Church Teachers’ College Course Title: Genetics Course #: SC302SEB Lecturer: Mrs. A. Omoregie Student #: CH20149495 Student Name: CHEVANCE HENRY Student Year: COHORT 3 Group Name /Major: BIOLOGY IN SECONDARY EDUCATION Submitted to the Professional Studies Department in partial fulfillment of the requirements for the Bachelor of Education Degree Church Teachers’ College: Mandeville Assignment: POLYGENIC TRAITS Due Date: NOVEMBER 2, 2015 Define polygenic trait. A polygenic trait, is a trait that nonallelic genes control. These traits result from one or more genes contributing to the phenotype. An individual's physical appearance is determined by chromosomal inheritance and genotypic ratio. This phenomenon is known as Mendel's Laws of Inheritance. In terms of polygenic traits, an individual's characteristic features result from different genes interacting. The cumulative effects of genes will determine several different traits, such as height, color, weight, shape, and metabolic rate. Genetic Phenomenon of Polygenic Traits These traits are also known as multifactorial traits or quantitative traits. They are referred to as quantitative traits because their phenotype expression depends on several different alleles found on different chromosomes. These traits do not follow typical recessive and dominance patterns because they are a result of the contribution and combination of several different genes. Polygenic trait characteristics: Are recognized by the expressions they possess that occur from continuous variation gradation Rather than counting, are quantified through measuring the variation Do not follow the phenomenon known as Mendel's patterns of inheritance Are additive effects of at least two separate pairs of genes that control the continuous variation A result of contributing pairs of genes is a varying wider range of phenotypic expression Describe examples of polygenic inheritance in agriculture (include both plants and animals). The Main Examples of Polygenic Inheritance in both Plants and Animals are listed below: Grain Color in Wheat: It was Swedish scientist Nilsson – Ehle (1909) who first studied the inheritance pattern of the colour of the grain in wheat. In some types, the kernel color is red (aleurone color) and in others white and in still others which may be regarded as intermediates different, shades of red appear in the kernel color. Nilsson – Ehle crossed a variety of wheat having dark red kernel with the one having white kernel. The red color is incompletely dominant over white hence the F hybrid had a kernel which was intermediate between red and white. These F hybrids were bred among themselves to get the F2 generation. In the F, progeny the kernel color ranged from dark red, to white, between them there were at least three grades of color. These shades of color can be graded as follows: Red, Reddish, Intermediate (pink), light and white. According to the law of probability, it can be assumed, that two pairs of segregating genes are responsible for the color variation in the wheat kernel. The red kernel wheat has two pairs of genes (two pairs of alleles) both of which contribute some quantity of redness to the grain. These genes are duplicates of each other the white kernel wheat had recessive alleles of both these pairs (Q R,) and does not contribute anything to red coloration. The F possesses two dominant genes (Rt r R2 r2) hence it is intermediate between red and white. In the F, generation the color varies depending on the number of dominant genes the offspring gets i.e. 4, 3, 2, 1 or zero. The ratio is 1:4:6:4:1. In many instances this ratio is also represented as 15:1, since except for the last one all others have some shades of red Nilsson-Ehle found in certain other crosses of wheat concerning the kernel color, in the F2 progeny only one in every 4 was white, with at least 5 intergrades between red and white. He proposed that in this instance there were three pairs of genes (R, Rt R2R2 R3 R3) responsible for the kernel color. Skin Color in Human Beings: The interpretation regarding the skin color in Negroes and Caucasoid whites was bit of a problem to interpret from the point of view of gene inheritance. Davenport (1913) first successfully explained the skin color inheritance in terms of polygene. He studied skin color of several people in Jamaica and Bermuda. In these regions intermarriages between Negroes and whites was quite frequent and the children were called Mulattoes. He hypothesized that Negroes of central and Western Africa differ from whites in having two pairs of dominant alleles which are incompletely dominant over the genotype of whites. In a cross between a Negro and a white, the F, called mulattoes is heterozygous to both P1 and P, with the result the skin color is intermediate. Just as in kernel color in wheat, here also the skin color depends on the number of dominant alleles. It is as follows – Negro – allele four dominant genes and whites all the four recessive genes. Mulattoes – Two dominant genes. When two mulattoes marry among themselves (a mulattoe man and a mulattoe woman) in the F2 generation 16 possible types could be found among children. Assuming that mulattoes are hybrids for both the color genes (P, and P,) there genotype should be while they get dominant allele from the Negro parent, the recessive