A simple question that many ask.

My own personal take on the whole line breeding, inbreeding thing is not based on any education, study or great intellect. To be perfectly honest I don`t even know where my idea of it actually came from at all.

Line breeding to me has always meant breeding close, but in a `safer` way than inbreeding. For instance, line breeding would be father x daughter, mother x son, grandoffspring to grandparent, or more distant cousins. Like breedings I have seen in some instances in cattle, horses and dogs.
Inbreeding would be a more risky, direct cross of full brother sister. Something that I have not seen in other animals much and would only do with chickens on certain occasions, but have done it a bit and may do it again under certain circumstances.

Like I say, this has always been my own little take on the difference. Not really based on anything concrete, just what has rattled around in my head all these years!

I found it interesting that there was different takes on it so I went to Word and looked in the dictionary and this is what I found:
Inbreeding - the mating of closely related members of a species, especially over many generations. It may be used to enhance desired traits in animals or plants but is avoided in humans as it increases the risk of unwanted inherited characteristics.
Line breeding - Form of inbreeding, the deliberate mating of closely related individuals in order to retain characteristics of a common ancestor

So there you go, that is as official as I get!

*just a note to say I am surprised how many times this thread has been read. Glad there is interest about it.

I apologise in advance for the tech-y language. I'm not trying to show off, it's just that it's pretty much impossible to say this stuff concisely without it. (And it's not THAT tech-y, really).

One main point I'm trying to make is that inbreeding is a continuum, ranging from very mild to extremely high. It's not a question of inbred or non-inbred, it's HOW inbred?

Ignore the next two paragraphs if you hate the tech-y language.
Inbreeding is measured by the inbreeding coefficient, F, or the probability that both alleles at a given locus are identical by descent (IBD). It ranges from just above 0 (very very mild) to 1, the point where both alleles at a given locus are identical by descent, i.e. are derived from the same ancestral individual. Obviously, F is never going to be 0, since everything is related to everything else at some point back there.

The more loci have both alleles IBD, the more likely it is that deleterious recessives, some of which are present in every genome (including yours), will be expressed, since there needs to be two copies of a recessive in order for it to be expressed.

And, in general, as you'll see if you google "inbreeding" and read a few things that come up, or, better yet, consult a basic genetics text, higher levels of inbreeding reveal more deleterious traits (inbreeding depression). As already noted, chooks are less prone than many animals to inbreeding depression; they are not immune, and I expect most of you have seen examples, in birds that "fail to thrive" or display genetic anomalies. (And we all know about those hip problems that some dog breeds have - they are a result of inbreeding and consequent fixation of bad alleles in the breed.) The only ways to combat inbreeding depression are through:
a) outcrossing, and/or
b) selection.

As Coopslave points out outcrossing is quite likely to bring in new unwanted traits, which is a problem if you care about show standards. It will, however, generally increase "fitness" traits, i.e. those acted on by natural selection.

Stringent selection can overcome inbreeding depression, again as Coopslave points out. It must be done on fitness traits (vigour, growth, fertility, etc.) as well as on the desired show traits, however, or you will end up with feeble undersized stock, and low fertility.

I've come full circle, and will shut up after one last comment (which is, I think, the first one I ever made on the topic) - inbreeding can be a useful tool. It can also be a dangerous one. It should NEVER be used for mere convenience, but must be accompanied by rigorous selection.

Not to get into an argument. But inbreeding isalways happening, period. So to say it can be a useful tool, is kind of a moot point.

The major thing is to accept what you are comfortable with as a level of inbreeding. To note, most times top individual animals of almost all species are linebred ones.

The biggest fallacy that keeps rearing its ugly head, is that linebreeding causes genetic defects. Wrong, wrong, wrong. Linebreeding does often ties reveal genetic defects quicker than siply outcrossing, but it does not cause the defect. To say hip problems in dogs are caused by linebreeding, is so inaccurate. Hip problems in dogs are caused by a genetic defect being in the breed, and the failure of breeders to cull it out.

Interestingly, the failure to remove the problem and rather try to outcross to eliminate it, usually results in the problem reoccurring in about 7 generations in animals that most wouldn't even consider linebred. So it is a matter of choice, do you tae the safe and somewhat head in the sand approach and just outcross continually, or do you take a slight risk and have faith enough in your genetics to linebreed. By linebreeding, you can hopefully reveal any defects before they are spread throughout a population, and eliminate the defect, at least from your line. Yes, nature has its limitations on how closely linebred you can get. Most often you lose either fertility or size/growth rate, when you get past a certain point (vary variable with each different line and how intensely selection for traits is). That is one of the reasons to try to keep the parent stock, and a time to consider outcrossing, even if only to a somewhat less linebred animal from the same line.

