part 3 of the 3-part article by Cecil A. Dobbin
- Mother Nature's Most Favored Member of the Equine Family
Jen Knutson submitted comments/response to the third portion of the above mentioned article (original article).
From Cecil A. Dobbin's article, pertaining to the mare Bright Delight:
["The important question here would be, how was this mare able to produce 100% color, especially so when she was mated to horses that were solid colored Thoroughbreds and Quarter Horses?
We first took this question to a man well versed in genetics, Jack Williams of Canada. After a few minutes of calculating her possible gene makeup, he stated, "The answer is very simple. Her Appaloosa genes have aligned themselves in the proper position on the chromosome. When this takes place it would be impossible for her to produce anything other than an Appaloosa, regardless of the outcross. It places her at the top in dominance."]
Jen Knutson comments:
First, a basic overview of the inheritance of genes:
-Genes do not "align themselves on the proper position on the chromosome" - rather, a given gene is always located on a specific part of a chromosome (otherwise, in the rare event that a gene has been relocated on a chromosome, some serious issues in chromosome pairing can occur during meiosis (the process that produces sex cells), generally resulting in semisterility of the individual).
-Chromosomes come in pairs of two (each pair is made up of two structurally similar chromosomes [called homologs/homologous chromosomes] which contain the same genes (but not necessarily the same alleles [see below...] of those genes).
-It's important to remember that genes come in different forms called "alleles." An individual has TWO alleles of each gene (ignoring the sex chromosomes, X & Y) - one allele per homolog, with one homolog (and therefore one allele) coming from each parent.
-During the production of gametes (sex cells), the homologous chromosomes line up and are separated, eventually resulting in gametes with only ONE of the homologs from each chromosome pair (and therefore half of the normal number of chromosomes).
-When fertilization occurs, a sperm cell (containing half the normal number of chromosomes) fuses with an egg cell (with half the normal number of chromosomes)... the resulting cell therefore contains the correct number of chromosomes again, with TWO homologs for each chromosome pair, one coming from each parent.
Second, the basic gene for appaloosa color/characteristics (ignoring other not-yet-identified genes that may modify the particular pattern, amount of white, etc.) comes in two alleles: Appaloosa color/characteristics, which is dominant, and Solid-colored (i.e. lack of appaloosa coloring), which is recessive.
I will refer to the two alleles as 'L' (uppercase-L... dominant/ Appaloosa coloring) and 'l' (lowercase-L... recessive/ solid (lack of appaloosa coloring)) from now on for convenience.
With 'L' being dominant, a horse with appaloosa color/characteristics MUST have at least one copy of 'L' - the other allele can be 'L' or 'l' ('L' "covers up" the expression of 'l' because 'L' is dominant over 'l') A solid-colored horse must have two copies of the allele 'l' ('l' is recessive - it's only expressed when not "covered up" by 'L').
Therefore, the genotype (the genetic makeup, or the particular alleles an individual's chromosomes contain) of a horse is as follows:
Appaloosa coloring - 'LL' or 'Ll' No appaloosa coloring - 'll'
*** Now, to finally answer the above question... Bright Delight must be of the genotype 'LL': one 'L' coming from her Appaloosa-colored father's sperm cell, and the other 'L' coming from her Appaloosa-colored mother's egg cell. Bright Delight (being 'LL') could therefore only contribute the 'L' allele of the appaloosa-color gene to her offspring (remember, only one allele per gene is contributed by each parent).
The sires of all but one of her foals were solid-colored QH's or TB's... being solid colored, all of the QH/TB sires must be of the genotype 'll' (and could only contribute the allele 'l' to their offspring). As a result, all of Bright Delight's foals by non-Appaloosa stallions would be of the genotype 'Ll' - that is, all would have appaloosa coloring.
The foal by Red Plaudit (a colored Appaloosa stallion of genotype 'Ll' as his dam was solid-colored ('ll')) could either be of the genotype 'LL' or 'Ll' - either way, the foal must have appaloosa coloring.
