Quick blip on heritability

geckolabs

New Member
Messages
327
Location
Virginia
The general biological concepts that govern the the odds of producing certain morphs is something that is often overlooked in the hobby today. People rely on the Reptile Calculator, or expect other people in a facebook group to give them the answer. People should, and need, to get back to learning how to do these things on their own, and actually develop an understanding of how traits are inherited.

So to give people a boost, I'm going to run through it with the accompanying diagram as an example.

Every animal possesses a large amount of genetic information, which ultimately, dictates how that animal will look, among other things. The entirety of the genetic information is called the genome. A genome is all ofthe genetic material an animalpossesses. Every cell of the animal (forthe most part) has an entire copy of theanimal’s genome, packed into the cell’snucleus (figure 1A-D.) The genome is split up in each cell, to smaller units, called chromosomes (figure 1D.) There are two copies of every chromosome, which do not always contain the same information. One copy of each is passed down from the mother, and the other copy is passed down from the father (figure 2.)


fig1.jpg
Figure 1. Taking a closer look. (A) Eublepharis macularius head shot,zooming in to detailed skin and outlined skin cells (B and C,respectively). (C) Individual skin cell is outlined and a nucleus isdramatized. (D) Skin cell from C is animated, showing the nucleus(gray dashed circle), and the chromosomes/chromatin (blue material withinnucleus). All of the chromatin collectively make the genome. (E) Asegment of chromatin/DNA is shown. (F) Linear sequence of DNA.


When you're trying to figure out the 'morphs' of offspring you can produce from a given pairing, you need to know what the pairing is, genetically. There are various types of traits, some of which are not directly heritable, in that they are not simply passed down from parent to offspring.

Of the heritable traits, traits are either recessive, dominant, or incomplete dominant. Traits that are not directly heritable are referred to as polygenic, or line-bred.

So what do all of these terms mean?

  • Dominant trait: A dominant trait only needs one copy to show as a phenotype. In other words, if an animal were heterozygous for a dominant trait, it would still show a phenotype.
    • For dominant traits:
      Dominant traits require only one copy of a trait to manifest as a phenotype.
      This is often where you will see 'c' or '2c' if you look around on the reptile calculator. Where 'c' means an animal has one copy of the trait (i.e.- mack snow), and '2c' means the animal has two copies of the trait (i.e.- Super snow).

  • Recessive trait: A recessive trait requires two copies to show as a phenotype.
    • For recessive traits:
      Recessive traits require two copies of the trait to visual manifest as a phenotype. An animal that is heterozygous for a recessive trait only carries one copy, therefore the trait is not seen as a phenotype in a heterozygous animal.

      1) Visual (VV) x Visual (VV)= 100% visual offspring (VV)

      2) Visual (VV) x Heterozygous (Vv)= 50% Visual (VV), 50% Heterozygous (Vv)
      Therefore, if your pairing is VV x Vv, 100% of the animals that are not visual will be heterozygous *This is where 100% het labels come from. Sometimes animals labeled as 100% het are ones that hatched as possibly heterozygous for a particular trait, and have since been proven to be het through test breeding.

      3) Heterozygous x Heterozygous (Vv x Vv)= 25% Visual (VV), 50% Heterozygous (Vv), 25% not het (aka normal, vv)
      Therefore, if your pairing is Vv x Vv, an average of 25% of the offspring will be visuals, and the remaining 75% will be unknown whether they are het or not. Therefore, non-visuals from this pairing would be labeled as 66% het, meaning there is a 66% chance that each of the non-visuals are heterozygous for the recessive trait in question.

      Examples:

      1) Rainwater albino x rainwater albino = 100% rainwater albino offspring

      2) Rainwater albino x het rainwater albino= 50% rainwater albino offspring, with the non visuals being het rainwater albino

      3) het rainwater albino x het rainwater alibino= 25% rainwater albino offspring, with the non-visuals having a 66% chance of being het rainwater albino

  • Polygenic [phenotype]: Polygenic, as we know it in our hobby, refers to a phenotype that is the result of line breeding animals for a desired look.

