Performance Line
Animal Breeding 101: Heritability
By Tonya Amen, National Center for Beef Excellence
Last month, I shared with you some information
about the value of heterosis. This month,
I’d like to discuss a similar sounding and
related word -- heritability. It’s a word that
comes up frequently when cattle genetics is
discussed. But, what does it really mean?
For starters, it is completely different than the
word inherited. It also does not mean that
describing a trait as 60% heritable (your height,
for example) means that 60% of the reason you
are as tall (or short) as you are, is due to genetics,
even though this is the most common way I hear
it discussed. Instead, heritability describes how
much of the variation for a trait that exists in a
population is due to variation in genetics that
can be passed on from parent to offspring.
To start, let’s take a look at why animals perform
the way they do for most traits of economic
importance: weight traits, reproductive
traits, carcass traits, etc. These traits are
referred to quantitative traits, and traits in
this category share several characteristics:
something like this: Var BW = Var G + Var E . Specifically,
the variation that we see in animal performance
for BW is due to the fact that there is variation in
animals’ genetic value for that trait as well as the
various environments the animals are exposed to.
Of course, there are two pieces to
the genetic component (G):
1. Breeding Value (BV) is the piece that can
be selected for. If we were able to add up
the effects of all of the alleles an animal has
for a specific trait, the sum would be it’s
breeding value. Half of an animal’s alleles
are passed on to its offspring and ½ of a
breeding value is referred to as a progeny
difference. You are well familiar with
Expected Progeny Differences – estimates
of the portion of an animal’s genetic worth
that can be passed on to their offspring.
2. Gene Combination Value (GCV) is the
piece that is due to the way the genetics
from sire and dam combine in the
offspring and is determined purely by
chance. Hybrid vigor and inbreeding
depression fall into this category.
• there are many genes that
impact performance • the traits are expressed on a
continuous scale (a range of weights,
for example), instead of lumped
into categories (red vs black) So, if we were to re-write our formula for the
variation that exists in a certain population for
birth weight, it would now look something
like this: Var BW = Var BV + Var GCV + Var E .
• the environment can play a role –
sometimes large – in how animals
perform for these traits. Recall that heritability (h 2 ): the amount of variance
in a trait (P) that can be explained by variance
in BV. Applying what we’ve learned above, the
formula would look like this: h 2 =Var BV / Var P .
Using birth weight as an example, if we were
to write a formula for variation in weights that
is observed in a group of animals, it would look
18 | APRIL 2019
Table 1 below shows the heritability estimates
for the various traits in the beef cattle, there’s