One way to improve guppies is to mate the best male to the best females; that is generally the way nearly everybody does it. But if we are to understand what we are doing, we must know first some of the terms of the student of heredity - the geneticist. Perhaps because I am a geneticist, as well as a veterinarian, I think the geneticist gets more enjoyment from his work than students in any other field of science.
 It is usually difficult for boys and girls (and grown-ups) who have had no training in biology, to grasp the principles discovered by Johann Gregor Mendel and now called Mendelism. This is possibly because the scientific terms used by the instructor when explaining the principle are not, but can be, put into everyday English so that the theory of Medelism can be understood by all.
 There are a lot of old wives' tales and nonsense about heredity which most older persons must unlearn before they can properly understand the latest findings of science. Here are some of them:
 Inheritance of Acquired Characters. If a fish tends to change its color to resemble its background, and this accommodation goes on generation after generation, there are those who say that the acquisition of the acquired shade will be inherited. It will not. This is an acquired character and acquired characters do not impress themselves on the germ plasm.
 Birth Marking. If a pregnant mother fish became frightened, even terrorized, until she hid all the time, our forefathers would have said that she would have marked her germ plasm or her embryos, and they would be naturally frightened. This is not so. Her being frightened would not mark her offspring. Every piece of research aimed at studying the problem has given only negative results.
 Telegony. If a female live-bearing fish is mated to an inferior fish once and bears three or four litters, and the female is afterwards mated to a male of excellent type, the first male cannot influence the offspring of the second. Our ancestors thought it could.       TOP
 The Blood Theory. We talk about blooded, pure-blooded fish when we mean highly bred or purebred specimens. The word blood is used mistakenly in place of the word heredity. This is a mistake and gives those who use it and those who hear it the idea that heredity is a case of dilution. Consequently it is difficult for them to grasp the fact that heredity is a case of presence or absence and not one of dilution.
 When you mate a grey guppy with a pink-eyed white (albino) you do not get a light grey but one like the grey parent. When you cross a gold with a grey you do not get a mixture; you get grey. There are few instances indeed, where the result of crossing involves single traits which give blends or dilutions, and none so far discovered in guppies. So it is better not to use the word blood when we mean inheritance-use terms such as pure-bred, breeding, heredity. How then do guppies inherit? Is there a formula? or is inheritance helter-skelter without rhyme or reason? There is indeed a pattern no different from that of all other creatures.