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Monozygotic twins derive from a single fertilized egg that divides in two and then goes on to form two separate embryos. This contrasts with dizygotic twins that arise from two eggs released in the same menstrual cycle that are fertilised by separate sperm and then form two separate embryos.

Contents

Twin Studies

References

Twin Studies

To determine whether a disease is caused by genetic factors, researchers study the pattern inheritance of the disease in families. This provides qualitative information about the disease (how it is inherited). A classic example of this method of research is inheritance of hemophilia in the British Royal Family. More recently this research has been used to identify the Apoliprotein E (ApoE) gene as a susceptibility gene for Alzheimer's Disease, though some forms of this gene - ApoE2 - are associated with a lower susceptibility. To determine to what extent a disease is caused by genetic factors (quantitative information), twin studies are used. 'Monozygotic twins' are genetically identical and likely share a similar environment whereas dizygotic twins are genetically similar and likely share a similar environment. Thus by comparing the incidence of disease (termed concordance rate) in monozygotic twins with the incidence of disease in dizygotic twins, the extent to which genes contribute to disease can be determined. Candidate disease genes can be identified using a number of methods. One is to look for mutants of a model organism (e.g. the organisms ''Mus musculus'',''Drosophila melanogaster'', ''Caenhorhabditis elegans'',''Brachydanio rerio'' and ''Xenopus tropicalis'') that have a similar phenotype to the disease being studied. Another approach is to look for segregation of genes or genetic markers (e.g. single nucleotide polymorphism or expressed sequence tag) (Fig. 2).
A large number of SNPs spaced throughout the genome have been identified recently in a large project called the HapMap project^[1][2]). The usefulness of the HapMap project and SNP typing and their relevance to society was covered in the 27th October 2005 issue of the leading international science journal Nature (Fig 3).
A large number of genes have been identified that contribute to human disease. These are avaialble from the US National Library of Medicine, which has an impressive range of biological science resources available for free online. Amongst these resources is Online Mendelian Inheritance in Man - OMIM that provides a very, very comprehensive list of all known human gene mutations associated with, and likely contributing to, disease. Each article at OMIM is regularly updated to include the latest scientific research. Additionally, each article provides a detailed history of the research on a given disease gene, with links to the research articles. This resource is highly valuable and is used by the world's top science researchers.

References

1. Perspectives on human genetic variation from the hapmap project, McVean G, Spencer CC, Chaix R, , , PLoS Genet, 2005 This review is free of charge
2. The effect of HapMap on cardiovascular research and clinical practice, Skelding K.A., Gerhard GS, Simari RD, Holmes DR Jr, , , Nat Clin Pract Cardiovasc Med, 2007