| T O P I C R E V I E W |
| Isoldael |
Posted - 08/01/2013 : 15:22:16
This is a guide I wrote to use on a facebook group and figured - why not share it here? Hope it helps. If it's not alright to post it here, please remove it ^^
Genetics is the study of heredity and variation of inherited characteristics. So what does this have to do with snakes? More than you might think! When breeding snakes, it's the genes that determine what the offspring will look like. In this document, I'll try to clarify some of the basic principles of genetics. For instance, how is it possible that, from breeding two normal corn snakes, you can get amelanistic offspring (red albinos)?
Thousands of genes, tiny units of heridity, can be found in DNA. DNA is basically a giant piece of code that determines the way every creature is built - it's unique for every person except identical twins. Understanding how DNA affects breeding, starts with understanding how DNA is passed on from parents to offspring.
DNA can be found in so-called chromosomes. These chromosomes come in pairs (corn snakes, for instance, have 37 pairs) - when reproducing, each parent will pass on one of the chromosome per pair to their offspring. This means that the offspring will inherit half of their father's DNA and half of their mother's DNA. A simplified representation of a pair of chromosomes:
Blue: DNA inherited from the father
Pink: DNA inherited from the mother
Many characteristics in snakes are determined by a single pair of genes, each of which is on one of the strands. The location of this pair of genes is called a locus (plural: loci). For instance, we could imagine the locus for a snake having black pigment or not as such:
Example:
There is a pair of genes that determines whether or not a snake has any black pigment. If both of the genes in this pair are "normal", the snake will have black pigment in its patterns. Examples of morphs that have black pigment are normals, anerythristic snakes, caramel snakes, etc. If the snake has two genes that are "amel" (short for amelanistic or missing black), the snake will have no black pigment. Examples of morphs that do not have black pigment are amelanistic snakes, snows, blizzards, etc. These can be easily recognized by their red pupils. But what happens when a snake has one "normal" gene and one "amel" gene?
First, we need to look at a few definitions that are commonly used when describing corn snake genetics.
Homozygous (short: homo or hom): when a snake has a pair of two identical genes for a certain characteristic. For instance, a snake that has two "normal" genes for the characteristic "expresses black pigment" or a snake that has to "amel" genes for that same characteristic.
Heterozygous (short: het): when a snake has a pair of two different genes for a certain characteristic, for instance, a snake that has "normal" gene and an "amel" gene for the characteristic "expresses black pigment".
So, in the case we are currently looking at, we have a snake who is heterozygous for amel - the snake has two different genes. What will this snake look like? Will it look normal? Will it look amelanistic? Or something in between?
That brings us to two more very important terms used in genetics - dominance and recessiveness. Certain genes will be dominant over others - this means that, when the snake has both a dominant and a recessive gene, the dominant gene will be expressed. Let's go back to our example of the snake that is heterozygous for amel. If we look at our "het amel" snake, we'll see that it looks like a normal snake that expresses black pigment. This happens because the "normal" gene is dominant over the "amel" gene. Consequently, the amel gene is recessive. This means that a snake will only really look like an amel when it has two amel genes (is homozygous for amel) on the locus for black pigment.
Interestingly, this means that you can have a snake of a certain colour or pattern (in our example, a normal looking corn snake) that still carries genes for a different colour or pattern! (in our case, missing black). What a snake looks like, regardless of its genes, is called the phenotype. So even our snake that has genes for both normal and amel still has the phenotype "normal". If we want to describe the genes a snake carries rather than its looks, we use the genotype. The genotype of our snake that is heterozygous for amel would be "normal het amel".
The fact that snakes can be heterozygous for certain characteristics is immensely important when it comes to breeding. As described earlier, snakes will pass on one of the strands of DNA (not necessarily the same one to each egg or sperm) when reproducing - this means that a snake that is heterozygous for amel could pass on the strand for amel, without passing on the strand for normal! So, say you have a male and female snake that are both normal het amel. If both snakes pass on the "amel" strand to a certain egg, then the hatchling will be a tiny little amel, even though both parents were phenotypically normals!
