Goat genetics, Purebred, Silky chickens, Dominance, Dominance Codominance and Epistasis

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PUREBRED!? Miller, W. J. 1977. Purebred!? Iowa Science Teachers Journal 14:37-38

It has a nice sound. "Purebred" stock sells better than "crossbred" or "mongrel". The connotations in people's minds about these terms seems pretty well fixed. But are they accurate?

A 1950 Webster's Unabridged Dictionary defines Purebred as "bred from pure blood"! Do your birds have parasites in their blood? They might, according to the parasitologists! So, therefore, they are not purebred. The dictionary also says "kept pure for many generations". Are your doves from species crosses? Well, they might be, but its not likely, and who could tell? Crosses of different stocks of the same species "back when" is more likely. Finally the dictionary says "of a lineage established by registration records". Who registers your birds? My doves may come the closest, since I keep single pairs per cage, and mine are pedigreed and the pedigree goes back to 1938.

Newer dictionaries are not much better. What do we mean by purebred? Probably no two fanciers quite agree. How about geneticists? Courses in Heredity use "purebred" in two major ways, not quite equivalent.

(1) Produces the same type (phenotype) and only that type generation after generation. Breeds true.

(2) The stock, or an individual, is homozygous for the genes controlling a particular genetic character. In fancier's terms (which ought to move closer to the scientific usage) such stock or individual is not "split" (is not heterozygous). "Split" implies not "pure". A purebred individual can make only one kind of gamete (sex cell= sperm or egg), that is, it cannot segregate genetic controlling elements (= genes). A "split" individual can segregate for a character pair such as silky plumage versus normal feathers.

First let's get out of the way why the two definitions are only nearly, but not exactly equivalent. In genetics a rare situation called "balanced lethal" kills the "pure" (homozygous) individual very early (usually before they can be seen to be classified), leaving only the "impure" type (heterozygote) to continue generation after generation. It seems to be breeding true, but half (1/4 + 1/4) of all its offspring in this stock die early. (Example: Curly wing and Star eye laboratory flies). Otherwise the two usages are equivalent. Since balanced lethals are rare, forget it for our purposes.

Now, if a stock is truly completely purebred, it will not segregate for any "character pair." A few very highly inbred lines of corn or mice or Drosophila come close to this, but they are not desirable specimens in themselves. Moreover, they still segregate for a few character pairs. "Well!" some will cry, "I have purebred dogs (or horses, or cattle, etc.), and they are all the same!" NO! THEY ARE NOT! They might be the same for one or a very few characters, but they are highly individualistic and do differ!

Examples of the more infrequent type of how a "purebred" can be not pure: "purebred" Shorthorn cattle segregate for red, roan, and white color; "purebred" palomino horses never breed true but segregate for chestnut, palomino, and cremello; tangerine doves never breed true, segregating for pearled, tangerine, and non-tangerine. Purebred blue Andalusian chickens are the classical example segregating for black, blue, and "splashed white" [flecked blue and black]. The white spotting pattern in "purebred" black and white Holstein-Friesian cattle is never quite the same from individual to individual. Moreover, a red and white segregant may "pop out" of black and white parents as an infrequent variant.

An example of the more common type of how "purebred" can be not pure could be Herefords looking all "alike" for white face, red sides and general size and conformation. They still segregate for alternative blood types, transferrins, hemoglobins..., [and maybe mulefoot (or whatever) will show up now and then]. Such generally hidden physiological characters will be found to segregate in any so-called purebred stock or species.1

SO! How will you use "purebred?" I vote for "homozygous for one or a very few characters." Your purebred fawn (blond) doves may be all fawn color, but they are likely to be segregating for one or more of the following: thin bill ring, yellow down, length of tail, weight, peanut extract agglutination of red cells, ....

Often I get asked a question like: "Hey Doc! Are your rosy doves purebred?" My hesitant answer is yes; they are homozygous for rosy, and they will breed true for rosy. BUT, they might segregate for ivory, Then you would get an interaction cream color. They might segregate for albino, in which case you'll get albino covering up (epistatic to) rosy, but they are still pure (homozygous for rosy).

"Got any purebred Frosty doves?" Nope! Not yet! All I have tested are heterozygous (split) for normal (= non-frosty). But I have three or four that might be purebred (homozygous). I need to test them in outcrosses to non-frosty since frosty is some kind of dominant. If any one dove is purebred, it will yield all offspring frosty.

Confused? Well, maybe some simple genetics should be looked at -- maybe a workshop at a big meeting every year. It's not that hard. Some pigeon breeders do it!

........W. J. Miller (modified 21 Aug 98)

1Further, any "purebred" stock is likely to carry (rarely or infrequently) one or more recessive detrimental characters. Breeders usually try to hide or choose not to reveal such occurrances. They hope to weed out carriers, but often the new selected individuals for breeding free of the first detrimental may carry an unrelated detrimental! So breeding to one famous bull, or dog, or whatever champion of show is discontinued after a period of time.

Miller, W. J. 1977. Purebred!? Iowa Science Teachers Journal 14: 37-38

[American Dove Association Newsletter] ADAN Sep/Oct 1991 p.7 ; The DoveLine Nov/Dec 98 p.5-6 with a title change to PUREBREED!?

(Note added 15 Sep 1998: Dr. Hollander would like to distinguish between "purebred" which is past history and purebreeding which is in the future.) Purebreds should be purebreeding, but note the exceptions in the above discussion.

Goat genetics, Purebred, Silky chickens, Dominance, Dominance Codominance and Epistasis

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