What is quantitative inheritance?
An understanding of the genetics affecting metric traits came with the unification of the Mendelian and biometrical schools of genetics early in the 1900s. The statistical relationships involved in inheritance of metric traits such as height in humans were well known in the late 1800s. Soon after that, Gregor Mendel’s breakthrough on particulate inheritance, obtained from work using traits such as the color and shape of pea plants, was rediscovered. However, some traits did not follow Mendelian inheritance patterns. For example, Francis Galton crossed pea plants with uniformly large seeds with plants that had uniformly small seeds. The seed size of the progeny was neither large nor small but intermediate. However, when the progeny were mated among themselves, seed size formed a distribution from small to large, with many intermediate sizes.
How could particulate genetic factors explain a continuous distribution? The solution was described early in the twentieth century when Swedish plant breeder Herman Nilsson-Ehle crossed red wheat with white wheat. The resulting progeny were light red in color. When matings were made within the progeny, the resulting kernels of wheat ranged in color from white to red. Nilsson-Ehle was able to categorize the wheat into five colors: red, intermediate red, light red, pink, and white. Intermediate colors occurred with greater frequency than extreme colors. Nilsson-Ehle deduced that particulate genetic factors, now known as alleles, were involved, with red wheat inheriting four red alleles, intermediate red inheriting three red alleles, light red inheriting two red alleles, pink inheriting one red allele, and white inheriting no red alleles. These results were consistent with Mendel’s findings, except that two sets of factors, now known as loci, were controlling this trait, rather than the single locus observed for the traits considered by Mendel. Further, these results could be generalized to account for additional inheritance patterns controlled by more than two loci. Quantitative inheritance was mathematically described by British statistician and geneticist Ronald A. Fisher.
Under many circumstances, the environment also modifies the expression of traits. A combination of many loci with individually small effects alone would produce a rough bell-shaped distribution for a quantitative trait. However, environmental effects are continuous and independent of genetic effects, allowing them to blur the boundaries of the genetic categories and potentially make it difficult or even impossible to identify the effects of individual loci for many quantitative traits. The distribution of phenotypes, reflecting combined genetic and environmental effects, is typically a smooth, bell-shaped curve.
Genetic and environmental effects jointly influence the value of most metric traits. The relative magnitudes of genetic and environmental effects are measured using heritability statistics. Heritability has several practical definitions that are essentially equivalent. One definition states that heritability is equal to the proportion of observed differences among organisms for a trait due to genetic differences. For example, if one-quarter of the differences among cows in terms of how much milk they produce are caused by differences in their genotypes, the heritability of milk production is 25 percent, and the remaining 75 percent of differences among the animals are attributed to environmental effects. An alternative definition is that heritability is equal to the proportion of differences among sets of parents that are passed on to their progeny. For example, if the average height of a pair of parents is eight inches (twenty centimeters) more than the mean of their population and the heritability of height is 50 percent, then their progeny would be expected to average four inches (ten centimeters) taller than their peers in the population.
Two relationships are fundamental to the understanding and application of quantitative genetics. First, there is a tendency toward likeness among related individuals. Although similarities of human stature and facial appearance within families are familiar to most people, the same is true for such traits in all organisms. Correlation among relatives exists for such diverse traits as blood pressure, plant height, grain yield, and egg production. These correlations are caused by relatives sharing a portion of genes in common. The more closely the individuals are related, the greater the proportion of genes that are shared. Identical twins share all of their genes, while full siblings or a parent and offspring are expected to share one-half of their genes. This relationship is commonly used in the improvement of agricultural organisms. Individuals are chosen to be parents based on the performance of their relatives. For example, bulls of dairy breeds are chosen to become widely used as sires based on the milk-producing ability of their sisters and daughters.
The second fundamental relationship is that in organisms that do not normally self-fertilize, vigor is depressed in progeny that result from inbreeding—that is, the mating of closely related individuals. This effect is known as inbreeding depression. It may be the basis of the social taboos regarding incestuous relationships in humans as well as the dispersal systems for some other species of mammals, such as wolves. Physiological barriers have evolved to prevent fertilization between close relatives in many species of plants. Some mechanisms function as an anatomical inhibitor to prevent union of pollen and ova from the same plant; in maize, for example, the male and female flower are widely separated on the plant. Indeed, in some species, such as asparagus and holly trees, the sexes are separated in different individuals, and all seeds must consequently result from cross-pollination. In other systems, cross-pollination is only required for fertile seeds to result. The pollen must originate from a plant genetically different from the seed parent. These phenomena are known as self-incompatibility and are present in species such as broccoli, radishes, some clovers, and many fruit trees.
