Basic Genetics

  Quality Captive Bred Burmese Pythons

What's all this mumbo jumbo about homozygous, phenotype, double heterozygous etc...? Genetic terms and concepts can be tough to understand if you don't come from a biology background. Luckily, if you're just trying to better understand snake breeding, the genetics involved are pretty basic. I'm going to attempt to explain the most basic form of genetic inheritance known as Mendelian genetics. When you come across a boldface-linked word, click on it to jump to the glossary at the bottom.
 

Greggor Mendel was an Augustine monk in the 1800s, and was the first person to carryout an experiment that demonstrates the basic principles of heredity. He conducted his experiments using pea plants because of their available varieties and their ease of controlled breeding. He started with two true-breeding strains of pea plants. One had purple flowers and the other had white. In a typical experiment, Mendel would cross the differing varieties in order to make a hybrid. The true breeding parents are known as the P generation, their hybrid offspring F1, and allowing the F1's to breed yields the F2's. What Mendel found, was that crossing a white plant with a purple one resulted in all purple offspring, but when those plants were crossed to themselves, some of the F2 offspring were white. To understand why this happens, we first must understand how sexual reproduction occurs (from a DNA standpoint). To start, take a look over this figure showing the differences between Mitosis and Meiosis. These are the two means by which cells divide themselves and their genetic contents.

The daughter cells from Meiosis II are our sex cells (eggs and sperm also known as gamete). The fact that they are only ½ of our DNA is important because when they combine, ½ the DNA comes from mom, and the other ½ from dad, so that we all have two copies of every gene to make one complete set. A gene is defined as the unit of heredity for a particular trait, like eye colour for instance. There are many different variations of the gene that codes for eye colour, from brown to blue - and each alternative version of a gene like this is known as an allele. It follows then, that we get one allele from each of our parents, as each of their genes could be slightly different for any particular trait. If the two alleles differ, then the dominant allele is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the appearance. When an organism has a pair of identical alleles for a character it is said to be homozygous for that gene. When an organism has one dominant allele, and one recessive allele, it is known as a heterozygous organism and it will look the same as the homozygous dominant because when ever the dominant form is possessed - it dominates the other form. In order to show a recessive trait, you must be homozygous recessive meaning you got a recessive allele from your mom, AND a recessive allele from your dad. Because of dominance and recessive ness, an organism's appearance doesn't always reveal its genetic makeup, so the two must be distinguished. Phenotype is the organism's appearance, and genotype is its genetic composition.

Geneticists have devised a naming convention of sorts to deal with these terms that affect an organism's genotype. The dominant trait is assigned a capital letter, the recessive trait a lower case letter, and one letter is assigned for each allele or parent. The genotype for a heterozygous pea plant with purple flowers is written as Pp, showing that the plant has one dominant allele, and one recessive. A white flowered plant is written pp. I write albino as aa, and labyrinth as ll. All of my breeders are homozygous recessive for albinism, and heterozygous for labyrinth. Thus, their genotype looks like this: aaLl. Take a look at the following figure which should clarify things a little bit.

You may wonder why the heterozygous form will represent 50% of the F2 generation. This is because there are two different ways to make the heterozygous (there are two arrows missing from the figure can you find them?). From that figure, it may become apparent that genetics is a game of probability. In crossing two heterozygous animals (F1 in the figure), the probability that a particular F2 snake will be albino is the chance that both the egg and the sperm have the a allele. Because the parent has just one copy of each version, the probability that an egg or sperm will have that allele is 50:50. The probability of the sperm having the a allele is ½, and the probability of the egg having it is also ½. By the rule of multiplication, to find the probability of both of these events happening we multiply their individual probabilities (½ x ½ = ¼). ¼ of the offspring should therefore be albino, however just like you can flip "heads" many times in a row, so can you have more or less than ¼ of your offspring end up albino. The more offspring you have though, just like coin tossing, the closer to 25% the ratio will approach.

One additional consideration I might mention is outcrossing. When dealing with inbred populations such as homozygous recessive albino pythons, who were virtually all sired by one snake, it becomes important to preserve genetic variability in the rest of the genome. If genetic variation is lost, offspring health declines to the point of functional mutation and fertility declines rapidly. This can become particularly important when dealing with more than one recessive trait. The way we prevent this from happening is a technique known as outcrossing. When we outcross, we breed a homozygous recessive to a non-related, as distant as possible, homozygous dominant. The offspring from this cross are all heterozygous, and we cross them to one another to recover the trait we're interested in. Doing this puts more variation into the mix, and offspring that lend themselves to be more valuable breeders. My snakes are the product of a recent outcross and so there should be no problem breeding their offspring to one another. Sort of related to an outcross is a testcross, where we're trying to find out if an animal carries a certain recessive allele. This is shown in this figure:

Many people ask me what happens when you cross different snakes to one another. If the traits are Medelian inheritance the answer can always be found by drawing yourself a punnent square. Write out the genotype for each snake, and when doing multiple traits make sure to include all the possible combinations. The punnent square will give you all the possible genotypes for the offspring of the cross, and from there you can figure out the phenotypes. If a trait is recessive, then you must have both recessive alleles to be that phenotype. To find the likely hood of any one thing occurring, divide its occurrence by the total number of outcomes. In the case below I assume I had a true breeding granite crossed with a true breeding albino, and so their offspring (F1's) yield 100% double heterozygous, (they are het for each trait). The diagram below shows what happens when we cross the F1's, and what the F2 generation looks like.

Please realize that I've just barely scratched the surface of basic genetics, there are many different types of inheritance, Mendelian is just one. However, it is the most basic, and the most frequent form found in the snake world, so an understanding of Mendelian genetics will help you out significantly. If you should have any questions just give me a call, I can help you with most genetic based questions as I have a background in molecular biology and love talking about genetics with anyone who's interested!

Glossary:

Allele: An alternative form of a gene.

Dominant Allele: The allele that takes precedence over the recessive allele, and therefore shows itself in the population much more regularly.

Gamete: A haploid (one copy of DNA) egg or sperm cell, gametes unite during fertilization to form a diploid (two copies) gamete.

Gene: One of the many discrete units of hereditary information located on the chromosomes and consisting of DNA.

Genotype: The genetic makeup of an organism, usually discussed in terms of the two alleles whether they are homozygous or heterozygous.

Heterozygous: An organism that has two differing alleles for a gene, one dominant, one recessive.

Homozygous: An organism that has two identical alleles for a certain gene. Usually followed by the form of the alleles, for instance homozygous recessive, which would show the scarce trait.

Hybrid: An organism that is a cross between two differing varieties. Often times, this organism's genotype will be heterozygous.

Outcrossing: The act of maintaining genetic variability in mutant animal populations by means of breeding into the population unrelated animals and later recovering the recessive traits in the F2 generation.

Phenotype: The appearance of an organism. Having purple flowers is a phenotype while the plant's genotype may be heterozygous.

Recessive Allele: The allele that only appears as a trait when both copies are in its form.

Strain: A group of organisms within a species that can be characterized by some particular quality.

True-breeding: When parents reproduce, all of their offspring are the same variety. For example, in Mendel's experiments, all of his plants with purple flowers, when self pollinated, produced plants with purple flowers.

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