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| genetics |
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Many shrink back when they see this word! But actually that is a shame because once one has understood the secrets of genetics there are many interesting possibilities in breeding reptiles and, of course, other species!
We try to give the reader an insight in genetics in a relatively simple way. Therefore we have created some diagrams that are supposed to illustrate what is said in the text.
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| history of genetics: |
Every living creature gives birth to creatures that are like it. Besides the parental characteristics the offspring show individual traits. Already back in antiquity people occupied themselves with family-similarities on a philosophical basis:
Platon (427-347 aD) believed in a natural unequality of all human beings that was inherited from their parents. That is why different man are made for different tasks. He demanded a separation of the best of men, with whom he wanted to form an elite that should save the nation from economical and political decline.
Aristotles (384-322 aD) noticed that children are similar to their parents, not only what the whole body and different characteristics are concerned but that: "furthermore descendants look like ancestors even though nothing from them got into the semen. Similarities are carried through several generations..."
It was not before the 18th and 19th century that people found out more about inheritation through systematic inheritation-experiments and breeding-results. But even though many experiments were made and some regularities could be found, Mendel (1822-1884) wrote in his paper "Experiments in Plant Hybridization" (1866):
"that of numerous experiments there was non that was carried out in a way and to such an extend that it would be possible to give any numbers when generally talking about the descendants of hybrids. But the given figures of different generations can be put in an order and like this, numerical relations can be found out."
In his crossbreedings Mendel was able to determine numbers that allowed him the formulation of his principles of heredity. The results of his research became the foundation of classic genetics. It makes use of many expressions that are important for it's comprehension.
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| basics of genetics / expressions: |
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All higher-developed species reproduce sexually. Sperm and ovum (eggcell) fuse and together form the zygote, from which the new individual will develop. To understand genetics you will have to know that each cell in the body is diploid, which means, it carries a set of doubled chromosomes (so called homologous chromosomes) in it's nucleus in which the totality of the genetic information (DNA) with all genes (codes for different characteristics) is saved. The Germ-cells, though (sperm-cell and ovum) are haploid which means that they only contain one half of the set of chromosomes. The nucleus of the sperm only contains the DNA of the father, the ovum's nucleus only carries the DNA of the mother. After the fusion the zygote is diploid, half it's DNA comes from the father (sperm), the other half from the mother (eggcell).
Two genes on the different halfs of the set of chromosomes that are responsible for the same characteristic are homologous genes. If the information for a characteristic is identical on both of the homologous genes, the individual is homozygot for this characteristic. That means that the individual will show the characteristic. If the informations are different from each other that is called being heterozygot for a certain characteristic. (In this case one of the characteristics will be expressed: The individual is heterozygot for the other, the hidden characteristic.)
dominant-recessive rule of heredity:
In this context it is important to know of the existence of dominant and recessive genes. Dominant genes are always expressed in the so called phenotype (outward appearance of an individual). Dominant genes prevent recessive genes from having a phenotypical effect. Only when both genes in the pair of homologous genes are recessive their characteristic is expressed in the phenotype.
The heredity of most color variations of snakes for example follow the dominant-recessive rule of heredity. (Since I don't want to confuse you, I am not going to talk about the few exceptions!)
The genes that are responsible for the color-mutations of your snake are mostly recessive genes. That means, that the color is only then expressed in the phenotype, when both, mother and father of the snake carry the gene for the color-mutation. Remember that the gene for the wild-type (the way a snake usually looks in nature) is always dominant!
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| symbolism |
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For explanation of genetics capital letters and small letters are used. Capital letters stand for dominant genes, small letters for recessive genes. Since there is always a pair of chromosomes in every cell (except for germcells) the diagram shows two of each color-gene.
- Wildtype (WW) - because the wildtype is always dominant capital letters for the two genes are used.
- amelanistic (aa) - amelanism is recessive, that's why two small "a" for both of the homologous genes are used.
The germcells of the two individuals look like that:
Wildtype: 1. ( W ) 2. ( W ) - Amelanistic: 3. ( a ) 4. ( a )
There is only a single chromosome in each germcell. Because germcells are made from "normal" cells, that contain a pair of chromosomes, one "normal" cell devides into two (!) germcells. I will always write down both germcells, because otherwise information that is relevant for the genetic would be lost! (You will see this later, for example when looking at the F2-generation of the second principle of heredity.
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| Mendel´s laws: |
First law (principle of uniformity):
If two individuals of a kind are crossed, that differ in one characteristic for which they are homozygot, their offspring, the F1 (first Filialgeneration) are all the same what this one characteristic is concerned.
example: if you crossbreed a wildtype male (WW) with an amelanistic female (aa), they differ in color, but are both homozygot for their color.
If you want to know what the results of this cross will be I recommend a cross-diagram (see diagram 1):
As you can see in the diagram, the result of this crossbreeding is that the F1 generation is phenotypical identical with their wildtype father.
Second law (principle of segregation):
If you cross two individuals from the F1-generation, their offspring, the F2-generation, is not uniform-looking but devides into a certain numerical relation. With the dominant-recessive heredity this relation is 3:1 phenotypical. (As you can see in diagram 2, three-quarters of the offspring will look like the wildtype-father. One-quarter will look like the amelanistic mother.)
Genotypical (DNA) the relation will be 1:2:1. This means that one-quarter of the offspring will have the genes of a wildtype animal. Half the animals will carry the wildtype gene as well as the amelanistic-gene (they look like a wildtype, because the wildtype gene is dominant). The last quarter of the F2 generation will be amelanistic animals, that only carry the amelanistic-gene.
Third law (principle of independent assortment):
If you cross two individuals of a kind, that both are homozygot for several characteristics in which the two differ, the two laws mentioned above work for each characteristic. Besides those combinations of characteristics in the P-generation (parental generation) there are a few new combinations in the F2 generation. (See diagram 3!)
Keep in mind that in this graph only those characteristics are shown that are relevant for the crossbreeding. In this example you will find the colors yellow and black. (The third color that can be found in a wildtype gartersnake, red, is left out to make it easier for you to understand. It will show in the phenotype of the offspring.) The colors you can see in the circles show, which color is expressed in the phenotype of which individual.
With these three laws every breeder should be able to know what the offspring of his snakes will look like and if they maybe are carrying a recessive gene for a special color-variation in their nucleus. |
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