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eye color is always dominant to blue, with 2 blue-eyed parents always producing a blue-eyed child, never one with brown eyes. As with many physical traits, the simplistic model does not convey the fact that eye color is inherited as a polygenic, not as a monogenic, trait (Sturm and Frudakis, 2004). Although not common, 2 blue-eyed parents can produce children with brown eyes. (McKusick, “Eye Color 1.”)

    Human iris color is a quantitative, multifactorial phenotype that exhibits quasi-Mendelian inheritance … To identify genetic features for best-predicting iris color, we selected sets of SNPs by parsing P values among possible combinations … These results confirm that OCA2 is the major human iris color gene and suggest that using an empirical database-driven system, genotypes from a modest number of SNPs within this gene can be used to accurately predict iris melanin content from DNA. (Frudakis, Terravainen, and Thomas, “Multilocus OCA2 genotypes specify human iris colors,” pp. 3311–26.)

    The highest association for blue/nonblue eye color was found with three OCA2 SNPs … The TGT/TGT diplotype found in 62.2% of samples was the major genotype seen to modify eye color, with a frequency of 0.905 in blue or green compared with only 0.095 in brown eye color. This genotype was also at highest frequency in subjects with light brown hair and was more frequent in fair and medium skin types, consistent with the TGT haplotype acting as a recessive modifier of lighter pigmentary phenotypes. (Duffy et al., “A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation,” p. 241.)
         Single-gene diseases do exist and account for roughly 5 percent of the total disease burden in developed countries : Khoury, Yang, Gwinn, Little, and Flanders, “An epidemiological assessment of genomic profiling for measuring susceptibility to common diseases and targeting interventions,” Hall, Morley, and Lucke, “The prediction of disease risk in genomic medicine.”
    Susan Brooks Thistlethwaite adds:
    Genetics is not merely a matter of single gene disorders or single gene traits, such as flower color and pod shape in Mendel’s pea plants. Mendelian genetics is about single gene disorders [that] occur in only 3 percent of all individuals born alive …
    Human inheritance is much more complicated. Most conditions are polygenic (involve many genes), and their expression depends on gene-gene and environment-gene interactions. (Thistlethwaite, Adam, Eve, and the Genome , p. 70.)
         “A disconnected wire can cause a car to break down ”: Oyama, Griffiths, and Gray, Cycles of Contingency , p. 157.
         “Genes store information coding for the amino acid sequences of proteins,” explains Bateson . “That is all”: Bateson, Design for a Life , p. 66.
    Similar statement: “All the genes can code for, if they code for anything, is the primary structure (amino acid sequence) of a protein.” (Godfrey-Smith, “Genes and Codes,”. p. 328)
         One of the most striking early hints of the new understanding of development as a dynamic process emerged in 1957 .
       There were much earlier hints. “For most of the past century,” says Penn State geneticist Gerald E. McClearn, “the evidence has been clear that a more collaborative model of coaction and interaction of genetic and environmental agencies is more appropriate. Even in the pell-mell pursuit of Mendelian phenomena in the post-rediscovery enthusiasm at the beginning of the last century, examples of the interdependence of genetic and environmental influences surfaced. One well-known early example is that of Krafka [1920], who showed that the effect of the bar-eyed genotype (now known to be a duplication) on eye facet number of Drosophila is strikingly dependent on the temperature at which the flies are maintained.” (McClearn, “Nature and nurture,” p. 124.)
         heights of Japanese children :