Tag: autosomal DNA

 

Not so long ago we just never heard of DNA being used in everyday situations. And then suddenly every detective story on TV seemed to mention the suspect’s DNA being collected from the crime scene.

In the world of family history, DNA has also emerged into the main stream. Today if you want to prove that you are descended from a certain line then you may be able to use genetics to prove it.

But then there is the shorthand that is used that can confuse us a little. You may have heard people talking about “snips” or SNPs and STRs and wondered what this has to do with anything!

I will now attempt to explain what I myself was uncertain of until I attended one of the talks by an academic at last year’s Who Do You Think You Are? Live show and then found it explained again in chapter 12 of Anthony Adolph’s book Tracing Your Aristocratic Ancestors. Pen and Sword 2013

Chromosomes are made up of special proteins and DNA. DNA is composed of numerous base-pairs of nucleotides all arranged in a double-helix shape.

In every human cell there is a nucleus that contains twenty-two pairs of chromosomes that bear autosomal DNA and one pair that decides on the gender of the human. The two sets of chromosomes are reduced down to one in a process of myosis that produces eggs in females and sperm in males.

When a man and a woman have a child the male passes on the one set of his chromosomes and mixes with the female’s one set and so the next generation inherits from its parents.

It is a complex process that causes some slight changes or mutations which are known as genetic polymorphisms. Some of these mutations include single nucleotide polymorphisms which are often abbreviated to the letters SNPs.

A single tandem repeat is known as a STR.

SNPs and STRs do not, it is believed by the scientists who understand such things, carry any useful codes needed in the creation of ourselves, but they are there.

 

Individual genes have two or more possible states of being and these are usually referred to by the letters A or T and C or G.

An SNP is a change detected in a gene’s state of being from, say, A to G and you may see it being called a “unique event polymorphism”. Once a SNP has occurred it will now stay the same as it is passed down the generations and so you can see how this can act as a reliable marker for “deep ancestry” haplogroup testing.

 

So what about STRs?

 

They are a bit different. STRs occur in a different part of the chromosome and they are a series of multiple changes caused by the addition or subtraction of the number of base-pairs. So by counting these base-pairs the DNA company get to a numerical code. The great thing about these mutations is that they occur over a shorter time than the SNPs do and so they can change over shorter spans of generations.

 

Y-STRs are taken specifically from the male Y chromosome and are only passed down by the father, making the Y chromosome in any paternal line practically identical.

What we are presented with is two complementary sets of results: SNPs define a person’s haplogroup, or the group of people that share the same markers that can go back many thousand of years. The second is the smaller group of people that share the same STRs who are related to each other over the last couple of thousand years or less.

 

The second exception is mitochondrial DNA (mt-DNA). This is only passed down from a mother to her child, but which only her daughters will pass on. This means that we have a definite marker for the female-line, in other words the mother’s mother’s mother’s (and so on) family.

As we get closer to Christmas I have noticed that www.familytreedna.com are offering money off their packages for the holiday season. Do you know anyone who would like to have a present of a DNA test as it would seem to be a good time to buy?

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