A polled Devon cow with a distinctive poll or knob on top of the head

A polled Devon cow with a distinctive poll or knob on top of the head

Here is a polled calf without the distinctive poll, but with the smooth rounded top of the head that some polled cattle can have.

Here is a polled calf without the distinctive poll, but with the smooth rounded top of the head that some polled cattle can have.

Answering a Question

A couple of weeks ago Guille and I received a question from a RDUSA member about polled and horned genetics.  After my feeble attempt to answer the question, Guille suggested that I post it on the blog in order to have some more informative type articles for newer members who may not have any experience with these concepts.  As most of you know genetics is a concept studied in Biology 101, so here is a retired math teacher’s stab at a lesson from Biology 101.  I will apologize now to those experienced cattle people who already have this knowledge.

Genetics with very little Math

Physical aspects of any animal is controlled by the genetic makeup of the animal which comes in a sequence of genes which are always in pairs.  Each pair contains one gene donated by the sire and one donated by the mother.  With cattle the poll gene is the dominant gene and the horn gene is recessive.  The term recessive seems to come with the connotation that it is undesirable. That is not always true as recessive means that it cannot override a dominant gene.  Since the poll gene is dominant we will designate it as (P) and the recessive horn gene as (h).  Since the poll gene is dominate any animal that carries at least one poll gene (Ph or PP) will always be polled.  For an animal to have horns it must have two horn genes (hh).

Here is a Devon cross oxen with the flat across the top of the head between the horns which is typical.  Even horn cattle that have been dehorned have this flat top except maybe those dehorned at birth with paste. Many older cattlemen call this type of head "square".

Here is a Devon cross oxen with the flat across the top of the head between the horns which is typical. Even horn cattle that have been dehorned have this flat top except maybe those dehorned at birth with paste. Many older cattlemen call this type of head “square”.

 Now just a little Math

Now lets find the probability (math) of some gene combinations from various parents.  Lets mate a homozygous (both genes are the same, PP) polled parent with a horn parent (hh).  There are only 4 outcomes (Ph, Ph, Ph, Ph) with each having a 25% probability of occurring.  In this case all the outcomes are the same with a polled calf guaranteed.  A homozygous poll parent will always have polled offspring.  Now lets mate two heterozygous polled parents (Ph).  We are mating  Ph x Ph and the outcomes are PP, Ph, Ph, and hh.  Now we have a 75% probability of polled offspring with a 25% probability of homozygous polled and 50% probably of heterozygous polled.  There is also a 25% chance of horned offspring (hh).  The only way two polled parents can have horned offspring is for each to carry one horned gene and horned offspring can show up after 2 or 3 or more generations later.

I guess the only mating remaining is a heterozygous polled animal with a horned animal (Ph x hh).  Now the outcomes are Ph, Ph, hh and hh.  We have a 50% probability of a polled calf, but that polled calf will have to be heterozygous polled.  We also have a 50% chance of a horned calf.

Oh my, the Memories

This reminds of the days when I was still teaching math and my students, knowing I raised cattle, would ask me questions about cattle to shorten the time in class.  I was amazed that the majority of them thought that all cattle were black and white and that you could easily tell bulls from cows because all bulls had horns and all cows did not.  I would inform them that horns had nothing to do with the sex of cattle, but refrained from offering any more information.

Having raised registered Herefords, which is another breed with both polled and horned genetics, before switching to Devons reminds me of listening in on some very heated discussions on whether the highest quality animals were from polled or horned genetics.  Fortunately, we have very high quality Devons from both polled and horned genetics.

(first two photographs by Georgia Heller, third by Roy)

Submitted by Roy