If you have a German Shepherd, you’re probably familiar with the damaging effects of inbreeding on fitness. The weak hind ends and other health problems of German Shepherds are the result of more than a century of breeding between close relatives. Inbreeding heightens the chance that harmful genetic traits will be expressed. Still, the dog-breeding industry remains popular worldwide.
A recent study published in PNAS analyzed when German Shepherds accumulated their “mutational load” of bad genes. Was it from when they first became a distinct breed? Or is it mostly from their continued inbreeding of late? Understanding the time frame for the declines in genetic diversity of German Shepherds may help restore their genomic health through the introduction of new genes.
Researchers from Germany, the U.K., the U.S., and Switzerland compared the genomes of German Shepherds from medieval times before breeds were formed; the historic breeding period from 1906 to 1993; and a dataset of more than 2,000 modern dogs. The medieval specimens came with an already published genomic dataset of Lithuanian dogs, whereas the 20th century genomes were extracted from museum specimens of German Shepherds.
The diversity of the medieval dog genomes suggested a large, randomly breeding population. Their genomes were mostly heterozygous—with a different allele from each parent—with few homozygous sequences (pairs of identical copies of genes that emerge with inbreeding).
Read more: “The Surprisingly Ancient Origins of Your Dog”
Then dog breeding became a “thing” in the 20th century, and the proportion of short sequences of identical allele pairs reached as high as 19 percent circa 1960. But inbreeding levels didn’t skyrocket until later in the 20th century. In modern dogs, the identical allele pairs show up as about 36 percent of the genome, including medium and large sequences of them.
As for who is to blame, the study authors pointed a finger at male dogs who were disproportionately used as sires. These macho males effectively decreased the genetic population size of German Shepherds. The first genetic bottleneck—or period of reduced diversity—showed up from 1906 to 1934, when the dog Pollux and his grandson Horand von Grafrath sired a bunch of offspring. In around 1927, the gene pool contracted again when Klodo vom Boxberg became a popular male, and again in 1946 with Axel von der Deininghauserheide. Because of World War II, novel genetic stock from Germany was unavailable outside of Germany until reunification in 1990.
And the rest is history. During the 1960s, a favored American sire, Lance of Fran-Jo, introduced the sloped hindquarters you see in modern American lineages of German Shepherds. Continued inbreeding has enhanced the slope while weakening the hips by lowering the pelvis.
“Strong artificial selection and inbreeding by humans during the second half of the 20th century to incorporate or maintain specific aesthetic criteria, rather than the formation of the breed itself, is responsible for the genomic health declines in contemporary GSDs,” wrote the study authors.
So, it’s a get-what-you-get when it comes to pedigreed dogs. If you want those iconic sloped backs in German Shepherds, you can expect to deal with canine hip dysplasia as well. If you want a more robust dog, get a mutt that originates from a more diverse gene pool. 
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Lead image: Tasnuva Elahi; with images by McCoy_LG and Who is Danny / Shutterstock.
