A comprehensive study of hybrid vigour and heterozygous advantage.
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Today, many accept racial mixing as the inevitable result of globalisation. Scientists are fascinated by theories that this mixing could lead to a genetic advantage. Despite this, the reality is that historical misunderstanding of racial differences sparked Hitler’s desire for a superior race and led to the Jewish Holocaust. With an increased knowledge of genetic diversity, scientific evidence now reveals that had Hitler got his wish the result might not have been superiority but instead a weaker race.
A Modern Genetic Perspective
For over a century animal and plant biologists have known that mixing two diverse strains of a plant or animal can result in more vigorous and healthy offspring. This “hybrid vigour” was first shown by American Plant Scientist George Shull at the Station for Experimental Evolution, Cold Spring Harbor, in 1908 when he crossed two different corn strains resulting in a more vigorous hybrid.
So, could mixed race children gain a noticeable genetic advantage and show degree of hybrid vigour?
Hybrid Vigour
Hybrid offspring are called the first filial or “F1” generation, hence the term gardeners are familiar with when buying seed; ‘F1 hybrid’. To produce F1 hybrids, the farmer crosses two pure-bred parent strains. Often, these parent stocks are relatively small populations and hence are genetically rather uniform. For this reason, the hybrid offspring tend not only to be more vigorous than their parents, but are also relatively uniform in appearance, a second desirable trait.
How far does hybrid vigour extend?
Hybrid vigour represents just one point on a spectrum of how related two parents are.
At one end of the spectrum is inbreeding, where the parents are closely related. This tends to produce very unfit offspring, many of which die young. Better is to choose an unrelated partner. When the partner is not only unrelated but comes from a different population, this is like being ‘super-unrelated’ and can lead to hybrid vigour. However, there must be an end-point where the parents are too different. Most obviously this end-points comes when the parents are so unrelated they are actually different species. Thus, when a donkey and a horse mate the offspring are called mules. Mules are interesting because the show some elements of hybrid vigour, being strong and hard-working, but they are also infertile.
What is the basis of ‘Hybrid Vigour’?
There are two main components of hybrid vigour, referred to as ‘outbreeding’ and ‘heterozygote advantage’. Humans have 23 pairs of chromosomes containing about 30,000 genes. Only the sex chromosomes break the ‘pairs’ rule, women having two ‘X’ chromosomes and men one ‘X’ and one ‘Y.
If you carry two copies of a gene that differ slightly in how they work, it is a bit like having an extra channel on your TV set: you don’t have to watch it but there may be times when the extra channel comes in handy. It gives you extra flexibility. An individual who has two identical copies of a gene (good or bad) is called a homozygote, while someone who has two copies that differ is called a ‘heterozygote’.
The benefit of outbreeding
Broken genes are usually rare, so to inherit two broken copies of the same gene is unlucky and can result in serious health problems. However, the chances increase considerably if your parents are related. With more genes that are identical by descent, a child will have more genes where both copies are broken.
If inbreeding is bad, outbreeding, marrying an extremely unrelated partner, should be good for the same reason. With a less related partner, the number of genes in your children that are identical by descent is reduced, and with it the chance that a gene has two broken copies. It is not that your children will inherit fewer broken copies in total, just that every broken copy has a much better chance of finding itself partnered with a good copy.
The benefits of heterozygous advantage
Outbreeding can be thought of as avoiding the bad effects of having two broken copies of the same gene. Heterozygous advantage deals with the extra flexibility one may get from having two good, but slightly different copies of the same gene.
Is there hybrid vigour in humans?
So, what happens when people from different human populations marry - is this likely to bring the benefits discussed above? Yes, it probably will, says Amos, both by reducing the number of gene-pairs that are broken and by increasing the number that are ‘both-good-but-different’. Combined, the result should be, on average, children who are genetically healthy, for example, who are less likely to catch ‘flu or who live a little bit longer. However, the size of this effect is extremely difficult to measure because so much of a human’s fate is due to the environment: having good genes won’t stop you getting run over by a drunken motorist!
The Future
In the 21st Century, as more and more populations move and mix with each other, scientists predict that genetic heterozygosity will increase. However, it won’t increase indefinitely, but should reach a peak and then either stabilise or decrease depending on how future generations choose to live.
To read the full story of Hybrid Vigour and a recent history of racial mixing, download the Is It Better to Be Mixed Race PDF
Discover more about the Race: Science's Last Taboo season.
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