What is polygenic inheritance?
Soon after the rediscovery of Gregor Mendel’s laws of heredity in 1900, Herman Nilsson-Ehle, a Swedish geneticist, showed in 1909 how multiple genes with small effects could collectively affect a continuously varying character. He crossed dark, red-grained wheat with white-grained wheat and found the progeny with an intermediate shade of red. Upon crossing the progeny among themselves, he obtained grain colors ranging from dark red to white. He could classify the grains into five groups in a symmetric ratio of 1:4:6:4:1, with the extreme phenotypes being one-sixteenth dark red and one-sixteenth white. This suggested two-gene segregation. For a two-gene (n = 2) model, the number and frequency of phenotypic classes (2n + 1 = 5) can be determined by expanding the binomial (a + b)4, where a represents the number of favorable alleles and b represents the number of nonfavorable alleles.
Subsequently, Nilsson-Ehle crossed a different variety of red-grained wheat with white-grained wheat. He found that one-sixty-fourth of the plants produced dark red kernels and one-sixty-fourth produced white kernels. There were a total of seven phenotypic (color) classes instead of five. The segregation ratio corresponded to three genes: (a + b)6 = 1a 6 + 6a 5 b 1 + 15a 4 b 2 + 20a 3 b 3 + 15a 2 b 4 + 6a 1 b 5 + 1b 6. Here, a 6 means that one of sixty-four individuals possessed six favorable alleles, 20a 3 b 3 means that twenty of sixty-four individuals had three favorable and three nonfavorable alleles, and b 6 means that one individual had six nonfavorable alleles. An assumption was that each of the alleles had an equal, additive effect. These experiments led to what is known as the multiple-factor hypothesis, or polygenic inheritance (Kenneth Mather coined the terms “polygenes” and “polygenic traits”). Around 1920, Ronald Aylmer Fisher, Sewall Green Wright, and John Burdon Sanderson Haldane developed methods of quantitative analysis of genetic effects.
Polygenic traits are characterized by the amount of some attribute that they possess but not by presence or absence, as is the case with qualitative traits that are controlled by one or two major genes. Environmental factors generally have little or no effect on the expression of a gene or genes controlling a qualitative trait, whereas quantitative traits are highly influenced by the environment and genotype is poorly represented by phenotype. Genes controlling polygenic traits are sometimes called minor genes.
Quantitative genetics encompasses analyses of traits that exhibit continuous variation caused by polygenes and their interactions among themselves and with environmental factors. Such traits include height, weight, and some genetic defects.
Diabetes and cancer are considered to be threshold traits because all individuals can be classified as affected or unaffected (qualitative). They are also continuous traits because severity varies from nearly undetectable to extremely severe (quantitative). Because it is virtually impossible to determine the exact genotype for such traits, it is difficult to control defects with a polygenic mode of inheritance.
The detection of genes controlling polygenic traits is challenging and complex for the following reasons.The expression of genes controlling such traits is modified by fluctuations in environmental and/or management factors. A quantitative trait is usually a composite of many other traits, each influenced by many genes with variable effects. Effects of allele substitution are small because many genes control the trait. Expression of an individual gene may be modified by the expression of other genes and environment.
Polygenic traits are best analyzed with statistical methods, the simplest of which are estimation of arithmetic mean, standard error, variance, and standard deviation. Two populations can have the same mean, but their distribution may be different. Thus, one needs information on variances for describing the two populations more fully. From variances, effects of genes can be ascertained in the aggregate rather than as individual genes.
The issues in quantitative genetics are not only how many and which genes control a trait but also how much of what is observed (phenotype) is attributable to genes (heritability) and how much to the environment. The concept of heritability in the broad sense is useful for quantitative traits, but heritability itself does not give any clues to the total number of genes involved. If heritability is close to 1.0, the variance for a trait is attributable entirely to genetics, and when it is close to zero, the population’s phenotype is due entirely to the variation in the underlying environment. Environmental effects mask or modify genetic effects.
Distribution or frequency of different classes in segregating populations—for example, F2—may provide an idea about the number of genes, particularly if the gene number is small (say, three to four). Formulas have been devised to estimate the number of genes conditioning a trait, but these estimates are not highly reliable. Genes controlling quantitative traits can be estimated via use of chromosomal translocations or other cytogenetic procedures. The advent of molecular markers, such as restriction fragment length polymorphisms, has made it easier and more reliable to pinpoint the location of genes on chromosomes of a species of interest. With much work in a well-characterized organism, these polygenes can be mapped to chromosomes as quantitative trait loci.
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