Those Tricky Statistics

Fell Pony stallion Lunesdale Mercury

I received a call from a student at one of my alma maters.  It was of course about fundraising, but then she asked for advice as a soon-to-be-graduating senior.  I asked what she was studying, and she answered that she was majoring in statistics.  My advice to her was to always use statistics honestly, to know what was truly meaningful and useful as information and to know what was not.

Around the same time, I read an article in Equus magazine about heritability of traits. (1)  It discussed the difference between dominant and recessive and homozygous and heterozygous genes.  It then tried to simplify things for the reader, and that’s where things unfortunately went astray.

It was because of Foal Immunodeficiency Syndrome (FIS) in Fell Ponies that I learned about the trickiness of statistics.  By making the same mistake that the article did, I learned how statistics are applied to recessive traits in equines like FIS.   If a clear and a carrier pony are bred, then each foal they produce has a 50/50 chance of being clear.  Sometimes, as happened in the article, it is stated that the foals resulting from this cross will be 50% clear and 50% carrier.  That is not the case, though.  It is a subtle but important distinction (each foal versus all foals) and one that illustrates how easy it is to misinterpret statistics and hence why I gave the advice to my caller that I did.

The Equus article talked about the inheritance of the gray color, which is dominant.  It stated that “…when heterozygous gray horses are mated to noncarriers of the gray gene, only half of the offspring will be gray.” (2)  As an example of the error in this statement, one can consider the progeny of Lunesdale Mercury, the gray Fell Pony stallion pictured here.  As of this writing, Mercury has sixty offspring registered with the Fell Pony Society.  The statement in the article would lead you to believe that thirty of the offspring would be gray, but in fact only 26 have been, or 43%.  That’s because each foal - not all the foals collectively - has a 50/50 chance of being gray when one parent is heterozygous for gray and the other is non-gray.

I knew a Fell Pony breeder once who played with the gray statistics in our breed.  They wanted a gray foal, so they purchased a gray mare in foal to a gray stallion.  In Fell Ponies, grays are typically heterozygous, having only one gray parent.  Therefore the chances of any foal being gray are typically 75% when two grays are bred to each other.  As you will have no doubt guessed, the breeder ended up with a non-grey (black) foal!

My collegiate caller indicated she intended to go into criminal justice, a field where statistics are heavily reported and used in decision-making.  She seemed very aware of how easily statistics can be misinterpreted.  Hopefully then she’ll avoid the mistake that was made in the Equus article that might incorrectly inform someone making a decision.

1)      Jeffries, Abigail.  “Applied Genetics,” Equus #479, August 2017, p. 37.
2)      Jeffries, p. 41.

© Jenifer Morrissey, 2017