The Danger of Inbreeding

Gregor Dike, Owner of Zhi-ba Shing-ga Yaks

Almost seven years ago, I purchased a yak heifer at the annual yak show held during the National Western Stock Show. I already had several yaks which came out of Colorado. They were just run-of-the mill-yaks, no pedigree, nothing special about them other than their quirky yak personalities. But at the NWSS there was this beautiful three-year-old heifer with flowing long hair. She was beautiful and was reluctantly for sale. She was from a great yak ranch, had great looks, a great personality and was pregnant from a very good bull. What more could you ask for?

So, I bought her and her baby grew up to be below average for my herd; grew on the slow side and never achieved any real size. I then bred her to a great bull I had purchased from Living Diamonds Ranch and got the same result. I tried again with a large Royal bull that had been siring great calves for me with the same result. I even bred her to a bull whose genetic history probably had no connection to hers based on where he came from, but again, the same result, a less than average calf.

She was beautiful and all the bulls were very good. Yet every time her calf was a dud. If I had looked at her pedigree, which I hadn’t when I bought her, I would have discovered that one very excellent bull appeared three times in her pedigree (maternal and paternal) and another great bull appeared four times in her pedigree. The result was not only a most beautiful yak, but also a yak that would never have quality offspring due to her genetic makeup.

This heifer had been produced by using the same two bulls again and again in earlier generations of her pedigree to “bring out” certain attributes.  This method of breeding when planned is called linebreeding.  The approach allows one to get certain characteristics to dominate or “breed true”. So, when she was born, she had all these wonderful visual attributes that made her so beautiful. However, while her genome was being “developed” by using these bulls again and again, at the same time these wonderful genes were locked into her genome other genes were being locked into her DNA which prevented her from having quality calves.

Suppose yaks had a recessive purple color gene and you discovered that your bull had this recessive gene and every so often one of your cows would give birth to a purple yak. It takes two copies, homozygous, of the purple gene to have a purple calf and animals with only one copy, heterozygous for purple, would be “normal” colored. If you got a purple heifer out of this purple bull she is homozygous meaning this baby now has two exact copies of this purple color gene. What if you bred the bull (dad) to the offspring (daughter), could you get a baby whose color genes were both purple? The probability is 50% now. And if this was a purple bull calf you now have a bull that will always contribute a purple color gene.

But suppose in trying to get more animals to be purple in color, other genes relating to smaller size and cleft palates were also getting locked into the genome. This undesired outcome is what inbreeding can result in.

The more one breeds yaks which have specific genes in common (good and bad), the greater the chance the offspring will have specific genes in common so that when the animal is used in a breeding program, these recessive undesirable traits have a greater likelihood of being expressed.  Instead of the animal having no alleles for cleft palate, the animal may now have one allele for cleft palate and one allele for normal palate and you can’t see the animal is carrying this undesirable recessive trait.  When you mate this individual (heterozygote) with another animal having one copy of the cleft palate allele and one copy of the normal palate (heterozygote), you have a 25% chance of getting an animal expressing cleft palate (homozygote for cleft palate), a 25% chance of the animal having no copies for cleft palate (homozygote normal cleft), and a 50% chance of the progeny being a carrier having one copy of each allele.  Without knowing, you are increasing the frequency of this undesirable trait as there is a 50% chance of having “carrier” animals that don’t show the symptom which may be retained in the herd.  This is a direct result of inbreeding, increasing the frequencies of undesirable traits.

This is how the scientists define inbreeding: “Inbreeding in Cattle”

By Sally L. Northcutt, Extension Beef Cattle Breeding Specialist and David S. Buchanan, Professor of Animal Science and Archie C. Clutter, Associate Professor of Animal Science.

“Inbreeding is the mating of individuals that are related. In the broad sense, all members of a breed are related. As a result, any seed stock producer is practicing some inbreeding. Therefore, we generally reserve the term inbreeding for the mating of animals that are more closely related than the average of the breed.”

Dr. Northcutt continues to explain that inbreeding does not create the undesirable genes.  These undesirable genes may occur from mutations of the DNA.  However, inbreeding can lead to these undesirable genes being expressed and more often.  As animals become more homozygous or less variable in their genetic make-up, average performance for various production factors can be reduced; this is referred to as inbreeding depression.  The greatest impact of inbreeding depression occurs with reproductive traits such as delayed puberty, poor fertility, and increased abortions due to lethal recessive traits.  However, growth and mature size can also be impacted while there is little impact on carcass traits. 

The impact of inbreeding depression is opposite that of the ability of parents to pass their genetic superiority to their offspring which is referred to as heritability.  Reproductive traits are considered to be lowly heritable meaning that it is difficult to improve reproduction efficiency quickly in a herd based on selection of the bulls used and heifers retained.  However, inbreeding has a large negative impact on reproduction efficiency which is exactly opposite of what is desired.  Why do we care?  Having our cows breed and give birth to  healthy  calves that mature well helps the pocketbook so we have something to market eventually.

In reality, the greatest strides in reproductive traits are made in mating animals that have less in common genetically such as outcrossing or crossbreeding.  When animals that are far apart genetically are mated, there is more hybrid vigor or heterosis which is the advantage the offspring exhibits over the average of the two parents.  For example, if a Yak bull weighed 275 pounds at weaning and the cow weighed 225 pounds, the average weaning weight expected of their progeny would be 250 pounds.  However, if the offspring weighed 260 pounds, this 10-pound increase over the expected average is the advantage from heterosis.   In contrast, inbreeding reduces genomic variability meaning heterosis is reduced or lost.

Consider the following diagram which shows two effects of an increase in the coefficient of inbreeding for a herd of cattle. The diagram shows that as the coefficient of inbreeding increases there is an increase in the percent of cows that do not conceive (are open) and in the number of calves that are stillborn.

Inbreeding depression for % open and % dead at birth.