In general, one gene affects a single character. But many genes are known to affect more than one character. Such genes are called pleiotropic genes and the condition is called Pleiotropy. An example of a gene in human beings is the recessive gene s ( sometimes denoted as Hbs) which produces sickle cell anemia in the ss homozygotes. More than 50% of the individuals homozygous for this gene (ss) die before the age of 20 years. The primary effect of this gene is the substitution of a valine molecule for a glutamic acid molecule at position 6 of the β-chain of hemoglobin. This mutant (sickle-cell) hemoglobin remains in solution as long as the oxygen concentration is high. However, at low oxygen concentrations, the filamentous aggregates of the sickle cell hemoglobin precipitate causing the RBC to assume the characteristic sickle shape. It is from this phenomenon the disease derives its name. In addition to the primary effect, a number of other characters of the individuals homozygous for this gene also are affected. These effects include, among other things, ‘tower skull’, dilation of heart and heart failure, poor physical development, impaired mental function, pneumonia due to lung damage, rheumatism, paralysis, kidney damage and failure etc.
A number of other recessive genes produce marked and, often, detrimental pleiotropic effects. Consequently, they are referred to as syndromes, e.g., Hunters’ syndrome, Huntington’s chorea, cystic fibrosis etc. Pleiotropic genes are known in many other organisms as well. In Drosophila, white eye genes (w) affects the shape of spermatheca in addition to producing white eyes. Several other gees of Drosophila, e.g., vestigial wings, notched wings etc., are also pleiotropic.
Genes showing pleiotropy produce a single polypeptide just like other non pleiotropic genes. But their polypeptides govern such a biochemical reaction which is basic to many developmental events. As a result, the impairment of this function interferes with a number of developmental events, which in turn leads to the pleiotropic expression of the gene.
The characteristic o a the pea plant that Mendel studied, as well as most of the traits discussed are known as qualitative traits. The phenotype in these examples appears as clearcut expressions of different alleles and can be distinguished qualitatively (e.g. yellow and green peapods, red and white flowers). Many traits however, cannot be classified into distinctive classes or sets of phenotypes. In these cases, the phenotype represents a continuous character of degree rather than a discontinuous one of kind. Such traits have been referred to as quantitative traits. The commonest examples of quantitative traits are height, weight, size, pigmentation and so on of the plants and animals, and of behavioral traits.
Biometricians like Galton had considered the existence of quantitative traits or gradual variation in expression of a phenotype, as a refutation of Mendel’s concept of inheritance of discrete genes. But this exception(like the ones posted at one time by the occurrence of linkage, multiple alleles and lethal genes) was shown to be not a contradiction but a further consolidation and extesion of Mendel’s principles.
It is assumed that quantitative traits are the results of the expression of many unrelated genes, modulated by interactions with the environment. The qualitative traits are, by and large unaffected by alterations in the environment. In diploid eukaryotes environmental fluctuations hardly reach the cellular domains. Quantitative traits on the other hand are very much influenced by non-genetic inputs. Phenotypes exhibiting quantitative traits are better considered as composite traits due to the interactions of many or multi or polygenes. The mode of inheritance of composite traits is variously referred to as Polygenic, multigenic or quantitative inheritance. Explanation for the lack of ‘absolute’ classes in the result of a cross is that the traits is the result of interaction of many genes, each one of which fluctuates in its degree of expression due to the modulation of individual genesby environment factors.
a) Wheat Kernel color
Nilsson-Ehle studied the color of wheat kernel and proposed the three pairs of genes were responsible for the coloration. (The loci have been since identified as r1,r2 and r3). When wheats of extreme colors, red and white, were crossed, the F1 showed an expected intermediate color of the kernel. But when F1s were obtained. Careful analysis revealed that all classes, which could be distinguished, (3 gene pairs involved) Nilsson-Ehle concluded that wheat kernel color was determined as a result of the expression of three gene pairs. This strengthened a hypotheses that Bateson had already advanced the polygenic nature of quantitative inheritance. (If R1r1, R2r2, R3r3 are the three pair of genes and each pair is concerned wit the production of a particular enzyme in the biosynthesis of the kernel pigment, workout for yourself the 64 genotypes that can be formed, and the phenotypic classes which show a gradation between the extreme classes.)
b) Corolla Length in Tobacco Flower
East crossed two tobacco plants each homozygous for a particular length of the corolla tubule (floral petal tube) – 41 cm and 93 cm respectively. The F1 flowers had a mean length. When F1 was inbred, a wide range of F2 was obtained but with further inbreeding of each succeeding generation, a corresponding increase in phenotypic classes was not seen. Rather, a gradation of colors between the extremes found in the F2 were found. East did not find any P1 parental types in almost five hundred progeny of the F2. He argued that more than four genes, at least, must be involved in the cross. Later more than 9 loci have been implicated in the determination of corolla length in Nicotiana tobacum.