The Colors of Life

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Beginning Genetics: Getting Started with Coat Color Genetics

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Color genetics is the study of the genes behind coat colors. Since genes determine the way colors express and the way they are inherited, in order to understand color genetics, it is necessary to understand some very basic genetics.

Genes are the building blocks that make up our DNA. Each gene is made up of two alleles. One allele inherited from the male parent and one from the female parent. Alleles are responsible for every physical trait. Those we can see and those that we can't see. Not every allele is expressed. Some are masked by other alleles but can still be passed on to offspring. Alleles can express in different ways and the first two we will discuss are dominant and recessive.

Dominant (sometimes referred to as simple dominant): Only one copy of the allele is required to express. For example lets consider a imaginary allele P that is dominant. If an animal (or person) carries this allele, and it is not masked by another gene, it will express. It doesn't matter if one or two copies are present it will still express the same way. The notation of this would be PP for two copies of the allele Pp for one copy (PP and Pp will look the same, the trait will express)and pp if the allele is not present.

Recessive: Two copies of the allele are required to express. This time we will designate our trait b. (Did you notice that it is lower-case this time? Recessive alleles are designated with lower-case letters while dominate alleles are upper-case.) Because the allele is recessive it will only express if the offspring inherited a copy from each parent bb. If the offspring only inherited one copy (Bb) the trait will not express. It will look exactly like an offspring who does not carry the trait at all (BB).

Now lets looks at some Punnett Squares. A Punnett Square is a tool used in genetics to calculate inheritance probabilities. They can get quite complicated but don't panic. We'll keep it simple.

In order to use a Punnett Square start at the top left dams allele. Travel across to the right and combine the dams allele with the first sire allele. Write them in the box. Combine the same dam allele with the next of the sire's alleles. Write them in next open box on the top row. Continue to the next row and do the same thing with the dam's alleles in that row and both of the sire's alleles. In this way you combine each of the dam's alleles with each of the sire alleles. Making four (4) possible combinations. Because there are four possible combinations and the total has to reach 100%. Each outcome will have a 25% chance of occurring. If one of the combinations occurs more than once, those possibilities are added. For example in the square we just completed, Pp occurs twice. This means that the chances of Pp occurring is not just 25% but 50%. We now have all the possible outcomes plus the possibility that they will occur in any breeding where the sire and dam are both Pp. Since the example we are dealing with involves a Dominant trait, the trait will show in 75% of the offspring (assuming there are not other genes that mask this trait). The trait will express in the offspring that are PP and Pp. How does that add up to 75%? Well, remember that each outcome has a 25% chance of occurring and since the Punnett Square has three boxes with the dominant allele we can simply add those chances together to get 75%. Clear as mud? Then lets move on...

With a recessive allele the Punnett square works the same way. The difference is how the allele expresses. Lets look at our recessive example with two Bb parents. The possible outcomes for offspring are BB, Bb(two of these), and bb. In this example only one out of four of the possible outcomes will express the trait, the bb offspring. Since each has a 25% chance of occurring and bb only occurs in one square, there is only a 25% chance of the trait expressing.

Now that you understand dominant and recessive we can move on to incomplete domiant and co-dominant.

Incomplete Dominant: This mean that when the allele is present only once Rr it will express but it will express incompletely. For example in a snapdragon flower if two copies of the red allele are present, RR the flower will be red and if the red allele is absent, rr the flower will be white. If however, there is one red allele present, Rr the flower will be pink.

It is common, even though an organism can only carry two alleles of any one gene, for more than two alleles of that gene to exist in the general population. It is standard practice to designate these alleles with the locus (the location on the chromosome where the alleles occur) and a superscript. For example, in humans blood type is determined at the I locus. Therefore, IA,IB,and IOare used to distinguish the alleles but only two of these alleles can be pressent in any one person. A person can be IAIA (Type A Blood),IAIO (type A blood since IOis recessive to IA), IBIB (type B blood), IBIO (type B blood since IOis also recessive to IB), or IOIO but not IAIBIO. A person can also be IAIB which is type AB blood. In this case neither the A or B is recessive to the other. They are what is know as co-dominant meaning that the traits combine to form the final expression. Co-dominance is different from incomplete dominance. In incomplete dominance the recessive allele doesn't produce red pigment this results in a pink flower because the recessive allele doesn't contribute. In co-dominance both alleles contribute to the final expression.

