Genetics

Today, many of the most popular reptile species exhibit a wide variety of morphs. Understanding the genetics behind the morphs can make this hobby even more enjoyable. It also makes predicting the outcome of breeding projects much easier.

Background with some definitions:

The information that will guide the development of an organism is contained in its DNA. Every species has a characteristic number of DNA molecules, called chromosomes. An individual receives one set of chromosomes from each parent, resulting in two complete sets of information (this condition is known as diploid or 2N). So, chromosomes occur in pairs - an individual will have a pair of chromosome 1, a pair of chromosome 2, etc. These pairs are called homologous chromosomes. One member of the pair comes from the female parent, the other comes from the male parent. Looking more closely at a chromosome, we find that it is divided into fuctional units called genes. Genes are the units of heredity. Each gene codes for a particular protein that can influence the way an organism looks or functions. A gene for a particular trait can always be found at a certain location, or locus, on a specific chromosome.

While homologous chromosomes contain the same genes in the same locations (loci), the version of the gene they contain may be different. Different versions of a gene are called alleles. With many of the mutations we are interested in, there may be only two possible alleles to work with (wild type vs amelanistic in African fat tails for example), but frequently in nature there are three or even more potential alleles. Motley - stripe - wild type pattern in corn snakes is one example where we find three possible alleles. Keep in mind that any one individual can only have two out of all of the possible alleles for a trait (one on each homologous chromosome).

An organism forms gametes (eggs and sperm) through a type of cell division called meiosis. Meiosis separates the homologous chromosomes so that the resulting gametes have only one set of chromosomes (this condition is known as haploid or N). Since gametes have only one set of chromosomes, an individual can donate only one of the two alleles for a particular gene to each gamete.

We are primarily interested in the phenotype of our animals - what they look like, but knowing the genotype (which alleles are actually present) is also important for our breeding plans. The genotype lists which two alleles the animal actually has (one from mom, one from dad). If they are both the same, they are homozygous (homo = same). If they are different, they are heterozygous (hetero = different). If the trait is only visible when the animal has two copies of the allele, the trait is recessive and the animal is homozygous recessive. If the trait is visible when the animal has one or two copies of the allele, the trait is dominant and the animal may be heterozygous or homozygous dominant. By convention, we assign a letter to represent a trait - "a" for amelanistic for example. The dominant condition is represented by a upper case letter, recessive by a lower case letter.

Using the recessive amelanistic trait as an example:
AA - homozygous dominant - normal pigmentation
Aa - heterozygous - normal pigmentation
aa - homozygous recessive - amelanistic (pigmentation lacking melanin)

Inheritance of Recessive Traits

A recessive trait is only visible when an individual is homozygous recessive - possessing two copies of the recessive allele. One of these alleles must come from each parent, so we can only get offspring that show recessive traits if both parents have at least one copy of the recessive allele. In other words, they are either homozygous recessive (showing the trait themselves) or they are heterozygous for the trait. In leopard geckos, albino (all 3 strains), patternless, blizzard, and eclipse are all examples of recessive traits.

Inheritance of Dominant Traits

A dominant trait is visible when an individual has at least one copy of the allele. Homozygous dominant and heterozygous phenotypes are the same, so you can't visually distinguish them. As long as at least one parent has at least one dominant allele, you will see the trait in some of the offspring. Enigma is an example of a dominant trait in leopard geckos.

Inheritance of Codominant or Incompletely Dominant Traits

Codominance is a condition in which each allele retains its own specific homozygous expression in a heterozygote (A & B blood types in humans). In incomplete dominance there is a blending of the separate homozygous phenotypes (red and white makes pink in four-o'clock flowers). The heterozygotes may appear to be intermediate or they may have their own distinct appearance (Andelusian blue chickens, where black x white splashed = blue). In both codominance and incomplete dominance, the phenotype of the heterozygotes is different from both homozygous recessive and homozygous dominant. The convention in herpetoculture has been to call all of these traits codominant. In leopard geckos, Mack snow and giant are examples.

Inheritance of Line Bred Traits

Some traits don't seem to follow the patterns that we've discussed. These traits are often called line bred traits because the trait can often be increased by breeding the best examples together and then selecting the best of those offsrping to breed in the next generation. A wide range of potential phenotypes occurs instead of simply an on/off situation. These traits are polygenic, meaning that the trait is controlled by many genes. If the trait is dominant, the more dominant alleles an individual has, the more it will express the trait. Height and skin color are examples in humans. In leopard geckos, tangerine color is polygenic.

What will I get if I cross....
Leopard Gecko Morphs

Looking for more information? The following resources can help!!

Genetics for Herpers - This book does a wonderful job of explaining the basics of genetics and how it applies to reptiles. I found the information to be very clear and accurate. It is not written specifically geared toward any particular species, and is applicable for any.

Biologica - this program is free to download to your computer. There are modules that walk you through the basics of genetics and some more advanced concepts as well. You may be especially interested in the mutations modules. I use this program with my students and I feel that it is a very useful tool for understanding genetics - try it out!

Larry and BT interviewed Randy Remington about Ball Python Genetics on Reptile Radio. The discussion of general genetics is valid for any species and I think Randy did a pretty good job of clearly explaining the basics. The genetics discussion starts about 5 1/2 minutes into the show.

All images & content copyright 2005-2008 Nancy Wheat. Please do not duplicate without permission.