Importance of Complete Pedigrees

Importance of Complete Pedigrees

By Leos Kral

The word "pedigree" does not carry the same meaning to a breeder as it does to a geneticist. To a breeder, a pedigree looks like the one shown below. The primary information conveyed by this type of pedigree include familial relationships of the dogs, titles earned, and kennel line ancestries.


		CH GrandmaX Aussie
	CH Mother Aussie
		CH GrandpaX Aussie
My Great Aussie (male)
		CH GrandmaY Aussie
	CH Dad Aussie
		CH GrandpaY Aussie

To a geneticist, the same pedigree would look like the one shown below. Circles represent females and squares represent males. A square linked to a circle by a horizontal line represents a mating of those individuals. The offspring of a mating are connected to the parents by a vertical line. The "genetic pedigree" is a simple diagram that makes it easier to visualize the familial relationships of the individuals. (For comparison, My Great Aussie in the "breeder pedigree" above, is the lone square at the bottom of the "genetic pedigree" below. Also, CH. GrandmaX Aussie in the "breeder pedigree" above, is the circle at the upper left of the "genetic pedigree" below.)

Of course, familial relationships are just one aspect to genetic analysis. The other aspect is the inheritance of some trait (in context of this web site, the inheritance of some genetically based disease). In a genetic pedigree, the individuals that have the disease are symbolized by a filled in square or circle. Individuals who do not have the disease are symbolized by a square or circle that is not filled in. For example, suppose that the owner of My Great Aussie tells us that his dog has the genetic disease hairy eyeballs. To indicate that fact, we would redraw the genetic pedigree as shown below. Note that question marks have been placed in the symbols representing parents and grandparents because we do not know if they have hairy eyeballs or not.

The above pedigree is essentially worthless for determining how a disease is inherited. There is not enough information. If hairy eyeballs is a genetic disease, we know that one or both parents have the bad gene, but there is no way to determine anything else. Now suppose that we do some detective work and find out that neither parent of My Great Aussie has hairy eyeballs and that none of the grandparents have hairy eyeballs except GrandpaX Aussie, who has hairy eyeballs. With this new information we can redraw the genetic pedigree to look like this:

This pattern now has some information we can analyze. For example, it is likely that the disease hairy eyeballs is a recessive disease. If it were a dominant disease than Mother Aussie would also have hairy eyeballs. However, there are two possible modes of recessive inheritance. Sex-linked or autosomal. Sex-linked means that the gene resides on the X-chromosome. Autosomal means that the gene resides on any chromosome except the X-chromosome. At this point we do not have enough information to determine if the gene for hairy eyeballs is sex-linked recessive or autosomal recessive.

Suppose we talk to the breeder of My Great Aussie and find out that the dog has 7 littermates. We track down the owners of those littermates and determine what their eyeballs look like. We find that My Great Aussie has 3 brothers and 4 sisters. All four sisters and two brothers have normal eyeballs. One brother has hairy eyeballs. We can then redraw the pedigree to look like this:

The bottom row of circles and squares represents all of the offspring of the mating between Mother Aussie and Dad Aussie. The arrow points to My Great Aussie. From this pedigree pattern, it is possible to conclude that hairy eyeballs is probably a sex-linked recessive disease. Diseases with sex-linked recessive inheritance characteristically display the pattern shown by this pedigree where an affected male produces a normal female who in turn produces half affected males, half normal males, and all normal females.

As can be seen from this example, genetic pedigree information must be as complete as possible in regard to all related individuals and their phenotypic (disease vs. no disease) status so that genetic analysis can be carried out. The Australian Shepherd Health Registry is designed to collect the necessary information so that complete pedigrees can be constructed for genetic analysis of the various heritable health problems.

Note that the given example is not sufficient to absolutely prove that hairy eyeballs is a sex-linked recessive disease. The given example only indicates the most likely possibility given the data. For absolute proof, a greater number of pedigrees would have to be analyzed to provide statistical significance.

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