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Using Mendel's Laws in our Tanks

by George J. Reclos & Marina Parha

My initial intention was to keep this article as simple as possible but, as I was working on it, it became evident that even simplicity has some limits which, if ignored, will lead to misunderstanding.  Thus, I decided to add some definitions of the terms most commonly used when we discuss this kind of issues.

Gregor Mendel was a monk at St Thomas Abbey in Brno of the Czech Republic. He studied garden peas from 1856-63 published his 'paper' in 1866, which was pretty much ignored until 1900 That year 3 scientists independently came to many of the same ideas. What you see in the two pictures below is the traits which Mandel used in his experiments which led to his laws.

Cross (or mating) -- sexual reproduction between 2 organisms

Hybrid-- This term is used to characterize an offspring of parents which are considered to be genetically dissimilar. It is commonly used in two ways. Firstly, to indicate a cross between two animals belonging to different taxa (thus we can have inter-generic, inter-specific or intra-specific hybrids). Secondly, to indicate a cross between populations or breeds of the same species (e.g. an Ankara Cat with a British Blue cat.).

Phenotype-- observable difference between 2 members of the species (see below).

True breeding-- only produce their own phenotype when bred to self.

parental strains (P0 or F0 generation )-- originally crossed organisms.

F1 generation--  offspring of the F0 generation (parents). F (Filial) generation numbers indicate the numbers of generations that its members are removed from the original parent generation

F2 generation-- offspring of F1 generation crossed to itself (two generations from the original parent generation) – see below for the correct use of the F numbers when labeling your broods.

reciprocal cross-- switching the phenotype of the male and female parents

dominant-- phenotype visible in the F1 generation

recessive-- trait which reappears in the F2 generation after self cross

Gamete -- reproductive cell

Zygote-- fertilized egg

Mendel's Principle of Segregation:  In the formation of gametes, the paired hereditary determinants separate such that each gamete is equally likely to contain either one. Gametes have 1 "determinant", zygotes and adults have 2 (somatic cells)

Capitalized traits = dominant phenotypes (as in “N” in our examples below)

lowercase traits= recessive phenotypes, the majority of known mutations are mostly or entirely recessive (as in “a” in our examples below)

Classical Punett's Square is a way to determine ways traits can segregate (used in this article, too)

backcross:  mating F1 generation to the parental (usually recessive) strain

test cross: mating to a homozygous recessive individual

gene-- hereditary element that gets segregated

allele : particular form of a gene - a single gene may have multiple alleles (two in the case of diploid organisms like humans and fish). Alleles carry specific information about a trait, eg eye colour. In a pair of chromosomes the pair of sites where this information can be found is called allele. If the information is the same at both sites the instructions are homozygous; if not, heterozygous.

Homozygous - organism carrying two copies of the same allele. In simple words, the organism has the same information encoded in both genes encoding a characteristic

Heterozygous -  organism carrying different alleles of the same gene. The organism has different information encoded in the two alleles of the gene encoding for a characteristic.

genotype- genetic makeup of an organism (what alleles it contains – expressed or not). This can’t be changed and characterizes this particular animal during its whole life.

phenotype-- observable properties of an organism (expressed alleles)

dihybrid cross-- parental generation differs in two traits

Principle of Independent Assortment:  segregation of any pair of alleles is independent of other pairs in the formation of gametes (meaning that each gamete can have any combination of alleles, although still one allele of each gene)

One gene, one enzyme hypothesis:  each gene that has been mutated coded for exactly one single enzyme. Each gene carries the information for one enzyme or protein. Therefore, phenotypes are created by the loss of a particular enzyme function. It should be noted that some enzymes may be used in more than one pathway (ie. isoleucine and leucine may have several steps in common) so modification or lack of an enzyme may affect more than one traits (characterstics).

F generation numbers are to be used cautiously, since misunderstandings may occur. In cases of back cross or out crossing, these numbers are not valid anymore. In general when the fish bred do not belong to the same bloodline or F generation it is better to indicate the two F generations of that particular breeding. For example, if you cross an F2 generation male with an F1 generation female of the same bloodline, it is better to label the fry as F2 x F1. If you cross an F1 parent of one bloodline to an F1 parent from another bloodline, then the fry should be labeled as F1 x F1 and not F2. If you outcross an F1 parent with a wild caught specimen, you should use the F1 x F0 label.

As we have seen before, characteristics are either Dominant (will show in the phenotype if they are present in either one of the homologous chromosomes; the organism may be heterozygous or homozygous for that particular characteristic and still show it) or recessive (will only be expressed if present in both chromosomes; the organism must be homozygous for that particular characteristic).

If both characteristics are Dominant but the organism is heterozygous (carries two different genes for the same characteristic) then the result may be a phenotype combining both of them. As an example, pink flowers may be the result of the combination of a red Dominant characteristic and a white Dominant one.  In fact, this was one of the experiments Mendel himself did.

In other cases of heterozygous organisms carrying two Dominant genes, one of them is “silenced”, so the phenotype is determined by the one which is not. If we have a brood of such fry, not all of  them will have the same gene “silenced” so they may show both phenotypes

All gametes (the cells which will fuse to form the new organism) contain only one gene for any particular characteristic, in contrast to somatic cells which have two genes for the same characteristic.

