Genetic diversity
Related Terms
Antibody diversity, eugenics, founder effect, genetic diversity, heterozygous, homozygous, hybrid, hybrid vigor, hybridization, inbreeding, inbreeding depression, outbreeding depression, outbreeding enhancement, purebreds, recessive genetic disorders, selective breeding, X-linked disorders.
Background
Heterosis is based on the notion that genetic diversity increases the health (or fitness) and survival of an organism. Genetic diversity is defined by the Unified Medical Language System of the National Library of Medicine as "the phenotypic and genotypic differences among individuals in a population." Genotype refers to the genes contained in a cell, while phenotype refers to how genes are expressed, such as in physical traits or abnormalities.
The term "heterosis" is often used in genetics and selective breeding, in which desirable traits are bred into a species, while undesirable traits are bred out of the species. In breeding, heterosis refers to the idea that a hybrid (an animal or plant of mixed origin) has greater genetic strength than organisms of a homogenous (similar) background. It also refers to the potential to combine the positive traits of the parents into "better" offspring.
In humans, heterosis refers to children of parents who do not share a blood line and thereby genetic material.
When offspring are considered to be better, or more fit for survival, than their parents, this is known as hybrid vigor. However, crossbred plants or animals are not always better than their parents. It is possible for a hybrid to be less fit for survival, which is called outbreeding depression.
Inbreeding depression, on the other hand, represents the decrease in fitness as the result of the breeding of organisms of similar genetic backgrounds. With inbreeding, rare genetic diseases become more common among populations. Not only are the chances of genetic defects in offspring increased, but researchers also believe that inbreeding may have long-term effects on health by increasing the risk of late-stage disease states, such as obesity, heart disease, and type 2 diabetes.
The opposite of a hybrid is a purebred. A purebred is the result of breeding by two organisms that have similar genetic material with no outbreeding (or breeding with those with different genetic material) over many generations.
Methods
General: There are two main hypotheses to explain the "fitness advantage" in heterosis: the over-dominance hypothesis and the avoidance of deleterious recessive genes hypothesis.
Over-dominance hypothesis: The over-dominance hypothesis states that an organism that descends from parents of different genetic backgrounds will have greater resistance to a broader spectrum of potential dangers. On the other hand, an organism that descends from parents of similar genetic backgrounds will have resistance to a narrower spectrum of potential dangers.
This concept is related to antibody diversity. Similar to genetic diversity, antibody diversity suggests that offspring from parents of different genetic backgrounds are more fit than those from parents of similar genetic backgrounds. This is because they have a greater ability to produce antibodies that can defend against a wider variety of pathogens, or harmful substances, such as bacteria and viruses. Based on this hypothesis, these organisms are more fit due to greater immunity.
Avoidance of deleterious recessive genes: In this hypothesis, an organism that descends from parents of different genetic backgrounds will have fewer harmful recessive genes.
A person inherits genes from his or her parents. One copy of each gene is inherited from the mother and a second copy is inherited from the father. Each parent can only pass one copy of their genes on to the child. Which gene gets passed down is determined purely by chance. When both copies (alleles) of a gene are the same, a person is said to be homozygous for that gene. If different alleles of the gene are inherited from each parent, the person is said to be heterozygous for that gene.
Recessive genetic conditions are caused by a mutation, or defect, in a gene. In order to inherit the condition, a person would have to receive two copies of the defective gene. If a person receives one copy of the defective gene and one normal copy, he or she will not have the condition and is known as a carrier.
An organism with genetically dissimilar parents is more likely to have fewer recessive genes than an organism born from closely related parents. Therefore, its decreased number of recessive genes may lead to increased fitness.
Summary: If over-dominance is the main cause of greater fitness, then certain genes should be overly expressed in offspring of genetically dissimilar parents when compared to offspring of closely related parents. If the main cause of fitness is the avoidance of recessive genes, fewer genes should be under-expressed in the offspring of genetically dissimilar parents when compared to their parents.
Research
General: Heterosis is widely used in agriculture and the breeding of various animals for the food supply as well as in scientific research. Heterosis in crops and livestock is different from genetically modified foods, in which new genetic material is introduced into living organisms.
Agriculture: Major crops, such as corn and rice, are examples of heterosis in agriculture. Many crops are hybrids of wild species that were selectively bred to produce desirable characteristics, such as greater nutritional value and yield.
The concept of antibody diversity is very important to crop science. Similar to genetic diversity, in which an organism that descends from parents from different genetic backgrounds is more fit than one that descends from parents from similar genetic backgrounds, antibody diversity also enhances fitness. This concept suggests that offspring from parents of different genetic backgrounds have a greater ability to produce antibodies that can defend against a wider variety of pathogens, or harmful substances such as bacteria and viruses.
In trying to understand the functional and evolutionary importance of heterosis, current research is exploring what determines the key steps in the creation of a hybrid organism.