alleles are obtained from the white parent. Theoretically mulattoes both male and female produce four types of gametes which on random combination would give 16 types. In the F2 progeny the skin color would be black (4 dominant genes), dark (3 dominant genes), Mulattoe (2 dominant genes), Fair (one dominant gene) and white (no dominant gene). Physiologically the skin color in human beings depends upon the amount of melanin (a pigment) deposited on the skin. The amount of melanin depends upon the genes and its development depends upon the amount of sun light received by the skin. According to Curt Stern, the human skin colour depends on gene loci located at four to several different loci. Height of Man: The height of the body in man is another typical example of polygenic inheritance. The functioning of these genes however greatly depends on environmental factors. The genes control the functioning of the pituitary thus controlling growth. An under secretion of the pituitary under the influence of a ‘dwarf gene’ would retard the growth. Ear Length in Maize: Emerson and East, the pioneers in the study of polygenic inheritance, observed many quantitative variations in ear size in maize they worked on two varieties of maize – long eared black Mexican sweet corn and short eared Tom Thumb popcorn. The first variety ranges in ear length from 12-21 cms with an average of 16.8 cms. The second variety has ears ranging in length from 5-8 cms, averaging to 6.6 cms. When these two varieties were crossed, the ear length in the F, progeny ranged from 9-15 cms averaging 12.1 cms. Selling the F, progeny, F, was obtained much in the same way as in inheritance of kernel color in wheat. Ray Size in Flower Heads of Compositae: A possible occurrence of polygenic system operating in the ray development has been reported by the author (Sundara Rajan.) in the plant Bidenspilosa of Compositae. Here there are two extremes. In the one represented by a normal plant, the rays in the heterogamous heads are of normal size i.e. they are formed by the fusion of three petals and the other 3 are reduced resulting in zygomorphy of the ray floret. In the other extreme which is a recessive mutant, the rays are completely reduced; with the result all the five petals of the ray floret are equal in size like the disc floret. The floret becomes actinomorphic. Between these, there are at least three intergradations representing cumulative interaction of these genes. The fully developed rays represent all the dominant genes. While the reduced rays represent all the recessive alleles. Explain continuous variation as it relates to polygenic inheritance. Continuous Variation is variation in phenotypic traits such as body weight or height in which a series or types are distributed on a continuum rather than grouped into discrete categories. These are "quantitative traits," so-called because their expression in any single individual can only be described numerically based on the results of an appropriate form of measurement. Quantitative traits are also called continuous traits, and they stand in contrast to qualitative, or discontinuous, traits that are expressed in the form of distinct phenotypes chosen from a discrete set. Continuous variation in the expression of a trait can be due to both genetic and non-genetic factors. Non-genetic factors can be either environmental or a matter of chance. The appearance of a quantitative trait usually signifies the involvement of multiple genetic loci or polygenic inheritance, although this need not be the case. In particular, a single polymorphic locus with multiple, differentially expressed alleles can give rise to continuous variation within a natural population. There may also be some instances where the expression of a quantitative trait is controlled by a mutant allele at a single locus with a high degree of variable expressivity. The term polygenic is used to describe traits that are controlled by multiple genes, each of which has a significant impact on expression. The term multifactorial is also used to describe such traits, but is more broadly defined to include those traits controlled by a combination of at least one genetic factor with one or more environmental factors. Not all polygenic traits are quantitative traits. A second polygenic class consists of those traits associated with a discrete phenotype that requires particular alleles at multiple loci for its expression. By increasing the number of genes controlling a trait, the number of phenotype combinations also increase, until the number of phenotypes to which an individual can be assigned are no longer discrete, but continuous. 