FORENSICS 127: DOUBLE TROUBLE

Identical twins can give forensic practitioners headaches. There have been cases where a felon could not be specifically identified as one or the other of a pair of identical twins, even if there were samples of DNA available for comparison. At least, such was the case until relatively recently.

Most of us are familiar with the twisted, ladder-like configuration of DNA. The rung-like structures of the ladder are known as nucleotides, and they form repeating DNA units. Some of us know that there are four types of nucleotides, namely, adenine, thymine, guanine and cytosine, commonly referred to as A, T, G and C. In the shorthand of biologists, A pairs with T and G pairs with C. Each side of a “ladder” is a sugar-phosphate, and the “rungs” are pairs of nitrogenous bases. Hydrogen bonds hold the pairs together, and it is the sequence of the nucleotides that actually carries genetic information. During replication, the rungs separate and each becomes a template for assembling free nucleotides in the same sequence as that present in the template. This might seem like a slow, tedious process; but some 50 nucleotides are added per second.

DNA segments can range in size from a thousand bases to millions of bases. Our human genome (the complete stock of our nuclear DNA) includes some 6 billion chemical bases of DNA nestled within two sets of 23 chromosomes. One set is inherited from each of our parents. Previous beliefs held that genes were almost always included in two copies in a genome. Following the fairly recent completion of the human genome project, however, it was learned that was untrue. Deletions, duplications, inversions and translocations can occur that cause the number to vary, which is known as copy number variation (CNV). It was also discovered that a small percent of the copy numbers of unrelated persons differed. Of importance here is the fact that copy numbers were found to vary in identical twins also. This provides forensic investigators means for identifying which of a pair of twins left a DNA sample at a crime scene. That fact might not have been included in the plots of very many novels yet.

Extra facts:

In addition to common fraternal and identical twins, there are other, relatively less-common, twins.

If a fertilized egg splits more than a week following fertilization, what are informally known as “mirror-image twins” can be the result. Having formed from a single fertilized egg, they are still identical twins, but they can have some reversed features. One might be left-handed and the other, right-handed. One twin might have a birth mark on one side and the other twin on the opposite side. Approximately one out of every four identical twins is a mirror-image twin.

Since identical twins are formed from a single zygote (fertilized egg) that contains only male (XY) or female (XX) sex chromosomes, they are always both male or both female. Well, almost always. In one instance, when a genetic mutation in one of a pair of male identical twin caused the loss of a Y chromosome, the twin developed as a female but was relatively short and lacked ovary development. Fraternal twins can both be male or can both be female. Also, one can be female and the other, male.

About one in two hundred identical twin births result in conjoined twins, also known as “Siamese twins.” A high percentage of them are stillborn or die within a few days. For an unknown reason, about 70 percent of conjoined twins are female. A fewer number of conjoined twins are known as “parasitic twins.” These twins are asymmetrical, with a smaller twin depending from a larger twin. There have been cases where abnormally formed cell masses have developed and grown completely within the body of its twin. They depend on a flow of blood from their twins and occasionally survive for some time. Records refer to a “fetus in fetu” having been found even within an adult male.

A persistent question some readers might have is whether or not twins run in families. There is a gene that influences women to have more twins. It does so only indirectly, however, by causing “hyper ovulation.” This refers to a tendency to release a number of eggs during ovulation. This, of course, increases the probability of having fraternal twins. Males can carry the gene but, of course, don’t ovulate much; and that might account for the belief that twin births skip a generation.

Copy number variation studies show promise in medical research as well as in forensics. If one identical twin has a disorder that the other does not, studying their CNV could reveal whether or not the disorder has a genetic link. Also, discovering the locations of variations can help pinpoint fruitful areas of future research to discover the role that genetics plays in the disorder.

In the United States, twins represent about one in fifty babies born. Fraternal twins make up about two thirds of all twins, and the remaining third are identical twins. Compared to twins, one doesn’t see very many triplets. They represent only one in about 8,000 births.