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Down Syndrome by Todd T. Eckdahl

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CHAPTER 2

Causes and Contributing Factors

The underlying cause of Down syndrome is the inheritance of one extra copy of most or all of the 250 genes found on chromosome 21. Our understanding of how the myriad of Down syndrome symptoms arises from extra copies of these genes is incomplete, but it is rooted in 150 years of genetics research. The concept of a gene as a heritable unit of information was published by Gregor Mendel in 1866. By studying garden peas, Mendel learned that genes can exist in different forms called alleles, and that an allele can be dominant over a recessive allele when both are present in the same individual. He established that genotypes, which are inherited alleles, affect phenotypes, which are observable traits. Mendel ...

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