Genetics can be a daunting subject but building anything including understanding starts with the foundation. The foundation we've just covered is all you need to get started learning color genetics. If you choose to go no farther you will still be able to understand how inheritance works and how it can be applied.

Comments

Hi I need help,I have a stallion,he is a black and white pinto.His sire was a solid black friesian and the mare was a pinto crossbreed. Is there a chance of him being homozyguos+?

This black and white stallion was bred to a bay pinto mare ,her dam was a friesian and sire was a bay pinto saddler.What is foal goin to be? Might the foal be homozyguos?

Your stallion cannot be homozygous for the pinto gene due to his sire being a solid Friesian. He may however have a chance of being homozygous for black (if dam was a black-based horse). Homozygous essentially means "two copies" of the same gene - so if both parents don't have the gene to pass on, the resulting foal cannot be homozygous.

You don't say what pinto pattern the parents express: Tobiano? Frame? Splash? Sabino?

The foal will likely be either bay or black (with a possibility of being homozygous for black) - but without knowing the pinto patterns of the parents, I cannot comment further on the likely pattern of the foal.

Lisa

Shirl's picture

Hi i have a Paint mare she is nn OLWS, nn 123 for Splash, and has come back SB1/N can anyone tell me what may be the other possible gene..and is there any test to find out thank you.

i have a female albino cat, she is just over a year old, i have never seen any others only on the net and would like to contact any other owners denise

I would like to use a photo of your cat; if you have one I may use please contact me here http://colorgenetics.info/contact

colorfan's picture

Can you give me an example of a co dominant color in horses please.  Would that be like a silver cream?

 

When I ride I feel His pleasure.

There are no co-dominant colors in horses.  Co-dominant has to do with two different alleles of one gene combining to produce a different effect than either produces alone.  Tortoiseshell cats are an example of Co-Dominance.  

colorfan's picture

Thanks

When I ride I feel His pleasure.

I have a breeding to a double dilute perlino Andalusian stallion.  I want to breed to my chestnut Oldenburg mare.  I was wondering what my foal color possibilities are. Here is some background on the mare.  She is out of a chestnut mare, who was sired by a chestnut .  they all look the same, the grandsire has more white markings.  The mares sire was a bay.

I have been breeding her for several years to my Friesian stallion, obviously black.  All her foals to date have been bay, some dark bay, some more of a bright bay, only one foal has had any white, being a simple star.

I'm guessing that her chestnut is dominant because of the consistent bay foals. Correct???

So if I breed her to the double dilute Perlino what would be my possible foal colors?

colorfan's picture

jlaberge,

I will take a shot at answering your question.    Since your mare has produced bay foals from a black stallion it seems she carries an agouti (bay) gene which she got from her bay sire. Agouti only affects black color which is why it can appear to 'skip' a generation and black is dominant over chestnut. Chestnut is recessive, in other words requires two copies to show,  your mare can only pass a chestnut gene. So the black stallion passes his dominant black, your mare contributes a chestnut and agouti gene which results in a bay foal.

Statistically the odds are 50/50 that a heterozygous mare will pass one or the other genes but some, like your mare seem to defy the odds and consistently produce one or the other.

Your mare will pass a chestnut gene and has a 50/50 chance of passing the agouti gene.

Now regarding the sire. Perlino means he is a bay with two copies of cream. Without knowing if he is homozygous for black or bay there are several possibilities for what he can pass on to the foal.

Since we know he is homozygous for cream he will pass on a cream gene.

He could pass a black gene which is dominant over chestnut so the foal would be black with one copy of cream which is smokey black.

He could pass a black and agouti(he could pass only black and your mare could pass the agouti but it ends up the same so it really is moot which parent passes the agouti)  which is bay plus the cream which makes buckskin.

If he is heterozygous for black, which means he carries a chestnut gene also, he could pass only the chestnut gene, so his chestnut(plus your mares chestnut) plus one cream would be palomino.

To summarize you could get one of three colors, smokey black, buckskin or palomino.

There is a lot of information here and I hope I have been able to write it clearly.  If something doesn't make sense please ask. I hope this helps.

Lucky you to find a nice perlino stallion!

When I ride I feel His pleasure.