One pair of chromosomes are uneven or unequal in size. All of the lovely characters of color, length of tail, dorsal fin, shape of tail, abide in the male, so it was natural to suppose that the genes for these traits resided on the chromosome which had no adequate mate to dominate it. But the early geneticists covered too much ground. It is true that the uneven pair do account for sex. Suppose that one chromosome which we call X is a little one and its partner, which we call Y, is a big one. If an egg has two of the X- chromosomes, it becomes a female, but if it has one X and one Y, the combination of XY produces a male. That is why there are approximately an equal number of males and females born. Half of the male sperms contain X and half contain Y. Every female sperm contains XX. The possibilities therefore work out to be fifty per cent XY and fifty per cent XX-half male and half female.
 In some species there is a large size difference between the X and Y chromosome; not so in Lebistes. In studying the chromosome appearances with a microscope, one finds every partner in each chromosome pair to be alike.
 One student has attempted to map chromosomes. He thinks that of the eighteen genes which affect color markings, several affect the shape of the tail as well as body colors. Some gene effects are recognized only in the male although they are carried by females. Sword tail is one of these characters. Females are not seen with swords but the characteristic is there undeveloped probably because of repressive hormonal influences. Until this fact is recognized, one cannot really understand Mendel's law as it works in guppies.
 At least nine genes are located on the Y-chromosome, but some, during the process of cell division, become incorporated in the X-chromosome; that is, some of the X and Y genes exchange, a process known as crossing over.
 X-ray irradiation interferes which normal inheritance in guppies by affecting genes and chromosomes. Recent studies of radiation effects when atomic stock-pile elements were used produced death in the fishes before any radiation effects were observed. Future studies may yield surprising results.
 They can cause you trouble.