Active Lecture PowerPoint ® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino Copyright © 2010 Pearson Education, Inc. Chapter 22 Quantitative Genetics

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

Copyright © 2010 Pearson Education, Inc. Figure 22-1

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 1–81–8 F 1 generation F 2 generation Fraction of population Skin color Eggs Sperm 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 1–81–8 aabbcc (very light) AABBCC (very dark) AaBbCc 1 –– –– 64 6 –– 64 1 –– –– 64 6 –– –– 64 1 –– –– 64 6 –– –– 64 A model for polygenic inheritance of skin color

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 Additive Alleles: The basis of continuous variation

Polygenic Inheritance

Mean & Variance Mean is the average. Variance is the average squared distance of all measurements from mean

Heritability (H 2 ) Phenotype = Genotype + Environment + Interaction between G X E V P = V G + V E + V G x E Proportion of total phenotypic variation in a population due to genetic factors Broad-sense heritability H 2 = V G /V P

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 H 2 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 (V P = V E ) –Epigenetic differences are included in the envt Dizygotic (DZ) or fraternal twins –Arise from two separate fertilization events –Variation is due to (V P = V G + V E + V G x E ) Comparison of the same trait in both kinds of twins is used to estimate broad sense heritability –H 2 = V G /V P

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QTL could be mapped, isolated, and transferred Read 22.8 in the textbook