Because snakes will typically pass on one strand equally often as the other, if you know the genes of the parent you can make calculations as to which offspring you can expect from a certain pair and in which proportions.
Example:
Imagine we have a female that is a normal that is "het" for amel. The male has the same genes. As a notation for the gene that decides whether or not a snake has black pigment, I'll use a capital B for the "normal" gene (capital because it's dominant), and a lower case b for the amel gene (because it's recessive).
Pink: The genes of the mother (normal het amel). I've written down the gene from each of the strands that she might pass down in a seperate cell - this will help us determine the genotype of the offsprine.
Blue: The genes of the father (normal het amel).
Orange and Green: These 4 cells together represent the offspring that these two animals would have.
Orange: All the offspring in the orange cells has at least one capital letter B. As the B represents the "normal" gene, which is dominant over the lower case b (for amel), all these snakes will look normal (phenotypically normal). As you can see there is one cell in which the snake is homozygous normal (BB) and two cells in which the offspring is heterozygous for amel (Bb), just like their parents.
Green: The offspring in the green cell is homozygous for amel (bb). This means that they will actually look like amels (phenotypically amel).
So what are the advantages of making a such a table? Not only can you figure out what kind of offspring you will get, but you can also estimate how many you will get of each! It's important to know that you won't get exact numbers - 4 cells in the table does not mean that you will get 4 eggs out of which 3 are normals and 1 is amel. It just represents the proportions of your nest that are likely to show. In the example above, 3 out of 4 cells are phenotypically normal. Using these proportions, you can calculate the percentage of your clutch that is likely to be normal - in this case, that would be 75%. One in every 4 hatchlings is likely to be amelanistic - 25%.
When looking at the "normal" hatchlings from the example above, we've noticed that two out of three cells contain hatchlings that are heterozygous for amel. Using this statistic, we can calculate the chance that a normal snake from this pairing will be heterozygous for amel. A 2 in 3 chance roughly translates to 66% - you could sell these normal hatchlings as "normal 66% phet amel" - here, phet stands for possible het. This means you can't be certain that the snakes carry the gene for amel - only breeding the snake with the right partner will prove whether or not the snake has the gene.
In corn snakes it's not uncommon for a snake to have more than one het. Regardless of the number of hets, you can still make a table - you just have to add more rows and columns. It looks a lot more complicated, but if you systematically write down all the possible gene combinations it's really not that difficult.
Example:
The previous example was about getting amelanistic hatchlings from two normal parents. Remember, amelanistic (melanistic stands for black pigmented) means having no black pigment. There is a similar gene that determines whether or not a snake has any red pigment. Snakes that lack red pigment are called anerythristic (erytristic stands for red pigmented). The gene for anerythrism (short: anery) is recessive, just like the gene for amelanism.
We have now seen a gene that removes all the black pigment, and a gene that removes all the red pigment. What happens if you combine the two? Well, a snake without any red or black pigment is called a snow - completely white snakes (with the occasional pink or yellow marking) with red eyes.
For this example, we're going to use both amel and anery genes. We'll take an imaginary female that is homozygous for amelanism (bb) and heterozygous for anery (Rr). Our imaginary male will be a normal that is heterozygous for both amel and anery (Bb and Rr). In this notation, I've chosen for an R to indicate whether or not the snake has red pigment - the capital R stands for the "normal" gene which is dominant, the lower case r stands for the "anery" gene, which is recessive. For black pigment I'll use the same notation as in the previous example - B and b.
Pink: The mother's genes (amel het anery).
Blue: The father's genes (normal het amel anery).
If you look at the parent's genes, you'll see that I've written out all possible combinations of the genes they could pass on. In the mother, there are only two options - she'll pass on the amel gene anyway as she doesn't have a normal gene for black pigment, but she can still pass on the gene for anery or the gene for normal. For the father, things are a little more complicated as he is heterozygous for both traits - for him, there are 4 different options. The more different genes you have, your table will get exponentially bigger - you will get more and more possible combinations. For the offspring, you'll see that each hatchling has 4 different genes to determine colour now - 1 from each parent for each trait, so two to determine if they have black pigment and two to determine if they have red pigment.