*** And why didn't Bright Delight's full brother, Mighty Bright, have the same 100% color production record? The answer is very simple. Mighty Bright had a genotype of 'Ll'. Why? Take a look at the grandparents of Bright Delight & Mighty Bright. Billy Maddon, the sire of Bright Eyes Brother, was a solid colored QH (genotype 'll'). Bright Eyes Brother, a colored appaloosa, must have inherited 'l' from Billy Maddon while receiving 'L' from his appaloosa-snowflake-colored unregistered mother, Plaudette.
Therefore, Bright Eyes Brother was of the genotype 'Ll'. Peggy's Delight, a roan mare, and the dam of Mighty Bright & Bright Delight, was out of two registered Quarter Horses, one of which probably had minimal appaloosa coloring or characteristics (carrying the 'L' allele) - my guess is Peggy's Delight's dam, Petey, who was out of a Coke Roberds mare. Peggy's Delight was most likely 'Ll'. While Bright Delight got 'L' from her sire and 'L' from her dam, Mighty Bright got 'l' from his sire and 'L' from his dam (or vice versa).
Therefore, half of Mighty Bright's sperm cells contained 'L' (so all offspring from those sperm cells were appaloosa-colored), but the other half of the sperm cells contained 'l'. If the sperm cell containing 'l' happened to fertilize an egg cell containing the allele 'l' (such as from a solid-colored quarter horse, or even another appaloosa of genotype 'Ll'), the resulting foal would be 'll' and SOLID-COLORED. Hence the not-anywhere-near-100% color production from Mighty Bright.
More from Cecil A. Dobbin's article:
[Next question to Jack. "If this can happen with one mare, what are the possibilities of developing other Appaloosas with the same factors?" Answer: The possibilities are unlimited. With proper breeding and understanding, the entire breed of Appaloosa could be brought to the homozygous dominant (or pure) state, therefore the breeders would be assured of color.
That is, breeding an appaloosa-colored/characteristics horse to another appaloosa-colored/characteristics horse is really the ONLY way to get a appaloosa-colored horse that produces 100% color regardless of the other parent's coloring! However, crossing colored appaloosa to colored appaloosa still doesn't always guarantee that the resulting foal will be 'LL' and a 100% color producer, unless both parents are 'LL'... If both parents are 'Ll', only about 25% of their offspring will have the genotype 'LL' and be 100% color producers.
We go to the books hoping to gain a better understanding as it relates to Bright Delight's ability to produce 100% offspring. It is my impression that she would be considered as Genotype which is defined as a gene type that an individual inherits from both parents. We add to this the possible mutation involvement which is defined as a sudden and permanent change in an individual genotype.
If the mutation occurs in the sex cell, the mutated gene may be passed to future offspring. We now compare Bright Delight's record to her full brother. Mighty Bright does not have a record of producing 100% Appaloosa offspring. The only possible reason we can think of as to why would be that he is minus the mutant factor.]
Jen Knutson comments again:
Technical terms are not used properly in the above passage:
*Genotype = the genetic makeup of an individual (which two alleles of each gene the individual has on its chromosome pairs) (note: phenotype = outward appearance, so an appaloosa-colored phenotype could have an 'LL' or 'Ll' genotype!)
*When a particular allele is inherited from BOTH parents, an individual is said to be homozygous for that gene/allele ('LL' - homozygous [dominant] for appaloosa coloring, while 'll' is homozygous [recessive] for non-appaloosa-coloring)
* Mutation is more correctly defined as: an alteration in a gene or chromosome which can be inherited. Mutations occur on the cellular level, changing an individual cell's genetic makeup, but NOT an individual's genotype!
*The real reason for why is Mighty Bright isn't a 100% color-producer, but Bright Delight is a 100% color-producer is explained above... Mighty Bright is 'Ll' while Bright Delight is 'LL'. Simple. There is no mythical 'mutant factor'!
*As a note: the 'L' allele probably did originate as a mutation of 'l' in the gamete(s) of some horse(s). (the original wild horses that modern domesticated horses descended from were solid-colored duns... all other modern colors are the result of inheritable allele mutations)
Carleton College - Northfield, MN
Ms. Knutson has a web site that is a wonderful "equine genealogy" resource for many of us. Visit herAppaloosa Pedigree Page.
This article first appeared on the world wide web in May 2001 at URL: barnlot.tripod.com
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