polygenic inheritance.pptx

Genetic

1. Polygenic or quantitative Inheritance

2. Polygenic inheritance: when one phenotypic character is controled by more than one gene,it is called polygenic inheritance. Discovery: Kolerenter is known as father of polygenic inheritance. The term "phenotype" refers to  the observable physical properties of an organism;i.e appearance , development, and behavior. An organism's phenotype is determined by its genotype, which is the set of genes the organism carries .

3. HISTORICAL WORK RELATED WITH POLYGENIC INHERITANCE : Yule (1906) stated that each of several genes affecting a quantitative trait had small and cumulative effect and do not show dominance but continuous variation. Nelsson Ehle (1908 ): Nelson Ehle (1908) demonstrated existence of several genes where small cumulative effects governing single trait commonly known as polygenes (Minor genes) because they give only small effect .

4. Monogenic VS Polygenic Inheritance Gregor Mendel: Studied many traits that have distinct alternative forms for instance Red vs white flowers and Tall vs dwarf plants. Monogenic Inheritance: Such phenotypic classes are called discontinuous, can be termed as qualitative traits showing discrete alternative forms ,are governed by one gene and its two alleles (R and r or T and t) . These are known as oligogenes or major genes .

5. Polygenic inheritance: But mant traits are more complex than these and basiclay can take a number of continuous value and controlled by many genes. Examples: Height yield in crops skin colour IQ etc. Polygenes : Genes that contribute to quantitative traits are referred to as polygenes (Minor genes) The study of such quantitative trait is controlled by many genes also by environment is called quantitativ inheritance.The inheritance of quantitatve trait is typically viewd in terms of polgenic inheritane.

6. Polygenic inheritance (Cumulative/Additive gene action ): For example skin colour in human has a number of shades of fair and dark. In these traits there are no alternative forms, instead different ranges are present and are called as continuous ranges. In plants also height, seed colour, seed size etc. are common examples. Such traits are known as polygenic or quantitative traits and are generally controlled by 2,3 or more genes and phenotype is resulted by cumulative effect of many genes Polygenic traits are not expressed as absolute or discrete characters as that of Mendelian pea traits. Polygenic traits are recognizable as gradation of small differences (a continuous variation). The results form a bell shaped curve having extremes on two sides and mean values.

7. Bell shaped curve: Polygenic inheritance often results in a bell shaped curve when you analyze the population. That means that most people fall in the middle of the phenotypic range, such as average height, while very few people are at the extremes, such as very tall or very short

8. Features of Quantitative trait: Some characters show discontinuous variation, while a majority of biologically important traits exhibit continuous variation and are subjected to measurements such traits are referred as quantitative (Metric) traits. Features of Quantitative traits 1 . Continuous variation 2 . Studied by measuring their intensity 3 . Markedly affected by environment 4. Genetic analysis a) Not possible by classical Mendelian approach b ) based on statistical (biometrical) procedure 5. Governed by several genes with small additive effects 6.These alleles neither dominate or supreess other allele . Each allele contribues its share 7. Positive alleles contribute in additive fashion .Negative alleles do not add 8. The Environment interacts with the genotype to produce the final phenotype.

9. Polygenic Inheritance example

10. color of skin in humans It is controlled by around 60 loci. Human skin color is regulated by three pairs of genes. When a negro black (AA BB CC) phenotype is crossed with white ( aa bb cc) phenotype, an intermediate phenotype ( Aa Bb Cc) is produced. In the F2 generation, ( Aa Bb Cc× Aa Bb Cc) will produce different skin colors (i.e., very dark to light). Skin color depends upon melanin pigment. A negro black (with all dominant alleles (AA BB CC) would have the highest amount of melanin, and light color ( aa bb cc) has a negligible amount of melanin. The ratio of skin color is following: Negro black (1): vary dark (6): dark brown (15): intermediate (20): fair light (15): light (1).

11. Human Height Human height is a polygenic trait determined by three genes that have six alleles (3 dominant alleles and three recessive alleles). A tall person has all dominant alleles; however, a short person has all recessive alleles. Human height follows a normal distribution curve (bell-shaped) wherein one end of the curve represents the tall population, and another end represents the small or short population, whereas the middle portion of the curve represents the average height population.  A recent study concluded over 400 genes are linked to variation in height. Extreme short Extreme Tall Average Height

12. Environmental Effect Polygenic inheritance (human phenotypes and other organism phenotypes) are highly affected by environmental factors. Various environmental conditions or agents are responsible for regulating the function of genes resulting in varying gene functions. The function of the gene may be to switch OFF or switch ON based on varying environmental conditions. The range of phenotypes based on the different environmental conditions from the genotype is known as the ‘norm of reaction’.

13. Importance of Polygenic Inheritance Polygenic inheritance determined quantitative traits rather than qualitative and does not lead to the sharp segregation shown by more familiar genetical differences. Polygenic inheritance shows a continuous variation of the traits in a population, polygenic theory shows the relationship between continuous variation and discontinuous variation and also biometrical and genetic variation. Polygenic variation or change is called polygenesis, which is responsible for the population variance or evolution of species. This is also referred to as the theory of polygeny for evolution.  The variation is essentially responsible for adaptative changes, that may doffers according to specific requirements and needs of individuals or species.