All serious butterfly collectors remember their first gynandromorph: a butterfly with a colour and pattern that are distinctly male on one wing and female on the other. Seeing one sparks wonder and curiosity. For biologist Nipam H. Patel, the sighting offered a possible answer to a question he had been pondering for years: During embryonic and larval development, how do cells know where to stop and where to go? He was sure that the delicate black outlines between male and female regions appearing on one wing — but not the other — identified a key facet of animal development. “It immediately struck me that this was telling me something interesting about how the wing was being made,” said Mr. Patel, a biologist who now heads the Marine Biological Laboratory, a research institute in Massachusetts, affiliated with the University of Chicago, in the U.S. The patterning on the gynandromorph’s wing shows that the body uses signalling centres to control where cells go during development and what tissues they become in creatures as diverse as butterflies and people, Mr. Patel said. Gynandromorph butterflies and other half-male, half-female creatures, particularly birds, have fascinated both scientists and amateurs for centuries. Scientists say these instances of split-sex animals and insects could offer clues to why some human diseases strike one sex more than the other. Researchers thought they had figured out the genetics of birds and bees, but gynandromorphs suggest that there is more to learn, said Jennifer Marshall Graves, a distinguished professor at La Trobe University in Melbourne, Australia. Mammals have X and Y chromosomes, birds and insects have Z and W, and some reptiles can change their sex depending on temperature, or a combination of temperature and sex chromosomes, she said. Proteins at work It was believed that the sex of a bird was determined by a protein made by the DMRT1 gene, which would reach all the cells of the bird through the bloodstream, Ms. Graves said. But for two sides of the bird to share the same bloodstream but not the same sex, there must be more to the story. Hormones cannot be the sole drivers of sex either, but they most likely play some role, said Arthur Arnold, a distinguished research professor at the University of California, Los Angeles. In a paper published in 2003 in PNAS , Mr. Arnold showed that in gynandromorphic zebra finches, brain cells on the female side were more masculine than comparable cells in a typical female. How gynandromorphs are born at all still remains a mystery. For birds, the most likely explanation is that a female makes an unusual double-nucleus egg cell, one with a Z chromosome and one with a W chromosome, and each is fertilized by a Z sperm, making some cells ZZ and others ZW in the same individual, Mr. Arnold said. The same process is very unlikely to happen in mammals, he said. Female mammals naturally have two of the same sex chromosomes, and the instant a mammalian egg and sperm fuse, “dramatic changes prevent the entry of a second sperm.” Gynandromorphs occur naturally, usually resulting from a random genetic error, Mr. Patel said. Mr. Arnold said his research on sex genes has implications for treating human diseases that seem to vary by gender. His UCLA collaborator, Rhonda Voskuhl, has found, for instance, that in multiple sclerosis, a genetically female mouse with two X chromosomes fares worse than a mouse with an X and a Y, even if they have the same hormones. Understanding why females fare worse could help explain MS in people, where there is also a gender difference, with women accounting for three times as many cases as men. Obesity, metabolic syndrome, autoimmune disease, Alzheimer’s, even ageing differs by sex, Mr. Arnold noted. Twenty years ago, he said, scientists did not think that sex chromosomes played any role in causing sex differences in these diseases. “But now we know it makes a difference in mice, so we can say: Where does it make a difference in humans?” he said.NY TIMES