# heterozygous and homozygous



## firebelliedfreak (Nov 3, 2008)

i used to think that a heterozygous just ment carring the gene for but not showing and homozygous showing the gene

but i saw a thread saying a pastel was a heterozygous form and the super pastel a homozygous form


could someone please claer this up for me,
thanks :2thumb:


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## eeji (Feb 22, 2006)

pastel is codominant to normal, so het (one mutated gene, 1 normal gene) looks different to normal and **** (2 mutated genes) looks different to both


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## vetdebbie (Jan 4, 2008)

Homozygous means 2 identical copies of a gene.

Heterozygous means 2 different copies of a gene.

This all refers to the genotype of an animal.
The phenotype is how the animal actually looks. This is affected by how the genes react. If you have different copies of a gene, one may be dominant over another, one may be recessive, they may be codominant and equally expressed, codominant and unequally expressed..... etc etc.

I know naff all about python genetics, so I have to use corns. Most genes that are 'different' to normal are recessive to the wildtype colouration, so any heterozygous animals (ie one normal copy of a gene and one differnt copy) will appear phenotypically normal, but are obviously carrying the 'different' trait.

Anery for example is recessive to normal - with 1 anery and 1 normal gene you get a normal looking snake, 2 copies of anery and you get an anery snake.

Ultramel is a good example of a codominant gene - you get two different copies of a gene at same place, 1 amel and 1 ultra, and in the phenotype you get an animal that is not quite an amel and not quite an ultra (to be fair it is much closer to looking ultra than amel so the codominance is unequally expressed).

As far as we are aware tessera is dominant to normal, so with1 copy of the tessera gene yout get a tessera snake, and with 2 copies you get a tessera snake.....

As far as I am aware, a lot of python genes are dominant, and I think that what happened was, for example, with the pastel form originated, we got heterozygous snakes (but didn't know it as, really how can you tell without genome mapping), so everyone assumed it was a straightforward dominance issue. Then the super pastel came about, so it was realised that the heterozygous animal (pastel) is again a 'halfway house' between normal and super pastel - ie the genes for normal and pastel are codominant.


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## firebelliedfreak (Nov 3, 2008)

ok thanks so i think i get it

if i breed a pastel to a normal
i will get pastels say N for normal and n for pastel?
...N...n
N NN Nn

n Nn nn

so if i bred a noral to a pastel i would get 
25% normal 50% normal hets? or pastel and 25% pastel
or 25% normal 75% pastel but only 25% will carry gene for super?


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## NBLADE (Apr 11, 2007)

firebelliedfreak said:


> ok thanks so i think i get it
> 
> if i breed a pastel to a normal
> i will get pastels say N for normal and n for pastel?
> ...


 
pastel is het for super pastel basically, its a visual het form, but pastel in itself is a co dominant morph, 

normal to pastel would result in 50% being normal, and 50% being pastel, breeding two pastels together will lead to super pastels, as super pastel carrys two copies of the gene, so is a homozygous form.


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## paulh (Sep 19, 2007)

firebelliedfreak said:


> i used to think that a heterozygous just ment carring the gene for but not showing and homozygous showing the gene
> 
> but i saw a thread saying a pastel was a heterozygous form and the super pastel a homozygous form :2thumb:


There are two genes in a gene pair. All gene pairs are either homozygous or heterozygous (slang = het). A homozygous snake has a homozygous gene pair. A heterozygous snake has a heterozygous gene pair.

A normal-looking snake looks like most of the snakes in the wild. A normal gene is the gene that is the most common in a given location in the cell chromosomes in the wild snakes. A mutant gene is a gene that is NOT the most common in a given location in the cell chromosomes in the wild snakes. 

The following definitions of homozygous and heterozygous are standard genetics. Heterozygous meaning carrying the gene for but not showing and homozygous showing the gene is a distortion based on early experience with only recessive mutant genes.

The two genes in a homozygous gene pair are the same. Either two copies of the same mutant gene or two copies of the normal gene.

The two genes in a heterozygous gene pair are not the same. The gene pair may have either two different mutant genes or a mutant gene and a normal gene. If a mutant gene and a normal gene, the mutant gene can be dominant, codominant or recessive to the normal gene. If two different mutant genes, one mutant gene can be dominant, codominant or recessive to the other mutant.

Two genes (A and a) make three gene pairs, AA, Aa, aa. By the way, I am using A and a like unknowns in algebra. They can be defined however you wish, as long as they are potential members of the same gene pair.

If A is dominant to a, then the AA and Aa animals look alike, and neither looks like the aa animals. Three pairs of genes that produce two appearances.

If a is recessive to A, then the AA and Aa animals look alike, and neither looks like the aa animals. Three pairs of genes that produce two appearances.

If A is codominant to a, then the AA animals have one appearance, the Aa animals have a second appearance, and the aa animals have a third appearance. In other words, you can look at the animals and tell what the genes are. Three pairs of genes that produce THREE appearances. Tiger in the reticulated python and lesser platinum in the royal python are codominant to their respective normal genes.

You can always or almost always tell the heterozygous animals when the gene pair contains a codominant mutant gene and a normal gene. The pastel mutant gene is a good example. A heterozygous snake (with one pastel mutant gene paired with a normal gene) is a pastel royal. A homozygous snake (with two pastel mutant genes) is a super pastel royal.

Dominant and recessive mutant genes always or almost always require a breeding test to identify the heterozygous animals.


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## paulh (Sep 19, 2007)

firebelliedfreak said:


> ok thanks so i think i get it
> 
> if i breed a pastel to a normal
> i will get pastels say N for normal and n for pastel?
> ...


This Punnett square is not from a pastel royal mated to a normal royal. A pastel royal has a pastel mutant gene paired with a normal gene (Nn). A normal royal must have two normal genes (NN). If it was either Nn or nn, the royal would not look normal.

The above Punnett square is from mating a pastel royal to another pastel royal. The result is 
25% super pastel (nn)
50% pastel (Nn)
25% normal (nn)
(probability per egg, not per clutch)



NBLADE said:


> pastel is het for super pastel basically, its a visual het form, but pastel in itself is a co dominant morph,
> 
> normal to pastel would result in 50% being normal, and 50% being pastel, breeding two pastels together will lead to super pastels, as super pastel carrys two copies of the gene, so is a homozygous form.


It is best to distinguish between genotype (the identity of the genes) and phenotype (the snake's appearance). Pastel is both the name of the appearance and the name of the gene. Pastel is a codominant GENE because it is codominant to the normal gene. And the only way to know whether a mutant gene is dominant/codominant/recessive is to find out what the animals with the heterozygous gene pair look like. A morph is the phenotype produced by one or more gene pairs containing mutant genes. A morph is dominant/codominant/recessive only in so far as the responsible mutant gene(s) are dominant/codominant/recessive to the appropriate normal gene(s). 

Super pastel is only the name of the appearance. There is no super pastel gene.

So a pastel royal has the phenotype produced by a pastel mutant gene paired with a normal gene. A super pastel has the phenotype produced by two pastel mutant genes. Pastel is a codominant mutant gene, and that does not change whether there is one pastel gene or two pastel genes in the gene pair.


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## firebelliedfreak (Nov 3, 2008)

oh ok now i get it thanks:2thumb::2thumb::2thumb::2thumb:


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