Many human birth defects arise because of chromosomal abnormalities such as deletions or insertions. For example, the most common deletion syndrome in humans - DiGeorge syndrome - results from the deletion of a large region within chromosome 22. Although the DNA sequence of the deleted region has been known for some time, progress in determining which genes in this sequence are critical for development has been slow. This is due to the complexity of human genetics and the limitations of working with human material.
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