With completion of the reference human genome sequence, much attention has turned to the discovery and cataloging of variation in sequence among different individuals (including both healthy individuals and those with various diseases) and among different populations around the globe. As we will explore in much more detail in Chapter 4, there are many tens of mil lions of common sequence variants that are seen at significant frequency in one or more populations; any given individual carries millions of these sequence variants. In addition, there are countless very rare variants, many of which probably exist in only a single or a few individuals. In fact, given the number of individuals in our species, essentially each and every base pair in the human genome is expected to vary in someone some where around the globe. It is for this reason that the original human genome sequence is considered a reference sequence for our species, and not one that is actually identical to any individual’s genome.

Early estimates were that any two randomly selected individuals would have sequences that are 99.9% identical or, put another way, that an individual genome would carry two different versions (alleles) of the human genome sequence at some 3 to 5 million positions, with different bases (e.g., a T or a G) at the maternally and paternally inherited copies of that particular sequence position (see Fig. 1). Although many of these allelic differences involve simply one nucleotide, much of the variation consists of insertions or deletions of (usually) short sequence stretches, variation in the number of copies of repeated elements (including genes), or inversions in the order of sequences at a particular position (locus) in the genome. These copy number variations account for at least 12 million bp of difference between any two unrelated individuals.

Fig1. A portion of the reference human genome sequence. By convention, sequences are presented from one strand of DNA only, because the sequence of the complementary strand can be inferred from the double-stranded nature of DNA (shown above the reference sequence). The sequence of DNA from a group of individuals is similar but not identical to the reference, with single nucleotide changes in some individuals and a small deletion of two bases in another.

The total amount of the genome involved in such variation is now known to be substantially more than originally estimated and approaches 2% between any two randomly selected individuals. As will be addressed in future chapters, any and all of these types of variation can influence biologic function and thus must be accounted for in any attempt to understand the contribution of genetics and genomics to human health.

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مواضيع ذات صلة

Abnormalities of Chromosome Structure

Impact of Genomic Variation

Variation in individual Genomes

Variation in gene Expression and its relevance to medicine

Allelic Imbalance in gene expression

Gene expression as the integration of Genomic and Epigenomic signals

Epigenetic and Epigenomic aspects of Gene expression

DNA Microarray Technology

RNA Splicing

Initiation of Transcription

Fundamentals of Gene Expression

Cell cycle checkpoints