Genetic variation means the following – there are various variants of gene abnormalities within a population or an individual. Each specific fish has only two copies of each gene, so it can have a maximum of two gene variants. However, the population or breed of the fish as a whole may have many different variants of each gene. If not all of the possible gene variants are inherited by the next generation, the breed loses its gene variability. In fact, all breeds of guppies that the breeder keeps without introductory crossing will lose gene variability for a long time. What does it matter, you ask, only one thing – the goal of breeding a guppy is to have the breed able to demonstrate the same characteristics over and over again, and all the fish of the same breed are as close to each other as possible. In some old articles on guppy breeding, you can read that the quickest way to get a pure breed is to closely interbreed (inbreeding) the most brightly colored individuals. All this in order to get a homogeneous fish in the next generations. The advice is correct if you want to get a pure breed. But there are also very disadvantageous effects of closely related crossbreeding. Here they are: reduced survivability; fertility decline; propensity to disease. To some extent, these effects can be reduced if the breeder chooses the most healthy and prolific guppy specimens as parents for the next generation.
Loss or reduction of fertility, for example, is the typical and widespread effect of closely related crossbreeding that most breeders experienced. Such a breed may have a very low fecundity, and then suddenly, in one of the generations, the very possibility of the continuation of the genus will most likely disappear. Weak immunity and low disease resistance, this is another problem of breeds supported by inbreeding. If the breeder, from whom you take the fish, recommends using salt, formalin, potassium permanganate, a wide range of antibiotics, and claims that all this is necessary for the guppies to survive in new conditions, you might think that he is wrong, and all this is a mistake , but in fact this is exactly how things are. Another undesirable effect of the loss of gene variability is that the breeder cuts the branch that he supports. Thus, genetic diversity is a prerequisite for breeding. If there is not enough genetic diversity, then there is no point in breeding. Then the question arises: how many variations are lost in each generation, and how long can we maintain the color of the fish without introductory crossing, but without serious effects? The answer depends on many factors, but here are two of them that are of paramount importance: the number of individuals used for breeding in one generation is very important and the number of males or females of guppy intended for the cross is outweighed.

The number of individuals for breeding

Let’s forget for a while about the sex ratio in breeding and dwell on the number of breeding individuals. Here it is important to understand that we will count only fish that are directly involved in the continuation of the line as breeding guppies. If you get 200 guppy fry in each generation, choose the best female and the best male to continue the line, then the number of breeding guppies is two, the number of gene variations is also two. If you get 200 guppy fry in each generation, and use 2 males and 20 females for crossing, but to continue the line you select only the litter of two crossed females, then the number of breeding fish and the number of gene variations will be equal to only four . Thus, in both examples, the population size is 200, but the number of different gene variations is much smaller. There is a simple formula that can tell you how many gene variations you lost when crossing, the loss figure directly depends on the number of fish used in the cross to continue the line. Here is this formula: 1 / (2 * n), Where n, the number of breeding individuals. For example, if two individuals are used for breeding, then 1 / (2 * 2) = 0.25 (= 25%) loss of gene variations. If you use four individuals, then 1 / (2 * 4) = 0.125 (= 12.5%) loss of gene variations. For a dozen guppies, this will amount to 1 / (2 * 10) = 0.05 (= 5%) of losses.
If instead of losses you want to know the number of saved variations in a generation, then you need the following formula: 1-1 / (2 * n). Naturally, the resulting numbers in this formula will be the exact opposite for the examples above and will be – 75%, 87.5% and 95%, respectively.
Now it is easy to calculate how many gene variations will remain after many generations (g). You can repeat the calculations for each generation or use the formula: (1-1 / (2 * n)) g now it looks somewhat more complicated, therefore I will give several examples: After five generations with two selection individuals (1- (1 / (2 * 2))) 5 = 0.237 (23.7%) of the genetic variability is preserved. With four guppies, 51.3% of gene variations were saved, and with ten guppies, 77.4% of gene variations were saved. We can see that the greater the amount of breeding material we use, the more gene variations we save, in fact, we are striving to preserve the gene variations. But how many variations can we afford to lose and how can we keep these variations? The last question is the easiest to answer. The only option possible for the distributor to increase or restore the number of gene variations is introductory crossing with the use of other, not related or distantly related individuals. Most successful breeders understand this well and hold introductory crossing from time to time. Still, one must be aware that the addition of “fresh blood” (introductory crossing) negatively affects the purity (homogeneity) of the line and should not be used more often than is really necessary. Let us, for example, limit the loss of gene variations to 50% and conduct an introductory crossing to achieve this figure. We can use the last formula given above to calculate the number of possible generations with different numbers of individuals used for breeding (at 50% loss). The result is presented in the table:

From this table we can see that if we take the loss of half of the gene variations as an acceptable value, then you can cross within the line (inbreeding) for 2 generations, if there are only 2 individuals, then you need an introductory crossing to restore the number gene variations. This means that you have one introductory crossing per year. If instead you use 30 individuals in each generation, then you can do without “fresh blood” in the line for more than 40 generations or about 20 years! But this is in theory, in practice, the breeder is very often limited in the number of aquariums and cannot contain such a number of fish. The table shows that the best option would be to maintain 10-20 breeding individuals in each generation, and this is quite possible for most guppy breeders.

