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Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino Chapter 22 Quantitative Genetics Copyright © 2010 Pearson Education, Inc. Mendelian Traits • Distinct and separate categories • Discontinuous variation • Qualitative inheritance • Single gene inheritance • Simple inheritance Quantitative Traits • No distinct and separate categories • Continuous variation • Quantitative inheritance • Polygenic inheritance – multiple genes can influence the trait • Complex or multifactorial inheritance – environment can influence the trait Quantitative Traits Plants: • Size of fruits, size of seed; weight of fruits, yield • Protein content, oil content, etc Animals: • Milk production, meat production (size) Humans: • Height, weight, IQ, proneness to some diseases Continuous Variation: has large number of phenotype classes Not all polygenic traits show continuous variation • Threshold traits are polygenic traits, but they have a small number of phenotypic classes • Environmental factors affect the phenotypes • Eg: Type II diabetes, multiple sclerosis, psoriasis, and schizophrenia. • See Fig. 22-1. Figure 22-1 Copyright © 2010 Pearson Education, Inc. Quantitative Traits Can Be Explained in Mendelian Terms • Multiple genes individually behave in Mendelian fashion in a cumulative or quantitative way • Multiple-factor hypothesis of Bateson & Yule Quantitative Inheritance Corolla size of Nicotiana Grain Color in Wheat Additive alleles vs. Nonadditive alleles P generation A model for polygenic inheritance of skin color aabbcc AABBCC (very light) (very dark) F1 generation AaBbCc 1 – 8 F2 generation 1 – 8 1 – 8 AaBbCc Sperm 1 – 8 1– 8 1– 8 1– 8 1– 8 1 – 8 1 – 8 1– 8 1 – 8 Fraction of population Eggs 20 –– 64 1– 8 1 – 8 1 – 8 1– 8 1 –– 64 6 –– 64 15 –– 64 20 –– 64 15 –– 64 6 –– 64 1 –– 64 15 –– 64 6 –– 64 1 –– 64 Skin color Additive Alleles: The basis of continuous variation 1. Phenotypic traits can be quantified by measuring, weighing, counting, etc. 2. Two or more gene loci scattered throughout the genome control the trait in an additive way 3. Non additive alleles do not contribute quantitatively to the phenotype 4. Contribution of each additive allele is small but equal 5. Together, additive alleles produce substantial phenotypic variation Polygenic Inheritance Polygenic Inheritance Polygenic Inheritance Polygenic Inheritance Polygenic Inheritance Mean & Variance Mean is the average. Variance is the average squared distance of all measurements from mean Heritability (H2) Phenotype = Genotype + Environment + Interaction between G X E VP = VG + VE + VG x E • Proportion of total phenotypic variation in a population due to genetic factors • Broad-sense heritability H2 = VG/VP Heritability • Heritability explains how much of the observed phenotypic variation in a population is due to genotypic differences • Heritability of 1.0 indicates that environmental conditions have little impact on the phenotypic variance. • Low H2 values, close to 0.0 indicates that environmental factors, not the genotype, are largely responsible for phenotypic differences. Interaction Between Genotype & Environment Twin Studies on Heritability • Monozygotic (MZ) or identical twins – Arise from splitting of a single fertilized egg – Variation between these twins is due to environment alone (VP = VE) – Epigenetic differences are included in the envt • Dizygotic (DZ) or fraternal twins – Arise from two separate fertilization events – Variation is due to (VP = VG + VE + VG x E) • Comparison of the same trait in both kinds of twins is used to estimate broad sense heritability – H2 = VG/VP http://www.guardian.co.uk/lifeandstyle/2011/sep/24/twi ns-black-white?fb=optOut QTL could be mapped, isolated, and transferred Read 22.8 in the textbook