SELECTIONS - Winter 2003
Putting Inbreeding Into Perspective
by Charles Sattler, vice president, dairy progeny testing and genetic research
Editor's note: This is the first of two articles about inbreeding. Part two
will be in the summer issue of SELECTIONS. To read both articles, visit
www.selectsires.com. Click on News & Info, then Animal Health, Diseases and
Genetic Recessives.
When inbreeding is mentioned, it usually conjures up unpleasant thoughts.
But, as with most things, reality is not as extreme as perception.
Improving dairy cattle genetics starts by increasing the percent of
good genes in the population and replacing bad ones. We also have goals
for consistency and uniformity. To do this, we make use of the
highest-performing cow families. As more animals become descendants of
these family lines, however, the chance that we are mating distantly
related animals goes up, leading to a gradual increase of inbreeding.
This isn't all bad, because clearly we have improved productivity and
uniformity. The challenge is to continue selection intensity without
"too much" inbreeding.
A closer look at the genetic process shows that an animal has two copies
of every gene, one each from her sire and dam. Inbreeding is what happens
when identical genes are inherited from the parents. For this to occur,
the sire and dam must have a common ancestor. If parents are completely
unrelated, there is no inbreeding (this is unlikely among animals of the
same breed). The chance of inheriting identical genes from both parents
increases as they are more closely related.
When mating two related animals, we never know exactly how inbred the
offspring will be, since only a full DNA analysis can tell us. However,
inbreeding probability can be estimated through pedigree analysis. This
number, the inbreeding coefficient, estimates the percentage of identical
genes that are inherited. Inbreeding coefficients are estimates, not
guarantees. Although we use estimates daily (e.g., genetic evaluations),
inbreeding coefficients are a bit different because they never are backed
up by real data. Typically genetic evaluations start with a Parent Average
(PA) and then track offspring’s performance to establish genetic merit.
Inbreeding coefficients remain PAs forever, since we don't know exactly
which genes are transmitted. If we mated the same parents 100 times, the
inbreeding coefficient is a good indicator of how often identical genes
would match up in the offspring for the group. But, individually, some
offspring will be less inbred than the estimate and others more so.
Also, the inbreeding coefficient does not identify whether the genes matching
up are undesirable or desirable. If an animal inherits good identical genes,
then inbreeding is beneficial. Results of individually mating related animals
vary based on the number of identical genes that get matched up by chance,
and the quality of those genes that happen to be identical.
The key to the inbreeding dilemma is to find a balance between genetic
selection and control of inbreeding. In fact, inbreeding can be a wise
way to make rapid gains in a single generation for A.I. organizations and
herds that are working to develop a particular cow family. The Jersey,
Golden MBSB of Twin Haven-ET, is an example (Table 1). She has a high 9.0
percent inbreeding coefficient, but is a genetically elite, profitable
producer. This shows the reward/risk of inbreeding. While some inbred
matings result in terrific performance, on average, inbred matings perform
below expectation due to inbreeding depression.
Table 1. Performance measures for Golden MBSB of Twin Haven ET.
| |
Genetic Values |
Actual Performance |
| Milk |
+1,174 |
30,380 |
| Fat |
+71 |
2,084 |
| Protein |
+53 |
1,251 |
| Type |
+2.5 |
Excellent (94) |
| Other |
+331 JPI
(10th in breed) |
National |
November 2002 data
Research conducted by Virginia Tech estimates that for each 1 percent
increase in the inbreeding coefficient, there is a corresponding loss of
$22 net income. This came from tracking performance of more than 2.6 million
cows after they entered the milking herd, and shows that inbreeding depression
reduces production, reproductive performance and longevity (Inbreeding losses
occurring before first calving are not in this estimate). Table 2 shows the
value that is lost due to inbreeding or gained due to genetic improvement
for several important traits.
Table 2. Values for genetic change in various traits.
| Trait |
Value |
| Inbreeding* |
-$22 per 1% increase in inbreeding coefficient |
| Milk+ |
+$24 per 100 pounds PTA Milk |
| Udder composite+ |
+$29 per 1 point of UDC |
| Feet-and-Legs composite+ |
+$15 per 1 point of FLC |
| Productive Life+ |
+$28 per 1 month of PTA PL |
| Somatic Cell Score+ |
+$15 per 0.1 point of PTA SCS |
| Calving Ease |
+3 per 1% DBH
(when breeding heifers) |
*Source: Smith et al., Virginia Tech. +Source: USDA.
With each replacement heifer so important to profitability, ignoring
inbreeding is not a good strategy. Neither is avoiding all inbreeding,
because you sacrifice more genetic improvement than what is gained by
minimizing inbreeding depression. The best strategy is to make sire choices
based on current genetic evaluations, and then mate these sires to specific
cows considering the inbreeding these matings may cause. For specific
guidelines you can use to manage inbreeding in your herd, visit www.selectsires.com
and watch for the next issue of Selections.
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