Statistical Analysis of Genetic Data in Twin Studies and Association Studies
In studies in human genetics we want to answer questions such as: how important are genetic effects on a phenotype; what kind of action and interaction exists between gene products in the pathways between genotypes and phenotype; are the genetic effects on a phenotype consistent across sexes; do some genes have particularly outstanding effects when compared to others; what are the locations of the genes involved in the phenotype of interest ? Twin studies and association studies as described in this thesis are designed to answer the first and the last questions in this list. A twin study}is designed to determine genetic and environmental influences on phenotypes by comparing the similarity of monozygotic (identical) and the similarity of dizygotic (fraternal) twins. A study designed to test the association between a phenotype and a specific candidate gene or region in the human genome is called an association study. This study can use Single Nucleotide Polymorphisms (SNPs) in the human genome as markers to find which locations on the chromosomes are associated to the disease. A review of twin studies and association studies is described in the first chapter. In the second chapter, we describe statistical modeling in twin studies to estimate the genetic component of phenotypes of interest. We develop a statistical model for the menarcheal status in girls as a dichotomic trait and estimate the tetrachoric correlation and the heritability using the Markov Chain Monte Carlo method implemented in WinBUGS. The model can be extended to polychotomic trait, such as breast development and pubic hair development. We also propose a model of two traits and the model can be applied to breast-pubic hair development traits, breast-menarcheal development traits and pubic hair-menarcheal development traits.
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