Genetics Guide Draft (community contribution welcomed!)
Jan 23, 2015 18:24:40 GMT -8
Post by muirgen on Jan 23, 2015 18:24:40 GMT -8
This should be...mostly good. Posted this for community contributions/edits/etc to make sure the information is as accurate as possible (or as desired).
VOCABULARY
Gene a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Allele forms of a gene that are found on the same chromosome.
Dominant a trait that will always be expressed, regardless of how many copies are present.
Recessive a trait that requires two copies in order to be expressed (there are exceptions. Some recessive genes will be expressed if other specific genes are present, ex. Pearl)
Incomplete Dominant a trait that will always express itself, but whose expression will change depending on how many copies are present (ex. Cream)
Phenotype the appearance of the organism, determined by its genes.
Genotype the genes carried by an organism, not all of which may be expressed.
Dilution/Dilute how the coat color is affected or changed by the gene presence.
Homozygous two copies of the same allele are present.
Heterozygous one copy of one allele is present, and one copy of another allele (or none at all) is present.
What's up with n? In basic genetics, most of the time you're shown a dominant gene, if only one copy is present, as something like Ff. "n" is a way of saying negative, so that the organism is positive for one allele but negative for a second copy. This is mostly used for alleles represented by more than one letter, but most alleles represented by only a single letter will use the lowercase as the absence of the dominant trait. . To clarify: You would write one copy of the Dun gene as Dd, but one copy of the cream gene as nCr. This makes it easier to read. Don't worry about being confused, you're still completely allowed to write "Crcr" instead of "nCr."
For each trait, there will be examples of how the gene can be visible. Recessive traits will have two copies, and dominant traits will show as either one copy or two.
BASE COATS
The two base coats for all horses are Black and Chestnut. Bay is sometimes referred to as a base coat, but isn't really because it requires a black base with another gene to affect it.
Black EE, Ee Dominant
Chestnut ee Recessive
MODIFIERS
Modifiers are responsible for producing horses with diluted coat colors, such as palominos, silver dapples, and grullas. Some are recessive, some dominant, and others incomplete dominant. Modifiers will appear differently on a horse's coat depending on the base color and other modifiers present (along with individual variation).
Agouti (Bay) AA, Aa Dominant Responsible for bay horses, but only on a black base.
Cream CrCr, nCr Incomplete Dominant A single copy will produce palomino, buckskin, and smoky black. Two copies produce perlino, cremello, and smoky cream.
Dun DD, Dd Dominant Often produces leg barring and dorsal stripe, along with lightening the base color to some degree. Responsible for grullo/grulla horses (which are misleading without "dun" in the name).
Champagne ChCh, nCh(Chch) Dominant Dilution results vary depending on the base coat. Classic Champagne, Amber Champagne, etc.
Silver ZZ, Zz Dominant Creates silver dapples, etc. Will not affect any horse with a chestnut base, regardless of other modifiers.
Pearl prlprl Recessive Normally is only expressed phenotypically if two copies are present, but if the cream gene is present, the pearl will be expressed (though perhaps not as strongly).
WHITE & OTHER GENES
From here on out, it will probably only get more confusing, so be prepared! White markings are responsible for horses that are commonly called paint, pinto, piebald, skewbald, appaloosa, and white. In certain cases, two copies of a specific white allele will result in a horse that appears completely or almost completely white, or in Lethal White Syndrome. Depending on the trait, a foal with Lethal White Syndrome will either die as an embryo or within around 72 hours after birth due to digestive problems. This section also includes roaning and graying, as well as the dominant white mutation.
Leopard Complex (Appaloosa) LpLp, Lplp Incomplete Dominant There are two forms of leopard, one of which creates blanket and snowcap appaloosas and the other of which creates leopard and fewspot appaloosas. These depend on how many copies of the gene are present.
Tobiano TT, Tt (sometimes shown as TO or To) Dominant Creates more rounded looking white patterning. References are easy to find.