The corollary to inbreeding depression is hybrid vigor, a phenomenon of improved fitness that is often evident in progeny resulting from hybridization, or the mating of individuals less related than the average in a population. Hybrid vigor has been used in breeding programs to achieve remarkable productivity of hybrid seed corn as well as crossbred poultry and livestock. Hybrid vigor results in increased reproduction and efficiency of nutrient utilization. The mule, which results from mating a male donkey to a female horse, is a well-known example of a hybrid that has remarkable strength and hardiness compared to the parent species; unfortunately, however, it is also sterile.
Many traits of humans, like those of other organisms, are quantitatively inherited. Psychological characteristics, intelligence quotient (IQ), and birth weight have been studied extensively. The heritability of IQ has been reported to be high. Other personality characteristics, such as incidence of depression, introversion, and enthusiasm, have also been reported to be highly heritable. Musical ability is another characteristic under some degree of genetic control. These results have been consistent across replicated studies and are thus expected to be reliable; however, some caution must be exercised when considering the reliability of results from individual studies. Most studies of heritability in humans have involved likeness of twins reared together and apart. The difficulty in obtaining such data results in a relatively small sample size, at least relative to similar experiments in animals. One unfortunate response to studies of quantitative inheritance in humans was the eugenics movement.
Birth weight of humans is of interest because it is both under genetic control and subject to influence by well-known environmental factors, such as smoking by the mother. Birth weight is subject to stabilizing selection, in which individuals with intermediate values have the highest rates of survival. This results in genetic pressure to maintain the average birth weight at a relatively constant value.
The ability to meet the demand for food by a growing world population is dependent on continuously increasing agricultural productivity. Reserves of high-quality farmland have nearly all been brought into production, and a sustainable increase in the harvest of fish is likely impossible. Many countries that struggle to meet the food demands of their populations are too poor to increase agricultural yields through increased inputs of fertilizer and chemicals. Increased food production will, therefore, largely depend on genetic improvement of the organisms produced by farmers worldwide.
Most characteristics of economic value in agriculturally important organisms are quantitatively inherited. Traits such as grain yield, baking quality, milk and meat production, and efficiency of nutrient utilization are under the influence of many genes as well as the production environment. The task of breeders is not only to identify organisms with superior genetic characteristics but also to identify those breeds and varieties well adapted to the specific environmental conditions in which they will be produced. The type of dairy cattle that most efficiently produces milk under the normal production circumstances in the United States, which includes high health status, unlimited access to high-quality grain rations, and protection from extremes of heat and cold, may not be ideal under conditions in New Zealand, where cattle are required to compete with herdmates for high-quality pasture forage. Neither of these animals may be ideal under tropical conditions where extremely high temperatures, disease, and parasites are common.
Remarkable progress has been made in many important food crops. Grain yield has responded to improvement programs. Development of hybrid corn increased yield severalfold over the last few decades of the twentieth century. Development of improved varieties of small grains resulted in an increased ability of many developing countries to be self-sufficient in food production. Grain breeder Norman Borlaug won the Nobel Peace Prize in 1970 for his role in developing grain varieties that contributed to the Green Revolution.
Can breeders continue to make improvements in the genetic potential for crops, livestock, and fish to yield enough food to support a growing human population? Tools of biotechnology are expected to increase the rate at which breeders can make genetic change. Ultimately, the answer depends on the genetic variation available in the global populations of food-producing organisms and their wild relatives. The potential for genetic improvement of some species has been relatively untapped. Domestication of fish for use in aquaculture and the use of potential crop species such as amaranth are two possible ways to augment world food reserves. Wheat, corn, and rice provide a large proportion of the calories supporting the world population. The yields of these three crop species have already benefited from many generations of selective breeding. For continued genetic improvement, it is critical that variation not be lost through the extinction of indigenous strains and wild relatives of important food-producing organisms.
Molecular genetics and biotechnology have also added new tools for analyzing the genetics of quantitative traits. In any organism that has had its genome adequately mapped, genetic markers can be used to determine the number of loci involved in a particular trait. In carefully constructed crosses, geneticists look for statistical correlations between markers and the trait of interest. When a high correlation is found, the marker is said to represent a quantitative trait locus (QTL). Often a percentage effect for each QTL can be determined, and because the location of markers is typically known, the likely location of the gene can also be inferred (that is, it must be somewhere near the marker). A good understanding of the QTLs involved in the expression of a quantitative trait can help researchers determine the best way to improve the organism.
Although QTLs are much easier to discover in organisms where controlled crosses are possible, studies have also been carried out in humans. In humans, geneticists must rely on whatever matings have happened; due to ethical limitations, they cannot set up specific crosses. Studies in humans have attempted to quantify the number of QTLs responsible for such things as IQ and various physical traits. However, ethicists continue to worry that conclusions from such research will be used in a new wave of eugenics. In spite of the risk of misusing an improved understanding of human quantitative traits, human biology and medicine stand to benefit.
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