Hello,

 

I am having a difficult time understanding the concept of SNP's and polymorphisms in general.

 

So far, I understand that a SNP is a change in a single nucleotide in the DNA sequence that occurs in greater than 1% of the population.

 

If everyone's genome is not sequenced, then how does anyone know that this particular change in nucleotide is in fact in greater than 1% of the population?

 

If we have a paternal and a maternal chromosome, does the SNP have to occur in one of these or both of these?

 

So is there a chromosome set that everyone is comparing these changes in nucleotide to?

 

Any help would be appreciated.

Your definition of SNP is partially wrong. The percentage of occurrence is irrelevant. It only means that between two given sequences at the same locus a single nucleotide is exchanged. To find these, you only need to sequence(or hybridize) a given region. e.g. a PCR amplificate.

A SNP can thus refer to, say a polymorphism between you and your parents, or between any two individuals, or even between paired chromosomes. There is no "universal" reference.

Hello Charon,

 

OK. I am starting to understand a little more.

 

So, lets say were talking about the paternal and maternal chromosomes 15.

 

Now lets say we sequence these choromosome.

 

When there is a difference in single base pairs between these chromosomes this is considered a SNP?

 

So for example.

 

GGCCTTAACCTTGG

CCGGAATTGGAACC

 

GGCATTAATCTTGG

CCGTAATTAGAACC

 

At position 4 and 9 are the loctions of the SNP.

 

Is this right?

Sounds right.