Mitochondrial

(passed on complete, but only by daughters)

____________________________________________________________________i sexual assaults, involve males as the perpetrator, DNA tests designed to only examine the male portion can be valuable. With Y chromosome tests, interpretable results can be obtained in some cases where autosomal tests are limited by the evidence, such as high levels of female DNA in the presence of minor amounts of male DNA (Figure 9.2). These situations include sexual assault evidence from azospermic or vasectomized males and blood-blood or saliva-blood mixtures where the absence of sperm prevents a successful differential extraction for isolation of male DNA (Prinz and Sansone 2001). In addition, the number of individuals involved in a 'gang rape' may be easier to decipher

Figure 9.2

Schematic illustrating the types of autosomal or Y-STR profiles that might be observed with sexual assault evidence where mixtures of high amounts of female DNA may mask the STR profile of the perpetrator. Y-STR testing permits isolation of the male component without having to perform a differential lysis.

Female-Male Mixture Performance with Autosomal vs. Y Chromosome DNA Markers

Female Victim DNA Profile

Male Perpetrator DNA Profile

No signal observed

DNA Profile from Crime Scene

Autosomal STR Profile

Y Chromosome STR Profile with Y chromosome results than with highly complicated autosomal STR mixtures. Using Y chromosome specific PCR primers can improve the chances of detecting low levels of the perpetrator's DNA in a high background of the female victim's DNA (Hall and Ballantyne 2003a). Y chromosome tests have also been used to verify amelogenin Y deficient males (Thangaraj et al. 2002), as mentioned at the end of Chapter 5.

The same feature of the Y chromosome that gives it an advantage in forensic testing, namely maleness, is also its biggest limitation. A majority of the Y chromosome is transferred directly from father to son (Figure 9.1) without recombination to shuffle its genes and provide greater variety to future generations. Random mutations are the only mechanisms for variation over time between paternally related males. Thus, while exclusions in Y chromosome DNA testing results can aid forensic investigations, a match between a suspect and evidence only means that the individual in question could have contributed the forensic stain - as could a brother, father, son, uncle, paternal cousin, or even a distant cousin from his paternal lineage (Figure 9.3)! Needless to say, inclusions with Y chromosome testing are not as meaningful as autosomal STR matches from a random match probability point of view (de Knjiff 2003).

On the other hand, the presence of relatives having the same Y chromosome (see Figure 9.3) expands the number of possible reference samples in missing persons investigations and mass disaster victim identification efforts. Deficient paternity tests where the father is dead or unavailable for testing are benefited if Y chromosome markers are used (Santos et al. 1993). However, an autosomal DNA test is always preferred if possible since it provides a higher power of discrimination.

Figure 9.3 An example pedigree showing patrilineal inheritance where all shaded males have the same Y chromosome barring any mutations. To help identify the person in question, any of the other males with the same patri-lineage could provide a reference sample to assist in a missing persons investigation, mass disaster victim identification, or deficient paternity test (boxed region) where the father is deceased or not available for testing.

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