BBC NEWS | Health | Semi-identical twins discovered

BBC NEWS Health Semi-identical twins discovered: "These twins, who were conceived normally, only came to the attention of scientists because one was born with sexually ambiguous genitalia.
The child was discovered to be a hermaphrodite, and has both ovarian and testicular tissue, while the other child is anatomically male.
But genetic tests show both are 'chimeras', and have some male cells - which have an X and Y chromosome, and female cells - which have two X chromosomes.
The most likely explanation for how they were formed is that two sperm cells - one with an X chromosome and one with a Y chromosome - fused with a single egg.
The twins are now toddlers, and doctors say they are progressing well. "