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homozygous - is there a simple explanation

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kdtexas

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Someone asked me what homozygous meant and I had a difficult time trying to explain. I said it meant your horse had dominant gene for whatever he was homozyous in; like homozygous pinto meant he would always throw pintos if bred to another homozygous pinto.

Is this correct?
 

Jean_B

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Nope. It means he will sire only pintos, even if bred to a solid color mare that doesn't have a pinto gene in her body.
 
L

Lisa-Ruff N Tuff Minis

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hmm in laymans terms homozygous will be the boss of(not really but good way to explain it) other genes

a homozygous pinto will always throw pinto no matter what it is bred to may be minimal but will be pinto
 

Critterhugger

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To help explain it you should first understand that genes come in PAIRS, one comes from the mother and one comes from the father. So when a horse is "homozygous" for a gene what you're saying is that BOTH genes (alleles) that it got from it's parents are identical. Whether it be a recessive gene, or a dominant gene. For instance a red horse is homozygous for RED (ee) -which is a recessive gene. A horse that is "heterozygous" has two DIFFERENT genes...one black gene and one red gene (Ee). So then a horse that is Homozygous for BLACK has two black genes (EE).

In the case of a homozygous tobiano it means the horse has two tobiano genes (ToTo), as opposed to a regular (heterozygous) tobiano that has one tobiano gene and one non-tobiano gene (To to). A homozygous tobiano will pass one tobiano gene to each and every one of it's foals. Does this help?
 
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_minihorses4ever_

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Critterhugger said:
To help explain it you should first understand the fact that all genes come in PAIRS, one comes from the mother and one comes from the father.  So when a horse is "homozygous" for a gene what you're saying is that BOTH genes that it got from it's parents are identical.  Whether it be a recessive gene, or a dominant gene.  For instance a red horse is homozygous for red (ee) which is a recessive gene.  A horse that is heterzygous has two DIFFERENT genes...one black gene and one red gene (Ee).  Then a horse that is Homozygous for BLACK has two black genes (EE).
In the case of a homozygous tobiano it means the horse has two tobiano genes (ToTo) as opposed to a regular (heterozygous) tobiano that has one tobiano gene and one non-tobiano gene (To to). A homozygous tobiano will pass one tobiano gene to each and every one of it's foals.  Does this help?

455809[/snapback]

Wow, I think I actually got some of that.
Horse color is utterly confusing!
 

Dr. Pam

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Critterhugger said:
To help explain it you should first understand the fact that all genes come in PAIRS, one comes from the mother and one comes from the father.  So when a horse is "homozygous" for a gene what you're saying is that BOTH genes that it got from it's parents are identical.  Whether it be a recessive gene, or a dominant gene.  For instance a red horse is homozygous for red (ee) which is a recessive gene.  A horse that is heterzygous has two DIFFERENT genes...one black gene and one red gene (Ee).  Then a horse that is Homozygous for BLACK has two black genes (EE).
In the case of a homozygous tobiano it means the horse has two tobiano genes (ToTo) as opposed to a regular (heterozygous) tobiano that has one tobiano gene and one non-tobiano gene (To to). A homozygous tobiano will pass one tobiano gene to each and every one of it's foals.  Does this help?

455809[/snapback]

Good job! I'd only add that Tobiano is dominant, so you only need one "To" to get the coloring. I think I'll have to name my next pinto foal "SR something something ToTo"
 

Hosscrazy

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But what if both parents are homozygous? Let's say the sire is homozygous for pinto, and the dam is homozygous for red? Couldn't the offspring be homozygous for red, even though we're saying the homozygous sire will only throw pinto?

Or am I just confusing myself even more...
 

Songcatcher

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Hosscrazy said:
But what if both parents are homozygous? Let's say the sire is homozygous for pinto, and the dam is homozygous for red? Couldn't the offspring be homozygous for red, even though we're saying the homozygous sire will only throw pinto?
Or am I just confusing myself even more...

455836[/snapback]

If the sire is a red pinto, the foal would be a red pinto. Red is a color and pinto is a pattern. Red is recessive so it must have two red genes to appear red. Two red horses will only produce a red foal.
 

chandab

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Hosscrazy said:
But what if both parents are homozygous? Let's say the sire is homozygous for pinto, and the dam is homozygous for red? Couldn't the offspring be homozygous for red, even though we're saying the homozygous sire will only throw pinto?
Or am I just confusing myself even more...

455836[/snapback]

The pinto gene is seperate from the base color gene, so a horse could be homozygous for pinto and homozygous for base color (red or black).

Pinto is a color pattern, so far there are only tests for tobiano and lethal white overo.

Horses are two base colors; either red or black, all other colors are the result of a color modifier (agouti/bay, dun, creme, champagne, roan...) acting on the base color. Black is dominant, so a horse with one black gene and one red gene will appear black, but can pass either a red gene or a black gene to it's offspring.

There is a pretty good website for describing colors on-line http://www.equinecolor.com/

Check out the website, it's very informative and they have some great photos of the different colors and patterns.
 

Lesa

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Just to add a note on this subject. I know of someone who has a stallion that was tested homozygous and has thrown 2 solids. When they called the lab, they were told that there is still a 2% chance of getting a solid, even though the horse tested homozygous.
 

kdtexas

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[SIZE=14pt]Yes!! I got it!! Thank you all for your time in explaining this to me. Now I can inform and educate my friend. Knowledge can be dangerous; so I'll use it carefully!![/SIZE]

thanks to all again!!
 

Lewella

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Lesa said:
Just to add a note on this subject.  I know of someone who has a stallion that was tested homozygous and has thrown 2 solids.  When they called the lab, they were told that there is still a 2% chance of getting a solid, even though the horse tested homozygous.
455944[/snapback]

First up, the tobiano test doesn't test for the tobiano gene but for markers associated with it so there is a margin of error. A horse can test homozygous for the markers associated with tobiano but not be a homozygous tobiano though this is fairly rare.

Secondly, in minis in particular, minimal tobiano expression happens. A minimal tobiano will not have body spots but instead be considered by most to be "solid" because it only has leg white. There are known to be homozygous by both testing and progeny horses that have white on their legs no higher than socks.
 

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