Inbreeding in Chefs-de-Race, Their Sires and Their Dams

Jones and Bogart, in "Genetics of the Horse" (Edwards Brothers, Inc., Ann Arbor, Michigan, 1971), define inbreeding as the mating of two individuals more closely related than the average of the breed.  They highly recommend that serious breeders know the amount of inbreeding in their stock in order to appreciate the rate at which homozygosity is developing, homozygosity being the condition where genes inherited from the sire and the dam are alike.  The amount of inbreeding estimates the percentage of genes put in a homozygous condition.  This estimate is always less than the the actual because records will not allow for every relationship that exists.  Horses that are highly homozygous tend to stamp their own characteristics on their offspring, a phenomenon often called "prepotency".  Prepotency also may be considered a regular or predictable transmission of particular traits.

The amount of inbreeding can be calculated using a formula proposed by Wright in 1923 (Mendelian Analysis of the Pure Breeds of Livestock, J. Hered. 14:339-348).  Wright's formula generates a fraction (the coefficient of inbreeding) which, when multiplied by 100, affords the amount of inbreeding as a percentage.

The information presented below includes the amount of inbreeding for 202 chefs-de-race (not including Buckaroo, Chief's Crown or Pleasant Colony at this time), as well as for their sires and their dams.  The raw data are preceded by some general statistics.

Observations:

The general trend of inbreeding percentage among chefs-de-race declined steadily throughout the 20th century.  The relationship between the percentage of  inbreeding and time is expressed by the following formula:

% Inbreeding = -0.0109 * Year + 22.262

Therefore, the overall rate at which inbreeding has declined since 1900 is 0.0109 percent per year, although dramatic changes occurred until about 1940, followed by a relatively steady percentage since then.  Whether this pattern among chefs-de-race represents the patterns found among all Thoroughbreds is not known at this time.  In any case, chefs-de-race foaled in successive ten-year time frames display a diminishing amount of inbreeding.

The amount of inbreeding found among chefs-de-race covers the range from over 13% for Ultimus down to 0% for Speak John, with some of the most influential stallions being found at both ends of the spectrum.  For example, Havresac II (a key link to Ribot) and Bayardo (a key link to Hyperion) are among the most highly inbred, while Native Dancer (the close up ancestor of Raise a Native and Mr. Prospector) and Ribot himself are among the least inbred.

Arguably the best runners among the chefs-de-race - Secretariat (0.1953%), Man o' War (0.8350%), Ribot (0.1465%), Nearco (0.9839%) and Sea-Bird (0.3906%) - were all inbred below the average percentage of inbreeding for the time frame in which they were foaled.

There is no significant pattern covering the relationship between inbreeding in the chefs-de-race and inbreeding in their parents.  Most (73, 36.1%) are less inbred than either parent, followed by 66 (32.7%) that are inbred more than one parent but less than the other, and 58 (28.7%) that are inbred more than either parent.  The remaining 5 (2.5%) are inbred the same as one parent.

For 55.9% of the chefs-de-race (113), their sire is more highly inbred than their dam.  For 43.1% of the chefs-de-race (87), the reverse is true.  There are two examples (1.0%) where the sire and dam of the chef-de-race exhibit the same level of inbreeding.

There are observable differences in inbreeding among chefs-de-race sorted by aptitudinal contribution; however, these differences are not statistically significant.

One may conclude from the data that the amount of inbreeding in a chef-de-race or his parents is unrelated to his ability to transmit consistent aptitudinal type to his progeny.  Prepotence for aptitudinal type apparently has a different origin.

General Statistics:
Avg. % inbreeding of all chefs-de-race: 1.1181
Avg. % inbreeding of the sires of  all chefs-de-race: 1.1558
Avg. % inbreeding of the dams of  all chefs-de-race: 1.1884

% Inbreeding over Time
Avg. % inbreeding of chefs-de-race born before 1910: 1.8091
Avg. % inbreeding of chefs-de-race born 1910-1919: 1.5773
Avg. % inbreeding of chefs-de-race born 1920-1929: 1.3128
Avg. % inbreeding of chefs-de-race born 1930-1939: 1.0025
Avg. % inbreeding of chefs-de-race born 1940-1949: 0.9687
Avg. % inbreeding of chefs-de-race born 1950-1959: 0.9521
Avg. % inbreeding of chefs-de-race born 1960-1969: 0.9114
Avg. % inbreeding of chefs-de-race born after 1969: 0.9409

Inbreeding in Chefs-de-Race Relative to Their Sire and Dam
Chefs-de-race with % inbreeding < % inbreeding in either sire or dam: 73 (36.1%)
Chefs-de-race with % inbreeding > % inbreeding in either sire or dam: 58 (28.7%)
Chefs-de-race with % inbreeding > % inbreeding in sire and < % inbreeding in dam: 27 (13.4%)
Chefs-de-race with % inbreeding < % inbreeding in sire and > % inbreeding in dam: 39 (19.3%)
Chefs-de-race with % inbreeding = % inbreeding in either sire or dam: 5 (2.5%)

% Inbreeding in Chefs-de-Race by Aptitudinal Group Representation
Avg. % inbreeding for chefs-de-race in Brilliant: 1.3653
Avg. % inbreeding for chefs-de-race in Intermediate: 1.1377
Avg. % inbreeding for chefs-de-race in Classic: 0.9255
Avg. % inbreeding for chefs-de-race in Solid: 0.8945
Avg. % inbreeding for chefs-de-race in Professional: 1.0043