Sex ratio

Everything said above, about the number of breeding individuals and the size of the gene pool, implies that you use an equal number of males and females per generation and that after spawning, an equal number of fry of both sexes is selected for the next cross. When breeding guppies, the breeder often uses a different sex ratio, usually the predominance of females over males. The effect of sex skewing is the following – reduction of gene variations with the same number of breeding individuals. The loss of gene variations is greater, the greater the bias achieved in the sex ratio. The size of the gene pool is called the “effective population size” ( ne ). Depending on the number of males ( m ) and females ( f ) you use for breeding, it is easy to calculate the effective population size. Here is the formula: ne = (m * f * 4) / (m + f) .
Let’s use one of the examples above to see how it will look like: five males and five females, the effective population size ( ne ) will be the following number: (5 * 5 * 4) / (5 + 5) = 100/10 = 10 .
If you use two males and eight females, the effective population size ( ne ) is: (2 * 8 * 4) / (2 + 8) = 64/10 = 6.4 . If you use one male and nine females, the effective population size ( ne ) is: (1 * 9 * 4) / (1 + 9) = 36/10 = 3.6 . When choosing 10 individuals, you can take 5 males and 5 females, then the effective population size ( ne ) is: (5 * 5 * 4) / (5 + 5) = 100/10 = 10 . If instead you take two males and eight females, the effective population size ( ne ) will be: (2 * 8 * 4) / (2 + 8) = 64/10 = 6.4 . If you use one male and nine females, the effective population size ( ne ) will be equal to: (1 * 9 * 4) / (1 + 9) = 36/10 = 3.6 . Thus, even if you choose the same number of breeding fish, the effective population size will be the smaller, the greater the bias in the sex ratio you will have. The effective population size is not a specific number, it only determines the number of gene variations that will be inherited by the next generation of guppies. Table 2 below shows the examples we have already reviewed, as well as several other options for the cross and the resulting effective population size.

The table shows that a breeder using 50 fish of one and 50 fish of the other sex with a cross, in one generation, can keep the line for 140 generations or approximately 70 years and at the same time keep 50% of gene variations in the line! This is possible with commercial breeding, but not at all realistic for the average amateur. The table also shows that the effective population size ( ne ) cannot be greater than the total number of fish involved in the cross. On the contrary, it may be much less, but vice versa never. A small bias in the sex ratio does not matter much, but at a ratio of 1: 3 and more, there is a significant loss of gene variations.

Variations first or where to start

Prior to that, we considered how much gene pool we can hold, for how long, and under what conditions. But if you initially start with a limited amount of source material, then it will be impossible to keep the line for as long as possible. When receiving a new breed, you should take as many individual individuals as possible and try to use them all in the cross to get a new generation. So, if you have the opportunity to purchase 10 – 20 young fish, this is a very good start. Getting a pair or a trio from a line already using inbreeding is not a good start. And yet, the latter option is not so rare, especially among beginners. Very soon they begin to encounter various diseases and infertility in fish. Sometimes you simply have no choice and are forced to purchase a limited number of guppies, for example, this can happen during auctions after exhibitions. In this case, it will be better if you try to buy guppies from different lines (from different breeders), but the same breed and with a minimum number of differences. It would be even better if you refuse to purchase an auction fish at all, but instead make personal contact with the breeder and take more good, young fish from him but later, after the auction. Displays, it is always stress for the fish, the same equipment is used in a large number of aquariums, it is very likely that the fish can pick up the sores carried by this equipment from other aquariums. If you take fish from a breeder, try to take it from someone who says that makes occasional introductory crossing, from time to time, adding “fresh blood”, and not from the one who brags about the cleanliness of the line and the preservation of color forever and ever. An additional way to get more gene variation is this: when buying, try one of the females in the trio to be fertilized by another male, but not by their brothers. It is very good if this female is fertilized by several and possibly different males. Also try to get females from different litters, and, if possible, generally from different lines. By following these recommendations, you will receive the maximum possible amount of the gene pool from the former breeder.

Breeding lines
The general practice of guppy breeding dictates the following rules: Contain the maximum possible number of lines and cross them together at different stages. Keeping multiple lines is not a magical way to avoid the loss of gene variation. Different lines are closely related, and the gene variations in them are lost, regardless of whether many of these lines or few. However, spreading three lines, you use 6 individuals in each generation, even regardless of the possible bias in the sex ratio. This is better than breeding one narrow line (minimum two individuals), because the effective population size (ne) is large in the first case. With the content of several lines, the breeder can also focus his attention on some specific signs in each line, be it the shape of the fins or the intensity of body color, for example. Another way is the preference of one big line to several small ones. If a breeder sees that in a cross, in a large line, a sufficient number of individuals in each generation participates, and the sex ratio is approximately equal, then the content of one large line rather than a few small ones seems quite reasonable. The most common mistake here is to use only a few males and a large number of females in the cross. Using a larger number of males, the desired result is easily achievable, in a large group, each female will easily choose the most suitable partner. In practice, the utility of choosing a male by a female has been proven. If you give the female the opportunity to choose a male, this will cause a general positive effect. In particular, the fecundity of the female will increase and the health of the fry will be enhanced, but this is a topic for a separate article.

Source; Guppy Labs (published in 2002).