An interesting aside, is that the failure to linebreed in a breed with a small population, actually decreases the genetic variation within the breed. Also as much as we think that we are fixing genes and will lose diversity by linebreeding, cattle research has indicated that we may not be doing as much as we think, provided we are dealing with a reasonable sized population.

In closing, it is up to each breeder to decide what level of inbreeding they are happy with. And as IPF said, outcross and/or selection is the way to combat inbreeding depression. There is no exactly right or wrong way to run a breeding program. It is your choice and ultimately your decisions will determine your success.

Good post, lots to think about.

coyoteacres wrote:Not to get into an argument. But inbreeding isalways happening, period. So to say it can be a useful tool, is kind of a moot point.

The major thing is to accept what you are comfortable with as a level of inbreeding. To note, most times top individual animals of almost all species are linebred ones.

The biggest fallacy that keeps rearing its ugly head, is that linebreeding causes genetic defects. Wrong, wrong, wrong. Linebreeding does often ties reveal genetic defects quicker than siply outcrossing, but it does not cause the defect. To say hip problems in dogs are caused by linebreeding, is so inaccurate. Hip problems in dogs are caused by a genetic defect being in the breed, and the failure of breeders to cull it out.

Interestingly, the failure to remove the problem and rather try to outcross to eliminate it, usually results in the problem reoccurring in about 7 generations in animals that most wouldn't even consider linebred. So it is a matter of choice, do you tae the safe and somewhat head in the sand approach and just outcross continually, or do you take a slight risk and have faith enough in your genetics to linebreed. By linebreeding, you can hopefully reveal any defects before they are spread throughout a population, and eliminate the defect, at least from your line. Yes, nature has its limitations on how closely linebred you can get. Most often you lose either fertility or size/growth rate, when you get past a certain point (vary variable with each different line and how intensely selection for traits is). That is one of the reasons to try to keep the parent stock, and a time to consider outcrossing, even if only to a somewhat less linebred animal from the same line.

An interesting aside, is that the failure to linebreed in a breed with a small population, actually decreases the genetic variation within the breed. Also as much as we think that we are fixing genes and will lose diversity by linebreeding, cattle research has indicated that we may not be doing as much as we think, provided we are dealing with a reasonable sized population.

In closing, it is up to each breeder to decide what level of inbreeding they are happy with. And as IPF said, outcross and/or selection is the way to combat inbreeding depression. There is no exactly right or wrong way to run a breeding program. It is your choice and ultimately your decisions will determine your success.

You nailed it with that one, very well explained. Thanks

Yes, I agree that “inbreeding is always happening”. How does that mean “it is a moot point to call it a useful tool?” The degree, intensity and pattern depend on the practitioner, who uses his/her control over mating patterns. It sounds like you use inbreeding deliberately and with careful thought (some do, some don’t). This, in my opinion makes it a tool, and it is clearly a useful one.

Note, I didn’t say that “linebreeding causes genetic defects”. What I said was that some defects are “a result of inbreeding and consequent fixation of bad alleles in the breed”. To clarify, yes, it’s the genes (bad alleles) that cause the defects, but it’s the process that allows allele frequency to be driven to 100% (i.e. fixed). This travel to fixation is inadvertent, either though the random genetic drift that happens inevitably in small populations, or through linkage (being inherited on the same chromosome) with other, more desirable, alleles. But allele fixation does happen in small populations that are subject to inbreeding; there is no doubt about that.

When you say “Hip problems in dogs are caused by a genetic defect being in the breed, and the failure of breeders to cull it out.” you’re saying the same thing as I. What I add to that is that the frequency of the gene for the genetic defect has not only NOT been culled out, but has been allowed to rise (by the process decribed above) to virtually 100%, and that the process of inbreeding in a restricted population has facilitated this rise.

I like your caveat “. . . provided we are dealing with a reasonable sized population.”

Only a moot point, because if you are inbreeding whether you intend to or not, it is the only option. I agree that a planned program can be very useful.

I still do not like the way it says that some genetic defects are a result of inbreeding. It would be much more correct, that some genetic defects are displayed as a result of inbreeding. It is not the inbreeding itself that causes the defects, and as with dogs, it is up to the breeder to cull, and not propogate genetic defects.

In dealing with rare breeds, a favorite saying is the failure to cull, only makes the breed rarer.