumped doctors involved a woman who needed a kidney transplant. In order to determine if one of her family members could donate a kidney to her, she and her family members (husband and sons) underwent genetic testing using DNA isolated from their blood samples. To her surprise, the results indicated that she could not be the mother of her sons. The woman knew that they were her sons, and hospital records indicated that there had been no mix ups when they were babies. Her husband's DNA matched that of the sons. All the doctors were confused. A student who had taken this class, and who was also volunteering at the hospital at the time, suggested taking a cheek swab from the woman and repeating the DNA test. This time, it showed that she was indeed mother of her sons. Assuming that there were no errors in testing, what was the explanation for the discrepancy? How could this have happened? N M

Answer 1

3. 2.

The woman was a tetragametic chimera, wherein she had DNA from two different individuals. The blood sample has a mixture of two DNA. One set was similar to her children’s DNA while the other was not. The test with the blood sample detected the DNA that was not the same as her son’s DNA. When the cheek sample was tested, the cells tested had the DNA that was same as her sons, leading to a positive result.

A tetragametic chimera is any organism that has genetically different sets of tissues, that originated from two types of fertilized eggs. The women may have been a product of fusion of two nonidentical fertilized eggs. Hence, she has two types of maternal DNA and type types of paternal DNA from two fertilized eggs. The blood sample different from the cheek sample in the origin of DNA present. The blood sample has cells from one fertilized egg and the cheek cells had DNA from other fertilized egg. The cheek sample has cells of epithelial origin (epidermal origin) while mesoderm form non-epithelial blood cells. Her ovarian gonads producing eggs had the DNA that matched the DNA from cheek cells. Hence, her son’s inherited the DNA that was in cheek cells.