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"Law & Order: Special Victims Unit" Bad Blood (2000)

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genetic erratum

Author: estrognboy ( from United States
23 January 2007

*** This review may contain spoilers ***

In this episode (# 1.11), entitled "Bad Blood", the medical examiner's report makes a reference to mitochondrial DNA, denoted as mtDNA, (extranuclear DNA which is transmitted ONLY from a mother to her child). Detective Stabler MISTAKENLY reasons that this DNA might also match that of the (male) suspect's father or son, while Detective Benson correctly suggests that it could belong to that of his brother.

One small caveat to Detective Stabler's otherwise incorrect statement: if the person in question was a female, the mtDNA could indeed belong to her son (or daughter), as well as her siblings, her mother, her maternal aunts, uncles and grandmother, as well as any of her own daughter's descendants, and so on (both up and down the generational ladder). For a man, the match could exist only in the upwards direction of the generational ladder (siblings, mother, grandmother, and maternal aunts & uncles), as well as his sister's descendants, but NOT in his own descendants and NOT in his father.

Although a mitochondrial DNA analysis might show a match, it is inconclusive evidence because, as explained above, it does not single out one person, but does narrow the search down to a particular familial lineage. Further analysis of genomic (nuclear) DNA must be done to more accurately identify a potential match.

Other possible forensic DNA matches which might confuse the uninformed include sex-chromosome matches; i.e., the X and Y chromosomes. A brief genetics lesson: women have two X chromosomes, while men have one X and one Y chromosome.

The simpler of the two potential mix-up cases, the Y chromosome is passed from father to son, which establishes a very straightforward paternal lineage of man after man. So a man could be wrongly identified while the real culprit could be his father, brother, son, etc. It should be noted, however, that family lineages can be expansive, and fourth cousins could just as easily share the same Y chromosome markers. The simplicity is in the fact that the Y chromosome is exclusive to men.

Conversely, X chromosome transmission is similar to the pattern that mtDNA follows, in that a mother passes an X chromosome to each of her children, although there is variability in which of her two X chromosome gets transmitted to which child, a phenomenon which is well documented as "X-linked traits," whose distinctive patterns differ from traditional Mendelian patterns of autosomal (non-sex chromosomes -- basically, all the rest of the chromosomes) inheritance. Another twist, which both complicates and simplifies matters, is that a father passes his ONLY X chromosome to all of his daughters.

My educated guess is that paternity tests are conducted using markers on the X and Y chromosomes, depending on the gender of the child in question. A boy would require a Y chromosome match with his alleged father, while a girl would need a match on one of her X chromosomes to identify her father. Maternity tests, on the other hand, are as simple as mtDNA matches. Leaving aside the rare possibility of mutations (which further complicate matters), it is important to remember that these parental identification tests are not 100% conclusive, as one's relatives could just as easily provide matches. In other words, one might be found to be the father of a child when in fact it was his brother, etc.

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