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McCann's Twins: the Twin Method in Research

Express: Tests 'to unlock' Maddy Mystery
David Pilditch

SEVENTEEN human cells could provide a major breakthrough in the Madeleine McCann mystery, it was revealed yesterday.
The DNA samples are being examined by scientists in Britain and Portuguese detectives believe the evidence could unlock an
investigation which is now in its eighth month.
Experts are trying to establish whether the cells match Madeleine’s DNA and so help investigators discover what happened to the youngster.
The samples are being analysed by the Forensic Science Service laboratory in Birmingham which has developed sophisticated techniques not available in Portugal.
The development comes only days after it was revealed that months of painstaking analysis on DNA uncovered in Portugal had so far failed to produce conclusive evidence. But in a rare public pronouncement on the case, Guilhermino Encarnacao, head of the Policia Judiciaria in the Algarve, said: “We have asked the laboratory to carry out mor
e tests and exhaust every possibility of obtaining conclusive results.”
to read more

Introduction to the Twin Method in Research

Twins occur in about 1 in 85 human births. Twins come in two types: fraternal or dizygotic (DZ) and identical or monozygotic (MZ). The difference between the two types of twins stems from a difference in how they began life – from the fertilization of two separate eggs (resulting in DZ twins) vs. from the fertilization of a single egg that later split in two (resulting in MZ twins). This difference in fertilization results in one type of twins – DZ – sharing an average of 50% of their genetic material (much like non-twin full siblings) and the other type of twins – MZ – sharing 100% of their genetic material. This difference in amount of shared genetic material sets the stage for a marvellous natural experiment and it is the basis of the twin method for research.

The twin method is used to quantify the magnitude of the contribution of genetic and environmental factors to individual differences in a trait or behaviour. The twin method of research entails the collection of data on both kinds of twin pairs (MZ and DZ). Data from MZ pairs can then be statistically compared to the data from DZ pairs in order to assess the relative contributions of genetic and environmental factors to the trait or behaviour of interest. Given that MZ twins share twice the amount of genetic material as DZ twins, then a greater similarity found among MZ twins as compared to DZ twins would be indicative of genetic influence. However, if DZ twins were as similar on a trait or behaviour as MZ twins, then shared environmental influences (those factors shared by members of a family that make them more alike) would be indicated. Finally, if MZ and DZ twins did not look similar on a trait or behaviour, then non-shared environmental influences (those factors that are unique to members of a family which serve to make them different from each other) would be indicated. These types of conclusions can be drawn from twin data if we make some important assumptions. Namely, that MZ and DZ twins have equal environments (i.e., MZ twins are not treated more similarly in the environment than DZ twins) and that twins are representative of the population (i.e., there is nothing unusual about twins per se that would make their data on a trait or behaviour markedly different from data from singletons).

Degrees of Relation and Number of Genes Shared

see the table here*

Estimates of how many human genes exist range from 25,000 to 100,000. As the sequencing of the human genome nears completion, the consensus is closing in on the surprisingly low figure of around 30,000. The following table uses this number to estimate how many of an individual's genes are shared by their relatives.
Of course, any individual's mother and father share over 99% of their autosomal[2] DNA (the DNA not carried on the X and Y[3] chromosomes[1]). This means that the individual shares much more of each parent's DNA than the 50% indicated by the table.
Thus the table shows us that 50% of our DNA is directly received from each parent and not that the genes inherited from one parent are entirely different to those inherited from the other.

What the table also shows us is that, beyond 7-degrees of relation, the number of genes shared in this manner is rather few. By extension, an individual's sixteenth-century, 13 times great grandfather has less than a half chance of providing a single autosomal gene to that individual, assuming that there is no other relationship between the two.

In terms of genes, there isn't much significance in taking your family tree back to Norman times for, at that distance, our genetic heritage will long ago have merged into the population's gene pool[5]. In fact, ignoring the DNA that all human's share, it's possible to be genetically unrelated to a direct but distant ancestor even when hanky-panky can be ruled out!

There are, however, three genetic lineages which can be traced back much further.
The first is your pure maternal line, i.e. your mother's, mother's, mother's etc. lineage. This is because you inherit a small fragment of DNA, called mitochondrial DNA[4], solely from your mother.
The second is a man's pure paternal line. This is because a man inherits his Y-chromosome[3] solely from his father. This does not apply to women as they do not have a Y-chromosome.

The third are specific genetic markers that can be pinned down to specific populations. An example of this is the gene associated with cystic fibrosis, the origin of which can almost certainly be traced back to European populations. Most such genetic markers will not be associated with disease although, to date, these have been the best studied.
The amount of DNA involved with first two of these lineages is rather little compared to the rest of our DNA. However, because of its unique status, it is often analysed for genealogical purposes. The third group of lineages represent a much larger, more diverse, number of genes, but, as genetic typing advances, I believe it will play a much larger role in genealogy.

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