Sabino SbSb, nSb Dominant One copy can produce jagged white markings, which may be minimal, white two can result in a horse that is almost completely white (sometimes referred to as "sabino white" or "full sabino").
Frame Overo Oo Dominant One copy will produce the desired overo pattern, while two will produce a near completely white foal that will die of Lethal White Overo (generally within 72 hours).
Splashed White Overo SwSw, Swsw Dominant Horses appears to be "dipped" in white. Two copies are non-lethal. There are different forms of this gene, but here we refer to SW-1, which is non-lethal with the presence of two copies.
Dominant White A mutation of white patterning which results in a completely white horse. It has appeared in a few individuals and some of their offspring, but in each separate occurrence it was on a different chromosome. It is dominant in inheritance, but may result in a solid horse, white horse, or lethal white foal. The Camarillo White Horse is a color breed specifically bred to retain this white trait. [Not sure if we'll be using this?]
Gray GG, Gg Dominant Causes a horse's coat to gray out over time. Time period and graying effects will vary on the individual, but it appears to be somewhat inherited. There are certain stages of graying, but not all horses experience these (fleabitten, dapple, steel/mulberry, "white").
Roan Dominant The specific mutation has yet to be identified, so there is no direct test for the gene. Roaning is the mixing of white hairs into a horse's coat over time, but will never completely white out or gray out the actual coat color. The legs and head remain the initial coat color. [Wasn't sure how to list this one either]
Ok, so this is kind of the "basics." I also would like to expand upon the colors resulting from these genes. I think eventually it would be nice to have a sort of "cheatsheet" for common coat colors and the genotypes behind them, which would make it easier for members creating characters as well as anyone assigning genes for breeding to do so, and they would only have to decide on whether they want certain genes to be homozygous or heterozygous (if they care). I was wondering if maybe horses should be assigned genes upon being accepted and have them edited into their profile (if the member didn't list their own genes), or maybe if it should be edited into the profile when the horses are bred, and mark those profiles (something like a {G} ) so that new genes won't be reassigned and when taking care of breeding you already know what the horse's genes are? Just a thought. It would prevent inconsistency with foal results in repeated breedings involving horses with unlisted genes.
VOCABULARY
Gene a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Allele forms of a gene that are found on the same chromosome.
Dominant a trait that will always be expressed, regardless of how many copies are present.
Recessive a trait that requires two copies in order to be expressed (there are exceptions. Some recessive genes will be expressed if other specific genes are present, ex. Pearl)
Incomplete Dominant a trait that will always express itself, but whose expression will change depending on how many copies are present (ex. Cream)
Phenotype the appearance of the organism, determined by its genes.
Genotype the genes carried by an organism, not all of which may be expressed.
Dilution/Dilute how the coat color is affected or changed by the gene presence.
Homozygous two copies of the same allele are present.
Heterozygous one copy of one allele is present, and one copy of another allele (or none at all) is present.
What's up with n? In basic genetics, most of the time you're shown a dominant gene, if only one copy is present, as something like Ff. "n" is a way of saying negative, so that the organism is positive for one allele but negative for a second copy. This is mostly used for alleles represented by more than one letter, but most alleles represented by only a single letter will use the lowercase as the absence of the dominant trait. . To clarify: You would write one copy of the Dun gene as Dd, but one copy of the cream gene as nCr. This makes it easier to read. Don't worry about being confused, you're still completely allowed to write "Crcr" instead of "nCr."
For each trait, there will be examples of how the gene can be visible. Recessive traits will have two copies, and dominant traits will show as either one copy or two.
BASE COATS
The two base coats for all horses are Black and Chestnut. Bay is sometimes referred to as a base coat, but isn't really because it requires a black base with another gene to affect it.
Black EE, Ee Dominant
Chestnut ee Recessive
MODIFIERS
Modifiers are responsible for producing horses with diluted coat colors, such as palominos, silver dapples, and grullas. Some are recessive, some dominant, and others incomplete dominant. Modifiers will appear differently on a horse's coat depending on the base color and other modifiers present (along with individual variation).
