Why does heterosis exist? Theories?

[Brandonm2]

CB- You know, I can't find it, but it was something that a genetics professor told us based on the original "Eves" and the amount of genetic material shared by humans today. The field of genetics changes so fast that it may have been disproven by now. I'm not taking my graduate genetics course for another year, so I'll let you know then if it stills holds true! :lol: However, either way, we are still much more diverse genetically that cattle, but not as far apart as one would think. Genetically, chimpanzees are very closely related to us. Some scientists would like to put them in the same biological family as humans, (though a different species, of course).

Brandonm2- I respect your knowledge of early cattle breeding and history. Interesting phrasing on the reasons for not so many offspring in human 'sires', ( :lol: ).

I think, perhaps, we have crossed lines as far as the definition of heterosis. Probably my fault because I kept talking about heterozygosity, which is relative variability, (variety of the combination of alleles), that is measurable by percentage of genes that code for different traits.

You are absolutely right when you say that the children of relatively 'purebred' humans from two distinct races would greatly increase heterozygosity. Heterosis, however, is usually used to refer to specific desirable traits that are higher in performance in offspring of crossbred cattle. Not sure if we can make the jump to human genetics easily as it would require a specific trait we are desire in our offspring but humans don't follow selective breeding practices! In livestock, it is usually a trait, that has already been highly selected for, that makes a measurable 'jump' in crossbred offspring.

'Jumps', such as height, may indeed exist in humans, but there are too many exterior factors, such as nutrition, to allow for a true measurement that can be attributed solely to genetics. A good example is the increase in the heights of Japanese people born since WWII. Many wondered if it were due to radiation, genetics or ??? The acceptable explanantion is the increase in milk and milk product consumption among Japanese children since WWII has increased bone development and caused relative height increases.

Basically, we can't say for certain that heterosis occurs in humans because they are not acceptable lab specimens! :shock:

You would be hard pressed to find many studies showing heterosis in frame size in cattle. You GENERALLY don't cross a 4 frame Hereford cow with a 4 frame Brahman bull and expect to get mostly 6 frame progeny. What you are hoping to get is a 4 frame hybrid with a faster rate of growth than what would be expected from the pure bred parents and four frame daughters with more vigour, fertility, disease resistance, and longevity than the purebred parents. I don't know if we can scientifically PROVE that this IS due to heterosis or not; BUT in the early to mid 16th century much of the "PUREBRED" Indian population in Mexico died out due to disease and overwork at the hands of their European conquerors. However the Mestizo population that resulted from the mixing of the bloodlines SEEMS to have more hardiness and resistance to both European and native American illnesses than either of the 'purebred' ancestorial populations in their makeup did. This could simply be the result of natural selection OR there could be some kind of heterosis effects also involved. There is no way to know 500 years after the fact.

 

[Little Cow]

Brandonm2- Now, you're over my head! :lol: That's why you're the cattleman and I'm the biologist. I still have lots to learn about cattle specifics.

 

[Beefy]

Al Gore invented heterosis back in the 80s to combat global warming.

 

[Little Cow]

:lol: :lol:

 

[MikeC]

July 10, 2007
Heterosis is Real

Bob Hough, Executive Secretary, Red Angus Association of America

On April 9, the BeefBlog ran a column, "Is There Value Added Through Cross-Breeding or is Heterosis Just a Theory." There are two benefits to crossbreeding, breed complementarity and Heterosis. Breed complementarity utilizes the genetic differences between parent breeds to achieve a higher frequency of desired genes for specific traits in the crossbred progeny than could be found within a single breed, hence the strong points of one breed can compensate for weaknesses in another. Heterosis is defined as the percent superiority expressed by crossbred progeny over the average of their straightbred parents. The author confuses the two.

On breed complementarity I will use his own extreme example from 30 years ago: a very large, hard doing Simmental mated to a short "roly-poly" Angus. In this case, you would get excellent breed complementarity with a resulting progeny right for the market. He rightfully points out that breeds look much more alike than they once did. However, this reduces breed complementarity not Heterosis.

