There are three methods for finding genes contributing to complex diseases: candidate gene screening, linkage mapping, and association (case-control) studies. Candidate gene screening relies on selecting potential disease-causing genes, for example, genes causing inherited forms of disease and sequencing these genes in patients with complex diseases. Linkage mapping follows the segregation of chromosomal regions marked by random genetic variants in families with complex diseases, in search of regions that co-segregate with the disease trait. Finally, case-control association studies look for differences in the frequencies of common genetic variants between ethnically matched cases and controls to find variants that are strongly associated with the disease. The eventual goal of each method is to identify either definite mutations with a strong causal relation to the disease or polymorphic variants which have a weak causal link to the disease.
An extremely wide range of techniques is now available for the diagnosis of genetic disorders, a review of which is beyond the scope of this chapter. The Human Genome Project showed that the current genome sequence contains approximately 2.85 billion nucleotides interrupted by only 341 gaps and seems to encode only 20,000-25,000 protein-coding genes . If only 5% of these genes have diagnostic significance, approximately 1200 gene-based tests should be available. No single laboratory will be able to offer the breadth of these testing requirements and close liaison with a clinical genetics unit is imperative to ensure that samples are sent to the appropriate laboratory.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.