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Inbreeding Basics
Grow Guide: Inbreeding Basics Category: Breeding
Author: web420 Post Date: 05-13-2006
Rating: 4.25/5 (4 votes) Views: 3339
Inbreeding
Defn: Inbreeding is the crossing of related individuals. The most severe form of inbreeding is selfing and then it graduates from there with less and less degrees of inbreeding as the crossed pairs become less related.

Inbreeding results in an increase in homozygous individuals but also an increase in variability between related inbred populations. I'll explain with an example. Say you only crossed siblings from generation to generation. Like take a mongrel mix of seeds and cross two keepers. From these offspring, pick 4 pairs and make 4 seedbatches. Then from each seedbatch, pick two keepers and cross them to make four more seed batches. Repeat this process for a couple of more generations and you would end up with 4 highly homozygous inbred populations that are related but probably very different from each other.

Here's another way of looking at it, let's say you start with seeds with the genotype AaBbCcDdEe, by the time you are done several generations of sibling inbreeding, you could have four seedlines; aaBBCCddee, aabbccDDee, AAbbCCddEE, AABBccddEE, or any one of the other possible homozygous combinations. The more related the parents are and the smaller the population size, the faster each line will approach homozygosity, selfing being the fastest.

Inbreeding's Effect on Vigour
With each inbreeding's path to homozygosity, it should be noted that the faster homozygosity is achieved, the more likely the genotype will be determined by random chance and not selective pressures, whether it be natural or human directed. This is because homozygosity is achieved before the forces of selection can take place or before recombination can rework the genetic combinations to come up with the best combos with respect to fitness. In other words, as a breeding population size decreases and the relatedness of the population increases, the less effective a breeder's selective efforts will be. The results will be determined more by random chance than anything. Regardless of the breeder's intentions.

But How's this Relate to Vigour?
Well, it matters more to outcrossers and less worked seedlines than to heavily worked selfing populations. This is because of what geneticists call "genetic load". Genetic load is all the recessive and deleterious genes that are contained in a plant's genotype that can hurt vigour. When heterozygous, they have no effect, but when homozygous, they can reduce a plant's fitness. The greater a plant's genetic load, the greater the chance is that an inbred line from that plant will be recessive for some of the hidden negative recessives that will reduce the seedline's vigour. When there is an accumulation of these homozygous negative recessives, we call this "inbreeding depression". Therefore, as breeding population sizes shrink, or the relatedness of parents increases, the effect of inbreeding depression increases.

Why Don't all Inbred Lines Suffer this Inbreeding Depression?
Agriculture is full of examples of crops and varieties that are severly inbred, homozygous, and still very vigorous. These are from genepools that were the result of hundreds of generations of inbreeding that allowed much of the genetic load to be removed. And this is the basic difference between inbreeding species and outcrossing species, the amount of genetic load contained in their genepools. Outcrossers don't really allow the genetic load to be removed and therefore it accumulates. Unfortunately, cannabis is predominantly an outcrosser, although not absolute. The natural occurance of hermies has allowed some of the genetic load to be removed and therefore cannabis is somewhat more tolerant of inbreeding than other species such as dogs and humans. But still much less tolerant than selfing species such as most field crops.

Will Inbreeding ALWAYS lead to Inbreeding Depression?
No, there is always a chance that the inbred homozygous population will be lucky enough to not have any of the deleterious recessives. Like take the AaBbCcDdEe heterozygous line in the example above, there is a chance that one of the inbred lines could be AABBCCDDEE and therefore not suffer any inbreeding depression. Once created, population size and relatedness of parents would never theoretically cause inbreeding depression. Sounds like the idea situation, right? The problem is finding that specific inbred line. As already mentioned, the results of the inbred lines is determined by chance, and such a result would occur in one of every 32 inbred lines. The problem is that the genetic load of cannabis is much in excess of 5 genes pairs. If we were only concerned with 10 gene pairs, we would find the homozygous dominant in only one of 1024 inbred lines. The chance of finding the right combo decreases exponentially as the genetic load increases, and cannabis, being an outcrosser, has a significantly high genetic load. Therefore, the chances of randomly finding the perfect purely homozygous plant in our generation, or even century, is pretty much non existant. Therefore, qualified breeder's of outcrossers such as cannabis don't try to remove the genetic load of the cannabis genepool, they choose strategies that try to manage it.

How Does this Relate to Adaption?
From a simple perspective, take the AaBbCcDdEe genotype again. Lets say that offspring wth AA are best suited for arid conditions, and those with BB are best suited for cold situations. In the first round of inbreeding, the breeder could, by random chance, end up with aa or bb and as a result loose the AA or BB genes. Once these genes are lost, the genepool is no longer able to perform well in arid or cold environements. Therefore, as inbreeding increases, adaptability decreases.

Can Inbreeding be Measured?
In a sense yes, one can calulate the estimated decrease in heterozygosity for a generation, or over several generations. To calculate the inbreeding coefficient of one generation, see section A on the attatched image, inbreeding1.jpg. Using sections B, C, and D of the image, the inbreeding coefficient can be used to calculate the inbreeding rate over multiple generations.

Inbreeding basics
Interpreted from pages 33-35 of Wright, 1976


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