Quantitative Inheritance - PDF

Quantitative Inheritance  TOPICS TO BE DISCUSS A. Inheritance of Quantitative Characters 1. Multiple Genes 2. Number of Genes in Polygene Systems 3. Regression to the Mean 4. Effects of Dominance and Gene Interactions 5. Effects of Genes in Multiplying Effects B...

Quantitative Inheritance  TOPICS TO BE DISCUSS A. Inheritance of Quantitative Characters 1. Multiple Genes 2. Number of Genes in Polygene Systems 3. Regression to the Mean 4. Effects of Dominance and Gene Interactions 5. Effects of Genes in Multiplying Effects B. Analysis of Quantitative Characteristics C. Components of Phenotypic Variance D. Heredity a. Heritability in Narrow Sense b. Heritability in the Broad Sense Quantitative inheritance, also known as polygenic inheritance, is a type of inheritance where multiple genes, each with a small effect, contribute to a single trait. Qualitative inheritance, where a single gene with dominant alleles determines a trait. Quantitative traits are often influenced by environmental factors and exhibit continuous variation, meaning they can take on a range of values rather than distinct categories. Qualitative vs. Quantitative Inheritance Qualitative genetics Quantitative genetics Characters of kind Characters of degree Discontinuous variation; distinct Continuous variations; phenotypic phenotypic class form a spectrum Single gene effects Polygenic control; effects of single genes too slight to be detected Concerned with individual mating Concerned with population of and their progeny organisms consisting of all possible kinds of mating Analyzed by making counts and Statistical analyses give estimates ratios of population parameters such as the mean and standard deviation Characteristics of Multiple Genes 1 Equal Effect 2 Cumulative Effect Each contributing allele in the series of multiple Effects of each contributing allele are cumulative or genes produces an equal effect. additive. 3 No Dominance 4 No Epistasis There is no dominance between alleles. There is no epistasis (masking of the phenotypes) among genes at different loci. 5 No Linkage 6 Environmental Influence There is no linkage involved. Environmental conditions have a considerable effect on the phenotypic expression of polygenes for quantitative traits. Kernel Colour in Wheat 1 Parental Cross A red kernel strain (R1R1R2R2) is crossed with a white kernel strain (r1r1r2r2). 2 F1 Generation The F1 generation is all light red (R1r1R2r2), indicating the cumulative effect of the alleles. 3 F2 Generation The F2 generation exhibits a range of kernel colors, with approximately 1/16 being white and 1/16 being red, demonstrating the polygenic inheritance pattern. F2 Genotypic and Phenotypic Ratios Genotype Genotypic Ratio Number of Phenotype Phenotype Ratio Contributing Alleles R1R1R2R2 1/16 4 Red 1/16 R1R1R2r2 4/16 3 Medium Red 4/16 R1r1R2R2 2/16 3 Medium Red 4/16 R1r1R2r2 4/16 2 Light Red 6/16 R1R1r2r2 1/16 2 Light Red 6/16 r1r1R2R2 1/16 2 Light Red 6/16 R1r1r2r2 2/16 1 Very Light Red 4/16 r1r1R2r2 2/16 1 Very Light Red 4/16 r1r1r2r2 1/16 0 White 1/16 Skin Colour in Man Davenport's Study Additive Effect Mulatto Phenotype Davenport (1913) studied skin These genes exhibit an additive A true negro has four dominant pigmentation in Jamaica, proposing effect, meaning each dominant allele genes (AABB), while a white has four that two pairs of genes, A-a and B-b, contributes a unit of pigmentation. recessive genes (aabb). The F1 contribute to the difference between offspring of a negro and a white are negro and caucasian individuals. all AaBb and have an intermediate skin color termed mulatto.  Male AB Ab aB ab Female AB AABB AABb AaBB AaBb Like negro Darker than Darker than like mulattoes mulattoes mulattoes Ab AABb AAbb AaBb Aabb Darker than Like mulattoes Like mulattoes Lighter than mulattoes mulattoes aB AaBB AaBb aaBB aaBb Darker than Like mulattoes Like mulattoes Like mulattoes mulattoes ab AaBb Aabb aaBb aabb white Like mulattoes Lighter than Lighter than mulattoes mulattoes Mulatto Intercross Parental Cross Two mulatto individuals (AaBb) are crossed. Offspring Variation The offspring exhibit a wide range of skin colors, from as dark as the original negro parent to as white as the original white parent, demonstrating the continuous variation characteristic of polygenic inheritance. Other Examples of Quantitative Inheritance Height Weight Human height is a classic Body weight is another example of a quantitative quantitative trait influenced trait influenced by multiple by genetics, diet, and lifestyle. genes and environmental factors. Milk Production Grain Yield Milk production in cows is a Grain yield in crops is a quantitative trait influenced quantitative trait influenced by breed, nutrition, and by genetics, soil fertility, and management practices. climate. History of Quantitative Genetics -Mendel observed seven clear cut visible traits in garden pea. -Mendel’s Law of Inheritance of Characters Francis Galton (1989) Biometrical technique – correlation and regression Fingerprints Galton's theory of fingerprints and physiognomy led to the application of mathematical models to fingerprints. He estimated that the probability of finding two identical fingerprints is one in 64 billion, even among twins Types of Quantitative Traits 1. continuous trait - continuous gradation from one phenotype to the next; continuum of phenotypes. example: growth rate, crop yield, weight Applications of Quantitative Inheritance Crop Improvement Understanding quantitative inheritance helps breeders select for desirable traits in crops, leading to increased yield and disease resistance. Animal Breeding Quantitative genetics principles are used to improve livestock breeding, selecting for traits like milk production, meat quality, and disease resistance. Human Health Quantitative genetics plays a role in understanding complex diseases like diabetes, heart disease, and cancer, which are influenced by multiple genes and environmental factors.