Orange (3/8): Phenotypically normal. Note that all the normals are heterozygous for amel - this is because the mother cannot pass on a "normal" gene for black pigment. As the offspring always has exactly 1 gene from the mother for this trait, they have to have at least one "amel" gene.
Grey (1/8): Offspring that is homozygous for anery. These snakes will actually look like aneries and lack all red pigment. As you can see, all these hatchlings are het amel too, for the same reasons as explained at orange.
Green (3/8): Amelanistic offspring. These are homozygous for amelanism. 2 out of 3 cells contain snakes that are heterozygous for anerythrism - you would call these amel het anery.
Yellow (1/8): These snakes are both homozygous for amel as for anery. This means that they are snows – they don't have any red or black pigment.
Again, you can extend these tables for as many hets as you would like. The more you add, though, the more confusing it gets. Easier than drawing these tables yourself, is to use a calculator. An example of such a calculator is the corncalc. There, you can simply indicate the genes of your snake and click "calculate". The program will tell you exactly what offspring you will get (both phenotypically and genotypically) and in which proportions. You can find the corncalc at http://www.corncalc.com/.
Most non-normal colours in corn snakes are recessive. That means that in most cases, the snake will have to be homozygous for a certain colour to actually be that colour. With all the different genes for colours that can be combined there is an enormous number of phenotypes and an even larger number of possible genotypes for corn snakes. A list of most of these phenotypes including pictures can be found at Ians Vivarium: http://iansvivarium.com/morphs/species/elaphe_guttata/.
Apart from different colours, corn snakes also come in different patterns. So far, we're aware of three genetically determined patterns: motley, stripe and tessera. These genes act a bit different from the recessive colour genes.
Tessera is the most recently discovered pattern modifying gene. Snakes that are phenotypically tessera will show two dorsal stripes that are the colour of the saddles as well as the normal lateral pattern any snake with a normal pattern would have. The special thing about this gene is that it is dominant to the gene for a normal pattern - even if your snake is only het tessera, the snake will still have the tessera pattern.
Example:
To show the difference, here is another table. In this example we will breed a normal patterned female (homozygous for a normal pattern) with a male that is heterozygous for tessera (here, the male will display a tessera pattern). I'll mark the gene for tessera with a capital T (dominant) and the gene for a normal pattern with a lower case t (recessive).
Pink: The mother's genes (normal patterned). Note that I have left out the bottom row, as it would lead to the exact same results as the first row and would not lead to any different proportions.
Blue: The father's genes (het tessera).
Green: Tessera offspring
Orange: Normal patterned offspring
Here, half of the offspring gets the tessera pattern even though the mother didn't carry any tessera genes! This is part of what makes the tessera gene so terribly interesting for breeders.
Motley and Stripe appear to work in the same way as the colour genes - they are recessive to the dominant normal pattern. What's special about these two patterns though, is that they take up the exact same spot on the DNA (the locus). This means that, when a snake is heterozygous for both motley and stripe, there is no room left on the locus for a "normal" pattern gene. In cases like this, the snake is almost always phenotypically motley, even though it only has one motley gene. Breeding with snakes like these will result in the snake passing on either a motley gene or a stripe gene.
