THERE has been some puzzlement as to the actual number of chromosomes humans have - is it 46 or 48? Here's the answer. Humans have 46 chromosomes - 22 pairs of non-sex chromosomes, and one pair of sex chromosomes. (Males have one X and one Y sex chromosome, and females have two X sex chromosomes.) But before 1956, scientists erroneously believed there were 48. Chromosomes are strings of DNA containing the genes that code for all the proteins that make up an organism. Inside a cell, chromosomes are usually stretched out and jumbled like a snarl of string, making them very difficult to count. But when cells divide, the chromosomes pull together into more easily distinguishable sausage shapes. To count chromosomes, scientists today treat cells in a way that spreads the chromosomes out, while keeping them grouped closely enough to tell which chromosomes came from a single cell. Before the 1950s, a researcher named T.S. Painter counted 48 chromosomes in cells from a slice of human testis. His report was worded cautiously because the chromosomes were bunched together and difficult to see, but the more it was quoted, the more it became accepted as true. Then in 1956, two scientists working in Sweden reported that they had used new methods for spreading out chromosomes. Their images were crystal clear, and finally nailed the number at 46.