Some commonly seen characteristics are recessive, one of them being albinism. We will use this as an example to see what takes place during breeding and what kind of phenotypes we will get in the end. This is a very simple and clear example because this characteristic is usually encoded in one gene, making it easy to analyze it genetically. Furthermore, it is important to some breeders which try to produce albino – generally sold at higher prices than the normal variety of the same fish.

In this case, if we breed two normal looking parents we may have the following combinations:

1)     Both parents are homozygous for the normal gene. Therefore, each parent will produce only one type of gamete, which we will represent as “N” for Normal.  We will use the classical Punett's Square we described above.  The first square is an “educational” one,  to show you what does each entry mean.

Gametes produced

Parent two,  gamete with allele 1

Parent two,  gamete with allele 2

Parent one,  gamete with allele 1

Offspring resulting from this combination of gametes (right x above)

1 1

Offspring resulting from this combination of gametes (right x above)

1 2

Parent one,  gamete with allele 2

Offspring resulting from this combination of gametes (right x above)

2 1

Offspring resulting from this combination of gametes (right x above)

2 2

 Now, we enter the gametes which will be produced in our case study, by breeding two normal fish which are both homozygous for the “normal” characteristic. Since they are homozygous, they only have ONE allele for this characteristic and since they are normal, this characteristic will always be “N

Gametes produced

Female  N

Female N

Male  N

N N

N N

Male  N

N N

N N

2) One of the parents is heterozygous, so although it looks normal, it carries the recessive gene for albinism, which we will represent as “a” – for albinism. We will also use a lower case letter to show it is recessive. If we assume that it is the female which carries it, then the female will produce two types of gametes while the male only one.

Gametes produced

Female  N

Female a

Male  N

N N

N a

Male  N

N N

N a

Thus we get fry with two different genotypes but all of them look normal, since a is recessive, so it must be expressed in both gametes in order to be shown in the phenotype. However, it is interesting to notice that half the fry will be carriers.

3) Both parents are heterozygous for albinism, so, although they both look normal, they both carry the a  gene. In this case, every parent will produce two different types of gametes and things start to get interesting.

Gametes produced

Female  N

Female a

Male  N

N N

N a

Male  a

a N

a a

Here, for the first time, we see the recessive characteristic being homozygous (a a ) so we will see this expressed in the phenotype. Since only one of the four combinations gives this homozygous result, 25% of the fry will be albinos, 50% will be normal but carriers and only 25% will be normal. This means that even if we presume that the albino fry will be eaten in the wild, due to their inability to blend in the environment, still, a large number of carriers will make it. In this context, it is evident that, even though wild caught albino specimens are hard to find, the genetic information for them is well established. However, one should not overlook the fact that natural selection keeps the possibility of an albino fry to far less than 10% since they will be produced in only one of the three combinations showed above and, even then, they will consist 25% of that brood.

In the photos below, you can see exactly what happens when you breed fish having this kind of combination - as Marina did in her tanks. A pair of normal - looking Ancistrus sp. 3 spawned (you can see the male guarding the eggs in the photo below and the female resting on the wood in the next photo) and the final brood consisted of almost 30% albinos and 70% normal coloured fry as you can see in the last photo (two of the albino fry are on the right side of the potato and one of the normal fry is in the middle).

Next, let’s assume that we found an albino specimen and we try to breed it with a normal specimen. In this case, there are only two possible combinations since the other specimen (the one which appears normal to us) can be either a homozygous specimen or a heterozygous one (a carrier)

If we have a homozygous normal specimen (F0 male) which we will breed with an albino specimen (F0 female) then the fry will have one of the following genetic combinations (4).

Gametes produced

Female  a

Female a

Male  N

N a

N a

Male  N

N a

N a

This is a very interesting combination. None of the fry will be an albino (all of them will look normal) although all of them are carriers. In this case, we will have a brood of fish which will seem normal but all of them can give birth to albinos if inbred. The funny thing is that by looking at the phenotype we get the impression that the albino characteristic is lost, while in effect it has been very well established since it is present in all the fry. We should also note that, no matter how many times we breed the parents, we will never see an albino fry.

Furthermore, if we try to breed any of the F1 fry with their normal father (F0), see case (2) above, we will end up with half the next generation (F0-F1) fry “losing” the albino gene and becoming absolutely normal.

In contrast, if we breed them with their albino mother, then we have the following combinations, which will eventually lead to half the F0-F1 fry being albinos and the other half carriers (selective breeding – (5)).

Gametes produced

Female  a

Female a

Male  N

N a

N a

Male  a

a a

a a

If we breed any two of the fry, we will be breeding two carriers (heterozygotes), which is the same as described in case (3) above.

Finally, if we breed any two albinos, we will be breeding homozygotes for this characteristic, so all the fry will be albinos (see below). 

Gametes produced

Female  a

Female a

Male  a

a a

a a

Male  a

a a

a a

This, rather simplistic approach makes apparent how a characteristic can be “lost”, “hidden” or even “reappear” during breeding. It also shows that what happens in captive breeding is different from breeding in the wild, since one key element is missing – natural selection. It is evident that selective breeding (or inbreeding) may quickly lead to homologous specimens for a recessive characteristic, desirable in this case. However, it will also increase the incidence of homologous specimens for other characteristics which may be undesirable or even bad for the health of the fish (deformities, enzyme deficiencies etc.). If we also take into account that some characteristics may be encoded in more than one genes, this process may increase them substantially, while in nature, the odds of getting a similar specimen may be practically negligible.

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