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Recessive Genes - An animal that looks Normal and carries no known recessive genes is called "Normal". - An animal that looks "Normal" but carries a recessive gene is called "Heterozygous" or "Het". - An animal that carries and shows the recessive gene, i.e. the snake IS an albino, is called "Homozygous" or "Homo". How recessive genes work Without getting too technical, this is the way it breaks down: each trait that an animal shows, such as color and pattern, is controlled by two genes. It gets one version of each gene from each parent. Recessive genes are genes that, when paired with a matching recessive gene, will show that trait. If a recessive gene is paired up with a dominant gene, the dominant gene will "override" the recessive gene and the dominant gene will show its trait. In the examples below, we are trying to determine whether or not we will produce any Albinos when we breed our pair of snakes and how many Homozygous and/or Heterozygous snakes we can expect to produce. So, let's say each parent carries the gene for Albinoism but doesn't show it, they are both Heterozygous or Het for Albino. This would be represented by an upper case "A" for the "Normal" version of the gene and a lower case "a" for the recessive "Albino" version of the gene. Each parent can either pass down the Normal "A" or the Albino "a" gene to its offspring. If both parents pass down the recessive Albino "a" gene to a given offspring, that offspring would show the Albino trait (aa). However, if one parent passes down the recessive "a" gene and the other passes down the dominant "A" gene, the dominant "A" will control how the snake looks (Aa) and the offspring would show the Normal trait. In order to see which genes a parent can pass down to its offspring and how those genes would interact with the other parent's genetic makeup, we will use a Punnett square. How the Punnett square works The Punnett square is a tool designed to show the traits that offspring can be expected to inherit from the pairing of two animals. It is not designed to predict exactly how many Homozygous or Heterozygous animals you'll get from breeding a pair. It's more to give you an idea of the ratios you can expect. In the examples we're using here, the genes one parent can pass down are shown in the grey boxes on the top row and the other parent's genes for the same trait are shown in the left hand column. Each possible combination of genes is entered into the parent's boxes. We will only be working with the Albino trait in the examples below. The genes one parent can contribute.
The genes the other parent can contribute.
The genes from each parent are then passed down/across to the corresponding boxes (white squares) until the table is complete. The white squares represent any possible combination of genes that can be inherited from this pairing. Once the table is complete, you will be able to see which genetic traits each offspring may inherit.
- A Normal will be represented by two uppercase A's = "AA". - A Het will be represented by one uppercase and one lowercase A = "Aa" - A Homo (Albino in this case) will be represented by two lowercase A's = "aa" Here are a few samples of the probable genetic outcomes when breeding snakes that carry recessive genes. Sample 1 A Punnett square showing Normal x Normal breeding.
Here we see that neither parent carries the recessive gene so there is no chance of any of the offspring inheriting the Albino trait. Sample 2 A Punnett square showing Normal x Het breeding.
In this case, we would have all of the offspring looking Normal (AA) but half of them would be Hets (Aa). Since we can't tell the Hets from the Normals, all of the offspring would be called "50% possible Hets" because each snake would have a 50% chance of being Heterozygous (Aa). Sample 3 A Punnett square showing Het x Het breeding.
Here we would have 25% of the offspring being Homozygous (aa) for Albino. The remaining 75% would look normal but two out of every three would be Heterozygous (Aa). Since it is impossible to tell which two of the three are Hets (Aa) and which one is a Normal (AA), all three are called "66% possible Hets". Sample 4 A Punnett square showing Homo x Het breeding.
From this pairing, you would have 50% of the offspring being Homozygous (aa) for Albino and the rest would be Heterozygous (Aa). Sample 5 A Punnett square showing Homo x Homo breeding.
All of the offspring from this pairing would be Homozygous (aa) for Albino. A few final notes I used some "extra" squares in some cases for the sake of continuity. When a parent is Homozygous, it is redundant to include both genes (AA or aa) in the parent's boxes, thus only one square is needed to represent that trait. When doing a Punnett square involvning one Heterozygous and one Homozygous parent, the table should look like this...
Note: The outcome here is the same as in Sample 4, only simplified. I opted to not use some of the more confusing but accurate "scientific" terms in an effort to keep things simple. Most of us merely want to be able to predict the outcome of single or double trait pairings and a lot of the technical and more detailed aspects of genetics are not necessary to accomplish this. One web page that covers more about the complex aspects of snake genetics is www.Serpwidgets.com. Please visit this useful and insightful site if you want to understand more of the complicated side of genetics. Double trait Punnett squares So now we can easily predict the probable outcome when breeding our pair of snakes using a Punnett square. But what if our snakes carry two recessive genes? Now we will breifly go over how to setup a Punnett square for animals with more than one recessive gene. The first step is to determine the possible genetic combinations each parent can pass down to the offspring. Let's say we are trying to produce the highly sought after Snow Ball Python. A Snow Ball Python is Homozygous for both Albinoism and Anerythrism, i.e. they are Amelanistic and Anerythristic. We will do this by breeding a pair of "Double Hets" As above, we will use the letter "A" to represent the Albino trait and we will now use the letter "E" to represent the Anerythristic trait. So the genes each parent can contribute are slightly more complicated than the single trait examples above. Each parent possesses the following genes: "Aa" and "Ee". So the possible combinations are as follows: - AE - Ae - aE - ae Those combinations are entered in the parents' boxes in the Punnett square as shown below.
Then as in the examples above we simply fill in the white boxes to determine the traits each offspring may inherit.
So, there you have your double trait Punnett square. You can see that there is only a 1/16 chance of producing the Double Homozygous "Snow Ball Python". You also get much more complex odds as to which of the normal looking offspring would carry which traits. Hopefully now you better understand how recessive genes work and can start to experiment with your own Punnett squares to understand some of the "science" and odds behind producing the latest morphs. Thanks, | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