Heterosis is just as valid now with today's breeds as it was 30 years ago. Kress and Nelsen reported calving rate can be increased 6%, calf survival to weaning 4%, weaning rate 8%, weaning weight 11%, milk production 9%, and yearling weight 4%. These are real life numbers in which a crossbreed bred to a third breed increases lifetime production 20 to 25%. And this does not add in the breed complementarity that still exists (although to a lesser degree) in crossbreeding.

Is Heterosis just a theory? No it is a fact, and a grossly underutilized one.

 

[TedH71]

Scientists have been doing research and have said that humans have had two genetic bottlenecks in which the population went down to a very few individuals and then back up again. How did they do the research.....don't ask me. I read that a few years ago..maybe in the Discovery magazine.

 

[backhoeboogie]

How did they do the research.....

Y DNA passes from mother to daughter. A son will have his mother's Y DNA but does not pass it on to his offspring. There is a great deal of info out there on the net. Y DNA mutates about once every ten thousand years. If the carrier doesn't have female offspring, the strand ends.

With Y DNA traces, we can all be tracked back to what they call the "Daughters of Eve".

 

[Brandonm2]

How did they do the research.....

Y DNA passes from mother to daughter. A son will have his mother's Y DNA but does not pass it on to his offspring. There is a great deal of info out there on the net. Y DNA mutates about once every ten thousand years. If the carrier doesn't have female offspring, the strand ends.

With Y DNA traces, we can all be tracked back to what they call the "Daughters of Eve".

I am not familiar with the "daughters of Eve" show but no woman has every had a Y Chromosome. That is only transmitted from Father to Son and the Y chromosome mutates at the drop of a hat. I "think" you mean mitochondrial DNA. Which is ONLY transmitted by the mother.

 

[SCRUBS620]

I am late to this post but I would like to offer a few thoughts. First the Y-DNA (I have not seen this designation before though) that was mentioned is mitochondrial DNA. It is totally separate from regular DNA. It resides only in the mitochondria (organells that help with aerobic respiration). It does not mix with regular DNA, divides on its own and reproductively is only present in the egg; not the sperm hence the mother to offspring but not father to offspring.

As far as heterosis imagine this. A woman has a terrible recessive disease designated aa she marries a man with a different disease bb. The woman is normal for the B gene (BB) and the man is normal for the A gene (AA). Their children would be AaBb. Assuming A is dominant to a and B is dominant to b the children would be carriers but normal in both characteristics. Cattle have been selected to carry multiple copies of desired genes. Each line and breed has been selected for different dominant traits. Unfortunately other traits sometimes have to be sacrificed to do this. I want to keep the black cow even though she does not milk as well, I want to keep the polled cow even though she has birthing problems, I want to use the small calving growthy bull even though his daughters feet go bad quickly. All the breeds had to make sacrifices to keep a certain look or characteristic. That is why chasing one goal in a breed is so dangerous; other important characteristics are sacrificed. One breed might be CCDDEEffgghh while another breed might have been selected for much different criteria ccddeeFFGGHH. The resulting offspring would benefit from the dominant traits of both breeds CcDdEeFfGgHh. There are thousands upon thousands of genes and there may be multiple genes coding for an observable trait so it get complicated quickly. Obviously the example I gave is an idealized oversimplification but I hope it helps.

 

[Little Cow]

Well said, SCRUBS!

 

[Santas and Duhram Reds]

personally I do not believe that Taurus and Indicus are separate species and their scientific name implies. If they were truly separate species, there would be fertility problems as seen in mules, ligers, wholphins, etc.

Of course then again, dogs can breed with wolves, etc and their offspring are fertile. By my definition, they would be the same species as well. I don't think a fox and dog produce fertile offspring...though I could be wrong

My wife is a biologist and I teach science (history too). We have had this argument before.

 

[Brandonm2]

personally I do not believe that Taurus and Indicus are separate species and their scientific name implies. If they were truly separate species, there would be fertility problems as seen in mules, ligers, wholphins, etc.

Of course then again, dogs can breed with wolves, etc and their offspring are fertile. By my definition, they would be the same species as well. I don't think a fox and dog produce fertile offspring...though I could be wrong

My wife is a biologist and I teach science (history too). We have had this argument before.

That is why they are referred to as two "subspecies" rather than two species.