Animal science: Heterosis is also applied to animals. In the case of poultry and livestock, hybrids of two types of animals are crossed to bring about a better product. For instance, the process of heterosis is used to create poultry that only lays white eggs, animals that are more "meaty" at an earlier age, and animals that gain weight at a specific rate, making them marketable sooner.
Heterosis may be applied to domestic animals, such as cats and dogs. It is generally well known that purebred dogs tend to have a higher incidence of specific health problems. On the other hand, hybrid or random-bred dogs ("mutts") tend to suffer from fewer maladies.
Humans: Some heterosis research in humans is focusing on how inbreeding depression, the decrease in fitness of offspring of parents from similar genetic backgrounds, may contribute to long-term increases in chronic disease incidence.
Heterosis research in humans is also examining how to improve disease resistance. Because heterozygotes (the offspring of parents with different genetic backgrounds) have a genetic advantage over homozygotes (offspring of parents with similar genetic backgrounds), researchers are looking at how this increased disease resistance can be applied to other populations.
Heterozygosity is also responsible for the advantage that females have over males in terms of X-linked recessive disorders like color blindness and hemophilia. If a condition is X-linked, the defective gene is located on the X chromosome, one of the sex chromosomes. Females have two X chromosomes, while males have one X chromosome and one Y chromosome. Females receive one X chromosome from each parent, while males receive one X chromosome from the mother and one Y chromosome from the father.
In females who receive one defective copy and one normal copy of a gene for an X-linked recessive disorder, the normal copy can compensate for the defective copy. This is why many females who have one defective copy of a gene for an X-linked recessive disorder have mild or no symptoms of the disease. Males, on the other hand, only have one X chromosome. If they receive one defective copy of a gene for an X-linked recessive disorder, they do not have another copy to compensate.
Implications
In the past, heterosis has been manipulated for the purpose of eugenics. Eugenics is the science of improving the genetic composition of a population. It generally refers to humans and has been the source of much controversy and ethical debate. While the philosophical standpoint of eugenics is that it lessens human suffering by preventing the spread of negative genetic traits, it is generally regarding as violating human rights. Historically, proponents of eugenics went so far as to forcefully sterilize individuals thought to have such negative genetic traits.
By applying heterosis, scientists can bring about more favorable traits in plants and animals used for human consumption while repressing less favorable ones. The other side of this is that breeding in desirable characteristics and breeding out undesirable ones contribute to a less diverse population.
There are also implications for recessive genetic disorders that occur in closely related populations. Tay-Sachs disease, for example, is a recessive genetic condition that is common among certain groups, such as French Canadian and Ashkenazi Jewish populations.
Couples from populations in which there is an increased risk of certain recessive genetic disorders may work with genetic counselors to evaluate the probability of having children with the disorder based on known risk factors. Counselors may also help prospective parents decide which testing methods are appropriate how to interpret results, and whether or not to terminate an affected fetus. It is important that patients realize that genetic tests cannot guarantee accuracy; there is always a risk of terminating a healthy unborn child. Also, clinical studies have shown that women who have had abortions suffer an increased risk of anxiety, depression, and suicide and are at an increased risk for breast cancer.
Limitations
There are two main hypotheses to explain the "fitness advantage" in heterosis: the over-dominance hypothesis and the avoidance of deleterious recessive genes hypothesis. However, a clear conclusion has not yet been reached.
As with all cross-breeding practices, there is some concern as to whether allergens and other harmful substances could be introduced to a previously harmless plant or animal, such as those in the food supply. Whether this is possible is not yet clear, and future research will aim to better understand these concerns.
Future research
Future research will examine ways to use heterosis to further improve the food supply. In addition, researchers will continue to look for ways to prevent the potential for cross-contamination through the introduction of harmful substances or allergens into cross-bred products.
Research in humans will continue to focus on enhancement of disease resistance and antibody diversity. Furthermore, ongoing research aims to produce improved diagnostic methods, thereby providing better ways to prevent recessive genetic disorders and other genetic conditions.
Author information
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
Bibliography
Genetics Home Reference. .
Hochholdinger F, Hoecker N. Towards the molecular basis of heterosis. Trends Plant Sci. 2007;12(9):427-32.
Lippman ZB, Zamir D. Heterosis: revisiting the magic. Trends Genet. 2007;23(2):60-6.
Melchinger AE, Utz HF, Piepho HP, et al. The role of epistasis in the manifestation of heterosis: a systems-oriented approach. Genetics. 2007;177(3):1815-25.
Natural Standard: The Authority on Integrative Medicine. .
Nonacs P, Kapheim KM. Social heterosis and the maintenance of genetic diversity. J Evol Biol. 2007;20(6):2253-65.
Nonacs P, Kapheim KM. Social heterosis and the maintenance of genetic diversity. J Evol Biol. 2007 Nov;20(6):2253-65