SNP's and Polymorphisms

[BStill]

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.

[CharonY]

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.

[BStill]

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?

[CharonY]

Sounds right.