4. Evaluate the influence of environment and genetics on the inheritance of traits. Modern evolutionary biology is founded on the Mendelian-genetic model of inheritance, but it is now clear that this model is incomplete. Empirical evidence shows that environment (encompassing all external influences on the genome) can impose transgenerational effects and generate heritable variation for a broad array of traits in animals, plants, and other organisms. Such effects can be mediated by the transmission of epigenetic, cytoplasmic, somatic, nutritional, environmental, and behavioral variation. Building on the work of many authors, we outline a general framework for conceptualizing nongenetic inheritance and its evolutionary implications. This framework shows that, by decoupling phenotypic change from the genotype, nongenetic inheritance can circumvent the limitations of genetic inheritance and thereby influence population dynamics and alter the fitness landscape. The weight of theory and empirical evidence indicates that nongenetic inheritance is a potent factor in evolution that can engender outcomes unanticipated under the Mendelian-genetic model. The decoupling of phenotypic change from genotypic change brought about by nongenetic inheritance can have some interesting consequences for the study of evolution. Generally speaking, all such effects stem from the fact that this decoupling partially removes some limitations of genetic inheritance, and thereby allows the phenotypic distribution of a population to change in ways that might not otherwise be possible. Indeed, strictly speaking, such phenotypic change need not involve evolution at all because the change might be mediated entirely by nongenetic factors. The fact that the resulting phenotypic change is heritable suggests that it is still appropriate to consider such nongenetic effects within the purview of evolutionary biology, and to regard them as being evolutionarily significant in a general sense, even if they do not affect evolutionary dynamics (that is, the dynamics of gene frequencies). It is also possible, however, for nongenetic inheritance to affect the evolutionary dynamics of gene frequencies per se. This can occur if fitness depends on the phenotypic distribution of the population. Under such circumstances, externally imposed selection can leave a signature in the fitness landscape via changes in the transmissible component of the phenotype, even after the external source of selection is removed. Evolutionary momentum results from a feedback between the genetic and nongenetic inheritance systems, whereby a change in a transmissible component of the environment, brought about by nongenetic inheritance, affects selection on genes. Although there are a variety of potential evolutionary implications of nongenetic inheritance, both in a general sense and in the strict sense mentioned above, a great many of these can be understood as arising from one of three general ways in which nongenetic inheritance overcomes some of the limitations of genetic inheritance: (a) Although genetic inheritance does not allow for traits acquired during the lifetime, such as high condition or learned behaviors, to be transmitted to offspring, nongenetic inheritance does. (b) Genetic recombination breaks up favorable combinations of alleles so that a parent with a high-fitness genotype is usually unable to transmit its full genetic potential to its offspring. In contrast, nongenetic inheritance permits the transmission of favorable trait combinations across generations. (c) Selection can deplete additive genetic variation, reducing heritability and the potential for phenotypic change. Empirical evidence shows that nongenetic inheritance can contribute to variation in phenotype and fitness. However, despite the many fascinating results produced by laboratory studies, few ecologically relevant traits have thus far been explicitly examined for nongenetic inheritance, particularly in natural environments. The extent to which some of the effects identified in the laboratory carry over to natural settings thus remains unknown. Natural environments also contain a much greater variety of environmental variation that can potentially result in nongenetic inheritance, including variation from interspecific interactions. Consequently, although the potential for nongenetic inheritance to influence phenotypic variation and fitness is no longer in doubt, its actual role and importance in natural populations remains uncertain. This knowledge gap must be addressed through experimental studies on nongenetic inheritance of ecologically relevant traits, using tractable laboratory and field models. Theory clearly shows that nongenetic inheritance can have wide-ranging consequences for long-term phenotypic change and evolution. Nonetheless, most past theoretical treatments have focused on particular mechanisms or examples of nongenetic inheritance, such as cultural transmission and maternal effects. Perhaps because of its presumed limitation to certain taxa and traits, this work has had relatively little influence on fundamental evolutionary theory. There is a need to develop a general, synthetic theory of evolution encompassing both genetic and nongenetic inheritance. References Elizabeth, R and Kitchen, C. Understanding Polygenic Traits. Bright Hub Inc. Retrieved from: http://www.brighthub.com/science/genetics/articles/80453.aspx Venkataiah, S. 5 Examples of Polygenic Inheritance in both Plants and Animals. Retrieved from: http://www.shareyouressays.com/112779/5-examples-of-polygenic-inheritance-in-both- plants-and-animals Silver, M. Mouse Genetics. Concepts and Applications. Retrieved from: http://www.informatics.jax.org/silver/contents.shtml BioNinja. Polygenic Inheritance. Retrieved from: http://www.ib.bioninja.com.au/higher- level/topic-10-genetics/103-polygenic-inheritance.html Bonduriansky, R. and Day, T. Nongenetic Inheritance and Its Evolutionary Implications. http://bonduriansky.net/AREES-2009.pdf