A few points on recent speculation about:

• Effect of metal (assumed brass) of sheath button on the DNA profile
• Possibility of secondary transfer of touch DNA (i.e. someone touched Kohberger and that person then touched the sheath)
• The sheath DNA match to Kohberger random match statistics (5.37 octillion to 1)

Brass Sheath Button - When Was DNA Deposited ?

I posted about the possible significance of brass last July. Since then it has been noticed and speculated on rather wildly.
DNA persistence on metal surfaces varies greatly - it is relatively stable on stainless steel or lead, much less stable on copper, zinc and their alloys. This is because copper and zinc catalyse oxidative degradation of DNA.

Recent studies, suggest DNA shows significant degradation on brass in 8-12 hours. While this period could be variable, if we use this -then Kohberger's DNA was deposited on the sheath button in the evening of November 12th or most likely given the complete DNA profile recovered, in the early morning of November 13th 2022. (Another 2024 study from University of Adelaide showed similar results - pre-print, not peer reviewed)

Secondary Transfer - When Could It Have Happened ?

Secondary transfer DNA (non-self DNA) has been shown to persist on hands for a maximum of 8 hours. Generally the actual person touching an object is shown to always be the major depositor, with secondary transfer being minor and already significantly reduced after 5 hours after the contact.

In most circumstances secondary transfer DNA is not detectable or is only detectable for a much shorter period than 8 hours, and is mostly eliminated by common activities30168-4/fulltext?uuid=uuid:9037ead5-91a4-4beb-a667-2d327059ee49) e.g. hand washing, touching objects/ surfaces, friction.

If we take the effect of brass and the persistence period of secondary transfer DNA on hands, these suggest any secondary transfer of Kohberger's DNA to a person who later touched the sheath happened late on November 12th after 11pm or early November 13th 2022. Combining the effects of rapid loss of non-self DNA for secondary transfer and the effect of brass suggests that transfer happened significantly later than 11pm on November 12th.

Note that secondary transfer is highly unlikely as no DNA from the primary depositor/ person who contacted the sheath, if that person was not Kohberger, was recovered. No reliable study using realistic conditions and a statistically robust sample size has shown transfer of a secondary person's DNA to an object without transfer of DNA from the primary person who touched the object.

https://preview.redd.it/cp4qkux1gsvc1.jpg?width=326&format=pjpg&auto=webp&s=9276d9fbaf41222ed1b484a130857831276987e0

DNA Match Statistics - Partial or Full Profile

The DNA match statistics for the sheath DNA with Kohberger (the 5.37 octillion to 1 random match probability) requires a full DNA profile. The 5.37 Octillion is in the typical range expected from the DNA profile kits used, based on validation including peer reviewed scientific studies. This statistic magnitude is also expected from simple calculation: The match statistic reflects the chance of any person matching at all of 20 areas of the DNA profile (STR loci, CODIS uses 20, typical DNA profile kits use 23 loci). Any random person would have a (roughly, average to illustrate calculation) 5% chance of matching one STR loci on a random DNA profile (the actual probabilities for the STR loci used for CODIS vary from c 0.007 to c 0.13). Multiplying that probability of 0.05 x 20 times gives a probability in the same order of magnitude as the 5 octillion.

Promega DNA Profile Kit - same as used by the ISP Forensics Lab

One point over-looked by those who argue, with no evidence, that the DNA profile was "partial" is that CODIS has specific rules on the minimum number of STR loci matches (i.e. the "completeness" of the profile) and the unique match probability for a profile to be uploaded. Only profiles with a minimum of 8 STR loci matches and a unique match probability of 1 in 10 million can uploaded to CODIS.

As the sheath DNA was uploaded to CODIS, even if was the most partial profile possible, it would still predict a possible match for this case, based on population statistics, of less than c 5 men in the USA.