  • Incomplete dominance: Incomplete dominance is when you cross two animals with different phenotypes, and the offspring demonstate a blend of the two phenotypes. An example of this in leopard geckos would be Mack Snow, which have been crossed in to many varying phenotypes, causing a blended phenotype from expressing the Mack Snow allele and the wild type allele. Because these traits express both alleles, Mack Snows also have a 'super' form, when there are two Mack Snow alleles.


Using the scenario laid out in figure 2 as an example, lets pair a male Tremper albino het Eclipse to a female Normal het Eclipse & Tremper albino. To break this down, that makes the male's genotype represented by ttEe, as it possesses two copies of the Tremper albino allele, and only one copy of the Eclipse allele (two lowercase 't's to represent the two copies of the recessive Tremper albino trait, and one lowercase 'e' to represent the presence of only one copy of Eclipse.)

The female is therefore represented as TtEe, as she only possesses one copy of each allele. This is depicted in the 'X' structures in figure 2, where the male possesses a green segment on both copies of the chromosome, but an orange segment on only one chromosome. Likewise, the female only possesses one green and one orange segment.

When reproducing, an animal will only pass down one copy of the chromosome. Note that just because the female seems to have the orange and green segments on opposite copies of the chromosome, the two can still be passed down together. This is because of a process called homologous recombination that happens when the sperm/egg are produced. I will not go into the process here, but here is a link that explains it for those interested (Genetics Basics Lesson 5: Meiosis.)

Now that we know that homologous recombination can occur, we can determine what possible combination of traits the male and female can pass down. The answer to this question is that the male can only pass down two possible combinations, Tremper albino (tE), and Tremper albino and Eclipse (te), whereas the female can pass down four possible combinations- TE, Te, tE, and te.

When the two copies of parental DNA mix to fertilize an egg, it determines what the hatchling will be. In this case, we end up with six potential combinations, with the odds of each in parentheses before them:
(1/8)TtEE- Normal het Tremper albino
(1/4) TtEe- Normal het Tremper albino & Eclipse
(1/8) Ttee- Eclipse het Tremper albino
(1/4) ttEe- Tremper albino het Eclipse
(1/8) ttEE- Tremper albino
(1/8) ttee- Tremper albino Eclipse

fig2.jpg

This always gets people asking "if I can produce Normal het Tremper and Normal het Tremper & Eclipse, how will I know which animals are het for Eclipse?"
The answer is that you don't know which are het and which aren't without test breeding to try and prove it out. The animals that are not visual Eclipse from this pairing will be 50% possible het Eclipse (refer back up to the section discussing recessive traits for an explanation on percentages.)

I hope this helps many. If you have questions, feel free to ask.

-Dillon @ Gecko Labs
 
Last edited by a moderator:

sapphira80

New Member
Messages
21
Location
Louisville, KY
Thanks for posting this! I totally agree that breeders should learn about the genetics of the animal they're breeding. I think the jargon scares some people, but once you get the hang of it, it's a pretty simple concept. Calculators are useful for double checking your own calculations, but relying on them completely probably isn't a good idea.
 

wicked gecko

New Member
Messages
58
Location
Wisconsin
I was lucky enough to find this on fb also but it's nice to see things broken down for us newbs from time to time! Still researching allot but got my first worthy male a few weeks ago and working on acquiring a great breeding group in time for the 2018 season, if not it will be 2019 lol
 

staceyleigh

New Member
Messages
369
Location
CNY
This is great!. I am one guilty of using the Reptile Calculator quite often! Is there a book you would recommend to further learning? Something not too advanced.
 

geckolabs

New Member
Messages
327
Location
Virginia
This is great!. I am one guilty of using the Reptile Calculator quite often! Is there a book you would recommend to further learning? Something not too advanced.
This book is one that should be pretty useful in regard to learning and understanding various concepts at play in our typical breeding scenarios. I don't know how well it goes into mendelian inheritance, but I think it likely covers it at least in some depth.

https://books.google.com/books?id=JPHcBAAAQBAJ&dq=basic+genetic+heritability
 
Top