The ancients imagined mythical creatures that combined human and animal elements, including mermaids, minotaurs, satyrs and centaurs. They also mixed up different animals to form new combinations, like the gryphon (eagle/lion) and the chimera or chimæra (lion/dragon/goat). Different species can sometimes mate and produce hybrid offspring, but there is also a stranger type of creature which, like the mermaid, is half one thing and half another. These are known to science as mosaics or chimeras.
Both mosaics and chimeras are individuals with two genetically distinct populations of cells. The difference between them is that in a mosaic the different types both arise from the same fertilised egg, whereas a chimera comes from more than one. In fact, in some cases a chimera is a pair of twins that has fused into a single person.
Chimerism generally passes unnoticed, but medical advances have caused an increasing number of cases to be discovered. One of them initially baffled Dr Margot Kruskall at the Beth Israel Deaconess Medical Center in Boston.  A middle-aged woman, referred to as Jane, needed a kidney transplant. Tests were carried out on her family members to see if they were sufficiently compatible to be donors. Bizarrely, two of Jane’s three sons were not only not suitable, but genetically they could not be her children at all.
At first it was suspected that both boys were switched with other babies at birth, however unlikely this might seem. However, further genetic testing showed that the boys’ genes had come from the grandparents. But why did tests not show the same genes in Jane herself? It turned out that Jane is a chimera, with two distinct sets of genes in different parts of her body. Under other circumstances, she might have been a pair of non-identical twins.
Like most chimeras, Jane was unaware of her condition. There are unlikely to be visible signs, though it’s possible that different populations of cells could have some sort of external effects. (Different coloured eyes – heterochromia – are not necessarily a signs of chimerism). This can occur with the marks known as Blaschko’s lines on the skin. These were first noted in the 19th century by German dermatologist Alfred Blaschko, who observed that skin diseases often seemed to form the same pattern, a kind of tabby striping. This pattern did not correspond to any known physiological feature such as the paths of nerves or blood vessels.
It is believed that the lines are caused by the way that cells from the fœtus are distributed over the body. In a chimera where one of the cell types codes for darker skin than the other, the results are (barely) visible. Blaschko’s lines may be more obvious under ultraviolet light. 
In some rare cases, chimeras may have significant abnormalities including ambiguous genitalia; they may even be true hermaphrodites; but generally the condition only shows up with DNA testing. The plot device of a chimeric individual having two different DNA types has already been used in both CSI and House and will doubtless become as popular a trope in forensic fiction as the multiple personality disorder in crime dramas.
The chimera raises some interesting metaphysical questions. Is the chimera one person or two? Does (s)he have a double soul? There is plenty of speculation on the Internet about missing twins and other halves but, in the absence of a DNA test, anyone who thinks (s)he is a chimera is more likely to be suffering from overactive imagination.
A different type of chimerism is common in cattle. When a cow has twins, the circulatory systems are often joined and there is an interchange of cells between the two growing calves. Both are chimeras. In other species such as marmosets, chimeras seem to be the rule rather than the exception – half of male marmosets carry their brother’s sperm. 
Mosaics are more common than chimeras; in fact all female mammals (including humans) are mosaics. This is because females possess two X chromosomes, and one of these is always inactivated – in about half the cells it will be the X-chromosome from the mother, in the other half the chromosome from the father. The individual is therefore a mosaic of these two types. This leaves no trace in humans, but can be readily seen in tortoiseshell and calico cats where the coat colour (orange or black) is carried on the X-chromosome. Their fur is an uneven, seemingly random patchwork of orange, white and black showing the distribution of the two types of cells.
These might seem a far cry from the mythological creatures blending different species, but thanks to science these chimeras are becoming a reality. In 1984, Cambridge scientists combined sheep and goat embryos and produced six animals, one of which was a true sheep-goat chimera, known as a Geep,  which has both sheep and goat cells in different parts of its body.
Now researchers are looking at chimeras as a way of producing useful amounts of human stem cells or even whole human organs.  A team in Nevada is looking at growing sheep with ‘human’ organs for transplants, while researchers at Newcastle are developing a new technique for growing stem cells. This would combine human and cow,  reinventing the Cretan Minotaur, this time as a potential healer (and sacrificial victim). With global warming and rising sea-levels, can mermaids be far behind?
1 Claire Ainsworth: 'The Stranger Within', New Scientist, 15 Nov 2003
2 Carl Zimmer: 'In the Marmoset Family, Things Really Do Appear to be All Relative', New York Times, 27 March 2007
3 Dr Barry Starr: 'Ask a Geneticist', Stanford School of Medicine Understanding Genetics
4 'It's a Geep', Time, 27 Feb 1984
5 Jamie Shreeve: 'Half Human, Half Beast?' New Scientist, 21 June 2005
6 Christian Nordqvist: 'Permission to Create Chimeras, Not Hybrids, For Stem Cell Research', Medical News Today, 8 Nov 2006