Studying Stud Dogs
Sire lines are one of the basic features defining a breed. The sire line is the paternal line of descent behind existing stud dogs in the breed; it can be found by tracing the far left (blue) line of a pedigree written out as a family tree as pictured here. The “Y” chromosomes in the breed genepool all come from this line. Questions to ask when studying a breed include:
- How many sire lines are available?
- How many breeding dogs are there from each line?
- What is the genetic relationship between these breeding dogs?
Accurate pedigree records provide a starting point for answering these questions; genetic testing of individual dogs provides more detailed information by identifying the “paternal haplotype” of male dogs.
How Many E.S. Sire Lines Exist?
How Many Dogs From Each Line?
Are These Lines Genetically Distinct?
When people talk about population size, most of us think about the “census size” of the group — we do a head count of dogs in the breed. Scientists studying breeds ask a slightly different question, however: they want to know the “effective population size”. The effective population size is smaller than the census count of dogs in the breed. The effective population size takes into consideration two things:
- Not all dogs in a breed will reproduce, and
- Dogs that do reproduce don’t so randomly and equally — breeders select particular dogs to breed and some dogs have more offspring than others.
Those two facts mean that the genes in a breed’s gene pool change from one generation to the next. Some genes are lost because the dogs carrying them never reproduce; some genes become more common because dogs carrying them produce more offspring than other dogs. As a result, genetic diversity in breeds diminishes over time. The bottomline is, the effective population size may be shrinking despite growing numbers of dogs in the breed.
Number of litters per stud dog, 2005 – 2015
The chart above shows the number of litters (red bars) and the number stud dogs (blue) that produced them. The x-axis is number of litters per dog (the range is 1 – 17). The red and blue bars are the same height for the dogs that produced 1 litter (140 dogs produced 140 litters). When the dogs produced 2 litters, the red bar is twice the height of the blue bar. And so on. This chart reflects the litters produced per English Shepherd stud dog from 2005 – 2015. In total, 290 dogs (blue) produced 730 litters (red); over two-thirds of the dogs (202) sired only one or two litters whereas a handful of dogs produced 10 or more litters.