Here's how to imagine how these two genes work. I've used N for a normal pattern (dominant), s for stripe (recessive to normal) and m for motley (recessive to normal):
NN: Normal patterned snake, no pattern hets
Ns: Normal patterened snake, het stripe
Nm: Normal patterned snake, het motley
ss: Stripe
mm: Motley
ms: Het motley, het stripe, phenotypically motley
Note that not all traits are passed down on a single gene - many traits are a combination of many, many genes and cannot be bred so easily. To get a certain trait in a line of snakes that isn't determined by a single gene, breeders use so-called selective breeding. This basically means you take two snakes that show the trait you want to express in your line and breed them together. From the offspring, you'll take the hatchlings that show the trait best and continue on breeding with those, until you get the looks you like. A very good example of such a selectively bred line is the Abbot's Okeetee: http://iansvivarium.com/morphs/species/elaphe_guttata/abbotts_okeetee/?sid=. These snakes are selectively bred okeetees with the intention of making the black borders surrounding the saddles much wider and more pronounced. Breeding this snake with a normal looking snake would likely result in normals that have a look somewhere in between the normal and the abbot's okeetee. A few other selectively bred types that you may have heard about are bubblegum snows, aztec / zigzag, bloodred, pastel, silver queen, etc.
These are some of the basics of genetics that you will hopefully find useful in breeding corns. Good luck with breeding! If you have any questions, comments or requests for more information to be added, don't hesitate to contact me or post questions here on the forum.
-Isoldael |
| 5 L A T E S T R E P L I E S (Newest First) |
| Isoldael |
Posted - 09/01/2013 : 09:56:17
The trick is to take some time in between readings, play around with the terms a bit to get familiar with them and only then read on. It's not that easy, so it's not weird that one wouldn't understand everything straight away! |
| sue2012 |
Posted - 09/01/2013 : 00:07:20
I got lost at the title genetics how does it work i did read everything but my brain fried by the end lol |
| Isoldael |
Posted - 08/01/2013 : 18:48:11
quote:
Originally posted by Charles
Excellent. However, as a Biology teacher I am afraid I can't help making a comment about this paragraph:
"DNA is built up out of two strands. When reproducing, each parent will pass on one of these strands to their offspring. This means that the offspring will inherit half of their father's DNA and half of their monther's DNA. A simplified representation of the strands of DNA:"
A molecule of DNA is made from two strands. However, after that you need to use the word Chromosome instead of strand. I would suggest leaving out the first sentance (Only relevant if you want to talk about DNA reproduction) and then start with something similar to:
"The DNA of an organism is found in its Chromosomes (46 in humans, 74 in corn snakes). The Chromosomes come in pairs, one inherited from each parent so a human has 23 pairs and a corn snake 37 pairs of chromosomes. A simplified representation of a pair of chromosomes:"
Hope you don't mind this suggestion.
I don't mind at all, thanks :). I was wondering if I should bring up chromosomes and I wrote the whole story including chromosomes first, then I realized it made things unnecessarily complicated and removed bits and pieces. Apparently I ended up mixing up two of my paragraphs... silly me. I would edit it, but I'm afraid it's impossible for me to edit first posts. Could one of the admins replace the "Dna is built up out of ..." sentences by the following?
quote:
DNA can be found in so-called chromosomes. These chromosomes come in pairs (corn snakes, for instance, have 37 pairs) - when reproducing, each parent will pass on one of the chromosome per pair to their offspring. This means that the offspring will inherit half of their father's DNA and half of their monther's DNA. A simplified representation of a pair of chromosomes:
Thanks for the correction and thanks in advance to whoever will correct my first post :) |
| a33272 |
Posted - 08/01/2013 : 17:21:22
 I got lost right at the top....... |
| Charles |
Posted - 08/01/2013 : 17:05:47
Excellent. However, as a Biology teacher I am afraid I can't help making a comment about this paragraph:
"DNA is built up out of two strands. When reproducing, each parent will pass on one of these strands to their offspring. This means that the offspring will inherit half of their father's DNA and half of their monther's DNA. A simplified representation of the strands of DNA:"
A molecule of DNA is made from two strands. However, after that you need to use the word Chromosome instead of strand. I would suggest leaving out the first sentance (Only relevant if you want to talk about DNA reproduction) and then start with something similar to:
"The DNA of an organism is found in its Chromosomes (46 in humans, 74 in corn snakes). The Chromosomes come in pairs, one inherited from each parent so a human has 23 pairs and a corn snake 37 pairs of chromosomes. A simplified representation of a pair of chromosomes:"
Hope you don't mind this suggestion. |
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