by Marit Slavin © Ha'aretz Monday, October 13, 1997 This material is distributed without profit to those who have expressed a prior interest in receiving the included information for research and educational purposes. |
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Is Margalit Umassi - the Yemenite woman whose daughter disappeared back in the early 1950s - really Tsilla Levine's mother? Marit Slavin looks at forensic testing |
Science is being asked to answer the emotionally charged question whether Tsilla Levine is really the daughter of Margalit Umassi. However, it has, so far at least, fallen short. To date, there are two contradictory results from two different laboratories. How can this be? Is science not supposed to provide us with clear, distinct answers? Last week, heredity jargon such as mitochondrial DNA and genomic analysis was being casually bandied about in the media. But what do they mean? How do molecular biologists determine that two people are related to one another? The prime player on this forensic science stage is the very stuff of heredity - DNA. The hereditary material found in the nucleus of every human cell is called genomic DNA, which contains a complete set of chromosomes along with their associated genes. Genomic DNA is a large molecule resembling a long chain of myriad links. Each link is made of nucleotides comprising three elements, two of which are identical along the entire length of the chain. The third element is a chemical material called the organic base, which appears in any one of four different combinations. Continuous segments of these links - which are created by combinations of the four organic bases - are the genetic codes that determine heredity. DNA consists of two linked molecular strands - Watson and Crick's famous double helix model. During conception, a child receives one strand of DNA from his mother and one from his father. However, DNA is also found in mitochondria - the tiny powerhouses within each cell, which contain enzymes for cell metabolism. Mitochondrial DNA is composed of the same building blocks as genomic DNA, but are different in that they pass directly from mother to offspring. Fathers do not contribute mitochondrial DNA to their children. Scientific examinations intended to confirm or rule out family relationship are based upon the degree of compatibility between the DNA of the parents and the children. Forensic medicine recognizes such comparisons as binding legal documents. In the case of Margalit Umassi and Tsilla Levine, two different tests were used to determine a familial relationship. Dr. Hasan Khatib of the Hebrew University Institute of Life Sciences conducted one set of DNA tests in which he compared genomic DNA found in blood samples from the two women. Khatib correlated the nucleotide strands found in the two samples, specifically comparing 15 sites situated along the strands. Khatib was looking for the same continuous sequence of four organic bases arranged along the strand. Feeding the results of his comparison into the computer, Dr. Khatib found a 99.9 percent probability that Margalit was in fact Tsilla's mother. Meanwhile, Dr. Maya Freund at the Institute of Forensic Medicine, took a slightly different tack, and examined mitochondrial DNA, as well. Freund asserts that this is the standard procedure used by forensic biologists when they are asked to determine the relation between a mother and her offspring. Since mitochondrial DNA is passed directly from mother to child, without intervention or splicing with the father's DNA, Freund claims that this is the preferred method for mother-child forensic comparison. The researchers do not have any samples of genetic material from the father, a fact which Freund says complicates genomic DNA comparison, since half of it comes from the father. Dr. Freund analyzed both mitochondrial and genomic DNA, testing pre-set locations along the strands that are accepted as comparison benchmarks by forensic scientists worldwide. In both cases, the samples were found to be statistically dissimilar. Geneticist Adam Friedman of the Hadassah-Hebrew University Hospital feels that the possibility of two people's DNA matching at 15 different sites along the strand, as was found in Khatib's test, provides convincing statistical proof of a family relationship. Friedman also pointed out that testing of mitochondrial DNA presents its own problems because, unlike genomic DNA, it can mutate or fractionalize with age. This in itself might explain why specimens from two related individuals might not match (c) copyright 1998 Ha'aretz. All Rights Reserved |
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