Agouti (Bay) AA, Aa Dominant Responsible for bay horses, but only on a black base.
Cream CrCr, nCr Incomplete Dominant A single copy will produce palomino, buckskin, and smoky black. Two copies produce perlino, cremello, and smoky cream.
Dun DD, Dd Dominant Often produces leg barring and dorsal stripe, along with lightening the base color to some degree. Responsible for grullo/grulla horses (which are misleading without "dun" in the name).
Champagne ChCh, nCh(Chch) Dominant Dilution results vary depending on the base coat. Classic Champagne, Amber Champagne, etc.
Silver ZZ, Zz Dominant Creates silver dapples, etc. Will not affect any horse with a chestnut base, regardless of other modifiers.
Pearl prlprl Recessive Normally is only expressed phenotypically if two copies are present, but if the cream gene is present, the pearl will be expressed (though perhaps not as strongly).
WHITE & OTHER GENES
From here on out, it will probably only get more confusing, so be prepared! White markings are responsible for horses that are commonly called paint, pinto, piebald, skewbald, appaloosa, and white. In certain cases, two copies of a specific white allele will result in a horse that appears completely or almost completely white, or in Lethal White Syndrome. Depending on the trait, a foal with Lethal White Syndrome will either die as an embryo or within around 72 hours after birth due to digestive problems. This section also includes roaning and graying, as well as the dominant white mutation.
Leopard Complex (Appaloosa) LpLp, Lplp Incomplete Dominant There are two forms of leopard, one of which creates blanket and snowcap appaloosas and the other of which creates leopard and fewspot appaloosas. These depend on how many copies of the gene are present.
Tobiano TT, Tt (sometimes shown as TO or To) Dominant Creates more rounded looking white patterning. References are easy to find.
Sabino SbSb, nSb Dominant One copy can produce jagged white markings, which may be minimal, white two can result in a horse that is almost completely white (sometimes referred to as "sabino white" or "full sabino").
Frame Overo Oo Dominant One copy will produce the desired overo pattern, while two will produce a near completely white foal that will die of Lethal White Overo (generally within 72 hours).
Splashed White Overo SwSw, Swsw Dominant Horses appears to be "dipped" in white. Two copies are non-lethal. There are different forms of this gene, but here we refer to SW-1, which is non-lethal with the presence of two copies.
Dominant White A mutation of white patterning which results in a completely white horse. It has appeared in a few individuals and some of their offspring, but in each separate occurrence it was on a different chromosome. It is dominant in inheritance, but may result in a solid horse, white horse, or lethal white foal. The Camarillo White Horse is a color breed specifically bred to retain this white trait. [Not sure if we'll be using this?]
Gray GG, Gg Dominant Causes a horse's coat to gray out over time. Time period and graying effects will vary on the individual, but it appears to be somewhat inherited. There are certain stages of graying, but not all horses experience these (fleabitten, dapple, steel/mulberry, "white").
Roan Dominant The specific mutation has yet to be identified, so there is no direct test for the gene. Roaning is the mixing of white hairs into a horse's coat over time, but will never completely white out or gray out the actual coat color. The legs and head remain the initial coat color. [Wasn't sure how to list this one either]
Ok, so this is kind of the "basics." I also would like to expand upon the colors resulting from these genes. I think eventually it would be nice to have a sort of "cheatsheet" for common coat colors and the genotypes behind them, which would make it easier for members creating characters as well as anyone assigning genes for breeding to do so, and they would only have to decide on whether they want certain genes to be homozygous or heterozygous (if they care). I was wondering if maybe horses should be assigned genes upon being accepted and have them edited into their profile (if the member didn't list their own genes), or maybe if it should be edited into the profile when the horses are bred, and mark those profiles (something like a {G} ) so that new genes won't be reassigned and when taking care of breeding you already know what the horse's genes are? Just a thought. It would prevent inconsistency with foal results in repeated breedings involving horses with unlisted genes.