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Asked by facefacter to Heather, Keith, Elaine on 18 Jun 2013.
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Elaine Marshall answered on 18 Jun 2013:
Insest, or inbreeding, is the term we use to describe mating of parents who are genetically similar. During the fertilization process, as you know, each parent provides 50% of the genetic information to for the zygote which will develop into the offspring or baby.
In a normal population there is natural variation and there will be many different forms (alleles) for each of our genes. However during inbreeding you have a very small selection of genes now available to you and if there are bad forms of a gene(s) (recessive) then inbreeding concentrates these genes and can lead to an exaggeration if you like of the recessive traits. This may be shown as poor physical/ mental development or disease resistance. Basically inbreeding can lead to a loss of “fitness” for the population .
That said if the alleles gives an advantage to the offspring (stronger/fitter/smarter) then this will be of benefit and farmers have been selecting crops and herds for thousands of years on this basis. However it is risky as you can sometimes concentrate bad alleles and involves a lot of culling of weaker offspring.
A population is stronger for natural variation through out breeding as it gives a population the ability to evolve in response to its environment rather than concentrate alleles and reduce the “selection pot”. Think of it like a huge bowl of sweeties with lots of choice compared to a small pot with only 1 type of sweet – which would you prefer???
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Heather Eyre answered on 18 Jun 2013:
Incest, or reproduction between two closely related people or animals, is a pretty bad idea.
There are lots of genetic diseases that are recessive, meaning it`s hidden if you only have one faulty gene.
Within a group of closely related people lots of them can have a copy of a faulty gene, so there is a bigger chance that their offspring would end up with two faulty copies, and the disease.
Many other problems are the result of many genes, but the same reason applies – incest makes the gene pool smaller and the risks of genetic defects showing get bigger.
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Keith Siew answered on 18 Jun 2013:
So like Elaine and Heather said the incidence of disabilities or disease are higher in the off-spring of consecutive generations of inbreeding due to a smaller gene pool which increases the chances of “bad” recessive traits being expressed.
But I’m going to play devil’s advocate here and say that there are times when inbreeding or incest can be a useful tool. For example, we want to research the effects of a certain gene, we’ll say ApoE in an animal model for the human disease Alzheimer’s. So if we want to delete or mutate that gene to cause or prevent the disease, we then want to make sure that the effects we see are a result of that gene’s influence or loss of it only! Therefore you want to minimise the amount of genetic variability in your population… so very often scientists will use an inbred strain of mouse to do their testing in which they have been inbred for at least 10 generations so that the genetic variability from one mouse to the next is very small and can be “ignored”.
Also inbreeding is useful for keeping a useful or desirable trait in a population. So if we take domestic pets, farm animals or plant crops for example there are loads of examples of inbreeding… the Labrador is a common example because people want either the brown or gold coat and the black coat is actually the dominant gene so sometimes inbreeding is needed to maintain the trait (though this is also the reason why they are at higher risks of health problems). The Belgian blue cow has a mutation in the gene for myostatin which is a hormone which keeps our muscle growth in check… so these cows naturally get muscular without having to do anything because they are deficient in myostatin! They’ve been inbred with one another to keep this trait to produce lots of meat per animal (there are a handful of cases in whippet dogs and two human cases that I know of). When it comes to crops maybe you want to selectively breed plants that grow faster, are resistant to frost damage or certain pests and this may require inbreeding… selective breeding as a strategy in farming and domestication of animals for thousands of years by humans (its how we got the dog from the wolf!).
Also if we’re talking about all being related in some way and inbreeding… then another interesting thing is “mitochondrial eve” who lived maybe 200,000+ years ago… in all our cells there are little energy factories called mitochondria that have their own DNA, however these only get passed down through the female line… so every living human on the planet has the same great, great, great, great, great, great, great, great, great, great, etc…. great, great, great, great, great, great, great, great, etc …..great, great, great, great, great, great, etc … grandmother!!!
Another cool example is blue eyes! About 10,000 years ago there were no people on the planet with blue eyes… and it turns out that the mutation that caused you to have blue eyes instead of brown can be traced back to a single ancestor… so people with blue eyes are all related and would have required some amount of inbreeding in a population to maintain the trait.
Comments
Elaine commented on :
Keith, thanks for expanding my point that sometimes inbreeding can enhance a good trait you have provided some really good examples.
I would like to sounds a cautionary bell though that the use of inbreeding in scientific models is only used in certain circumstances. It is particularly usefully when we are comparing 2 alleles of the same gene. I am thinking especially of the model used to look for the fat genes when they compared thin and fat mice and were able to map chromosomal locations for genes involved in regulating fat deposition. The use of inbreeding is a valuable tool but only when not deleterious. In most cases when we alter the genotype, concentrating the mutation by inbreeding can be very bad and in extreme cases lethal. In these situations, as you know, we always out-breed onto a constant background and maintain a population with only one copy of the altered gene (ie they are heterozygote).
Keith commented on :
Elaine, and thank you for expanding on my point too! An omission on my part!