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Old 15-02-2012, 11:12 PM
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Default Genetics 101

Genetics 101

Introduction

All animals are made up via a set of instructions known as DNA. DNA is simply a recipe book that gives the instructions on how to make the animal. Each animals DNA is made up of lengths of code that give the instructions to produce a particular pigment or to make a particular pattern. Each character code is called a gene.

All sexually reproducing animals have two copies of every gene. One of each gene pair comes from the mother and the other comes from the father. Sometimes there are different versions (or mutations) of the same gene (i.e. the gene for 'eye colour' in humans comes in different versions such as green, blue, brown etc).

We call different versions of the same gene, alleles. We call the position of each gene on the DNA strand the locus. If both of the alleles of a particular gene pair are the same, we say that the animal is homozygous for that gene. If the two alleles are different, we call the animal heterozygous at that locus
When we describe an animal in terms of its genetic make-up we say we are describing its genotype. When we talk about what an animal actually looks like, we say we are describing its phenotype.

Gene interaction

Before we get on to actually working out how different mutations are inherited, there is one more piece of information that we need. That is the correct terms to describe how gene pairs interact to produce different phenotypes and the names given to them.

Looking at a single gene with two alleles:
Let's code each allele with a letter.

N = Normal allele of a gene (i.e. most commonly found wild version)
A = Mutant allele of the same gene


Because all genes come in pairs there are three options for any particular gene that has two different alleles.

NN = Normal animal ('homozygous normal')
NA = Heterozygous animal
AA = Homozygous mutant

Now, if:

NN = looks normal
NA = looks normal
AA = looks different

Then we describe the mutation as recessive. In recessive mutations the heterozygous phenotype is indistinguishable from the homozygous normal phenotype and only the homozygous mutation's phenotype looks different.

however, if:

NN = looks normal
NA = looks different from normal
AA = looks the same as NA

then we describe the mutation as dominant.

You will notice that actually, the above two examples are exactly opposite to each other and in fact the terms recessive and dominant are not mutually exclusive. So in the first example, the mutation is recessive but equally the normal gene is dominant. In the second example, normal is recessive and the mutation is dominant.

Finally, if:

NN = looks normal
NA = looks different from normal
AA = looks different from normal AND different from NA

Then we describe the mutation as [B]co-dominant[/B. In this case, as the heterozygous form is different from normal and often gets named as a morph in its own right, when a homozygous version of the mutation is bred, it often gets given the term 'super'.



Inheritance


When sexually reproducing animals produce gametes (sex cells) each sex cell contains only one of each possible gene pair (so that when an egg cell meets a sperm cell, they combine to make a cell with a 'full set' ***8211; half from each parent). This means that for each gene pair, each sex cell can only contain one of them, and it***8217;s always completely random so each gamete has a 50% chance of containing one or the other version of each pair.

Let's start with a single gene pair.

In most animals there is are a number of genes that codes for a chain of substances that eventually lead to the production of black pigment (melanin). If a particular gene in this chain mutates so it no longer works the animal can no longer produce melanin. If an animal cannot produce melanin, we call it amelanistic. This is often referred to as an 'albino' animal.

The amelanism gene is recessive to the normal version of the gene.
Lets code the normal version of the gene as 'A' and the recessive mutated version as 'a'.

There are three possible combinations of these two alleles that will give two different phenotypes.

Genotype = AA phenotype = Normal/wild type
Genotype = Aa phenotype = Normal/wild type
Genotype = aa phenotype = amelanistic (no black pigment in animal)

The 'Aa' animal can be described as 'normal, heterozygous amelanistic'. This would normally be shortened to 'normal het amel'.



If you bred an albino (aa) to a normal (AA), each parent only has one version/allele to give to the offspring. Therefore all offspring will be identical both genotypically and phenotypically.

They will all be 'Aa' or normal het amel.


If you bred an Amel (aa) to one of those offspring (Aa) something different happens. The albino still only has 'a' allele to give to it's offspring, but the 'het amel' parent can give either an 'A' or an 'a'. Because of the way egg and sperm cells are made, this means that on average, 50% of this parents gametes (sperm if male, eggs if female) will carry the 'A' and the other 50% will carry 'a'.

Therefore, all offspring will get 'a' from the albino parent and then half will get an 'a' from the heterozygous parent and the other half will get the 'A' allele.

Therefore the expected offspring are

50% amel (aa)
50% normal het amel (Aa)



What about a normal het amel (Aa) x Normal het amel (Aa) mating?



Both parents have both alleles to give. This means 50% of each parents gametes will be 'A' and 50% will be 'a'.

So what are the possible combinations?

Well, 25% will get 'a' from both parents, 25% will get 'A' from mom and 'a' from dad, 25% will get 'a' from mom and 'A' from dad, 25% will get 'A' from both parents.

As 'Aa' is exactly the same genetically as 'aA', then the final result is:

25% AA = homozygous normal
50% Aa = Normal het amel
25% aa = Amel.


Now - here we have an issue; because amelanisim is a recessive mutation, the 'AA' animals will LOOK exactly the same as the 'Aa' animals. You will not be able to tell them apart. This is where 'possible het's' come in. If you take out the visual amels (aa) then out of the normal offspring left, ~2/3's will be heterozygous (Aa) and 1/3 will be totally normal (AA) . Therefore we describe all the normals in the litter as '66% possible het', meaning each one has a 66% chance of carrying the amel mutation and a 33% chance of not carrying it.


What about a co-dom mutation mating?


Well, the genotypes are absolutely identical to the previous recessive mating, they ALWAYS come out the same. The difference with a co-dominant mutation is that each genotype has a different phenotype, so they can all be distinguished.


Lets take the lesser platinum mutation in royal pythons.

Lets code the normal version of this gene as 'N' and the mutated allele that effects the pattern and colour as 'L'

NN = homozygous normal = normal
NL = heterozygous lesser = lesser platinum
LL = homozygous lesser = 'Super lesser' (AKA Blue eyed leucistic)



So:

normal (NN) x Lesser (NL)

50% lesser (NL)
50% normal (NN)

(note that this is exactly the same outcome as a 'normal (AA) x normal het amel (Aa)' that gives 50% 'AA' and 50% 'Aa'. The only difference is that because the lesser mutation is comdominant, you can tell the difference between the two outcomes just by looking.)


lesser (NL) x lesser (NL)

25% normal (NN)
50% lesser (NL)
25% super (LL)


(Again - exactly the same as a het x het mating of a recessive mutation, but this time the PHENOTYPES allow you to distinguish the different genotypes.



Super lesser (LL) x normal (NN) = 100% lesser's (NL) (same as amel x normal = 100% het amel)



NOTE: lesser platinum is a HETEROZYGOUS animal - it has two different alleles at this locus. Just because you can see the mutation in the heterozygous form doesn't mean you 'can't get a het lesser'. That statement only works if you add a bit - "you can't get a normal looking animal that is heterozygous lesser"





More to follow....
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Last edited by bothrops; 23-03-2016 at 02:16 AM..
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