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Atlanta Identical twins Ronald and Donald Smith Murder case: Twin DNA testing Proves Useless

Atlanta Identical twins Ronald and Donald Smith Murder case: Twin DNA testing Proves Useless
Twin DNA testing can today tell us a lot about the genetic makeup of twins. The DNA profiles of identical twins will be exactly the same whilst the DNA profile of fraternal twins will be different. In the case of identical twins Ronald Smith and Donald Smith, solving a crime case becomes a bit complicated.

DNA Testing to Determine whether Twins are Identical or Fraternal

Monozygotic twins: The terms Zygotic means ‘egg’ or ‘ovum’. The prefix ‘mono’ as used in words like monologue, implies ‘one’. Monozygotic twins are formed when a sperm cell fertilises an ovum and then within the first 13 days of pregnancy the fertilised egg splits to form two embryos. Identical, or monozygotic twins, will have the genetic material from the same egg and thus, the genetic profiles will be exactly the same for both twins.

Dizygotic twins: Again, if we break the word into its etymologically into its constituent parts we can see we have the term ‘zygotic’ deriving from Greek, meaning ‘egg’ and the prefix ‘di’ meaning ‘two’. Fraternal twins are conceived when two sperm cell fertilise two separate eggs to form zygotes. Each zygote is formed from a different egg and thus, fraternal twins will have different genetic makeup. Identical twins are much rarer than fraternal twins. The rate of incidence for identical twin is 3 in every 1000 births. Fraternal twins have varying rates of incidence depending on country. In the US it is around 24 births per thousand.

Identical Twins and a Murder Case: is DNA Testing Reliable?

In 2008, Donald Smith was arrested for the hijack and shooting of a pre-school teacher in Atlanta, USA. Police arrested him basing themselves on eyewitnesses, cameras, and DNA testing of forensic evidence. DNA found at the crime scene matched that of Donald Smith. However, Donald claimed he was innocent of the crime and that in fact it was his identical twin Ronald Smith who was guilty of it.

Police had already arrested Donald on drug charges and thus, they had his DNA in their database. Investigators went back to the crime scene as the DNA testing carried out previously was now useless as Ronald and Donald had identical DNA profiles and thus, if one were to rely on just DNA, then either twin could have done it.

Finger prints collected at the crime scene did in fact exculpate Donald and incriminate Ronald. Identical twins, or monozygotic twins, share the exact same DNA but not the same finger prints, at least not on all ten fingers.

Identical Twins have Identical DNA profiles but different Fingerprints?

This brings into the equation of twins, DNA and fingerprints the nature versus nurture debate.
Identical twins will have very similar contours and ridges in their fingerprints but they will not be the same. Even though identical twins are genetic duplicates, the shape of fingerprints is not solely linked to genes. Environmental factors are important. Fingerprints are referred to as phenotypes meaning that their formation is linked to external and internal environmental conditions. The nutrition the embryos receive, the blood pressure, differences in length of umbilical cords and the growth of the fingers in the first trimester are all factors influencing fingerprint shapes. Even in identical twins, there are bound to be the minutest differences.

Twin Testing using DNA

Twin Zygosity DNA tests are often carried out by twins who are unsure of whether they are identical or fraternal. Twin DNA testing is carried out for the following reasons:

  • For curiosity purposes; twins may often wish to know whether they are truly twins and thus, genetic replicas of each other.
  • Medical purposes; having an identical twin means that the blood groups will also be the same. This makes blood transfusion easier and safer as one twin can donate blood to the other. In cases where one twin donates an organ to the other, chances of the organ being rejected are dramatically reduced if the twins are identical.

Cases involving identical twins in murder cases are not very common because identical twins are very rare. Twin DNA testing is an important field in genetic research. Interestingly, some scientists claim that many of us had a twin which was lost in the earliest stages of pregnancy.