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Multifactorial Traits

7.1 Genes and the Environment Mold Most Traits
1. Polygenic traits are determined by more than one gene, with each gene providing a small, but additive effect.
2. Multifactorial traits are determined by interactions between a gene or genes and the environment, and do not exhibit Mendelian ratios.
3. Pure polygenic traits (i.e. no environmental influence) are rare.

Polygenic Traits Are Continuously Varying
1. Polygenic traits usually produce a continuum of phenotypes.
2. Individual genes of a polygenic trait follow Mendel's laws, but together do not produce Mendelian ratios.
3. A bell shaped curve often describes the distribution of phenotypic classes of a polygenic trait.

Fingerprint Patterns, Height, and Eye Color
1. Dermatoglyphics is a technique that compares the fingerprint patterns that identify and distinguish individuals. Fingerprint pattern is a multifactorial trait and environmental differences during gestation results in dermatoglyphic differences between identical twins.
2. Human height in a population varies continuously in a bell shape distribution. Diet and health are strong environmental factors in expressing genetic potential for height.
3. Eye color is a polygenic trait that has little (if any) environment component. A model using two genes with two alleles that interact additively approximates the distribution of the five human eye colors.

A Closer Look at Skin Color
1. Differences in skin color arise from the quantity and distribution of the pigment melanin in the uppermost layers of skin cells.
2. Exposure to sunlight is an environmental factor that can affect skin color.
3. From a genetic perspective, allele frequencies are the best indicator of shared heritage in a population. Skin color alone does not effectively indicate a person's ethnic and genetic background.

7.2 Methods Used to Investigate Multifactorial Traits

Empiric Risk
1. Empiric risk is not calculated, but is an observed population statistic.
2. Empiric risk is used to predict recurrence of a multifactorial trait in a family.
3. Empiric risk increases with severity of the trait, number of affected relatives, sex of family members, and increasing relatedness to an affected individual.

Heritability- The Genetic Contribution to a Multifactorial Trait
1. Heritability estimates the proportion of variation in a population for a multifactorial trait that is due to genotype.
2. The coefficient of relatedness indicates the proportion of genes relatives share.
3. The coefficient of relatedness increases as more closely related individuals are compared.
4. Studying multifactorial traits in humans is difficult since data must be pooled from many families.
5. Analysis of multifactorial inheritance has wide spread application in agriculture.

Adopted Individuals
1. To assess the influence of heredity and the environment, traits of an adopted child can be compared with those of the adoptive and biological parents.

1. Twin studies have been used to separate genetic and environmental contributions to a phenotype.
2. A trait that occurs more frequently in both member of identical (MZ) twins than in both members of fraternal (DZ) twins probably has a significant genetic component.
3. The concordance of a trait is the percentage of pairs in which both members express the trait.
4. The study of monozygotic (MZ) twins separated at birth offers some unique insights into the nature vs. nurture debate.

Association Studies
1. Single nucleotide polymorphism (SNP) mapping can be used to identigy multiple genes associated with a polygenic trait.
2. SNPs are useful in association studies where SNP patterns of two populations may reveal disease causing genes, susceptibility genes, or guide therapeutic approaches.

7.3 Some Multifactorial Traits

Heart Health
1. Genes that control cholesterol metabolism, leukocyte adhesion, and blood pressure greatly influence the risk of developing cardiovascular disease.
2. Allele specific probes may be used to generate multilocus genotype data for assessing genetic risk.

Body Weight
1. Obesity is a growing problem in the United States.
2. Leptin, a protein hormone, suppresses appetite and increases metabolic rate.
3. Neuropeptide Y, produced in response to low leptin levels, increases appetite.
4. Genes controlling leptin, neuropeptide Y and other related proteins exert a significant effect on body weight.
5. Studies on the Pima Indians of Arizona and Mexico demonstrate the role of environment in controlling body weight.

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