DNA: Basic Genetics
5/8/17
Judith Culver
Basics
of Genetics: All you need to know is 2, 3, 4, and 7.
1.
A Genome is all the genetic material of a cell
or an organism. Genes are the basic unit
of heredity.
2.
There are 23 pairs of chromosomes that reside in
the nucleus of our cells. One half of
the pair is inherited from your mother and the other half from your
father. No test results tell you which
is which.
3.
Chromosome 23 is the sex chromosome which
identifies gender. XX pair for women and
XY pair for men. One X chromosome is
inherited from the mother and either an X or a Y chromosome is inherited from
the father. If the father passes an X
chromosome, the child is female. If the father
passes a Y chromosome, the child is male.
4.
Chromosomes 1-22 are called autosomal. An autosome is a chromosome that is not a sex
chromosome. Autosomal DNA is the part of
your DNA that recombines between parents every generation. The segments are
random and differ between siblings. It
is used by genealogists to confirm connection with a specific ancestor and to
break down genealogy brick walls.
5.
There is a small DNA molecule outside the
nucleus called mitochondria. This is
passed from the mother.
6.
A person receives their DNA from their parents who
receive it from their parents and so on throughout history. These are their direct ancestors. When DNA of two people matches, they are
descended from a common ancestor. They
are usually looking for their “most recent common ancestor” or MRCA.
7.
Chromosome segments are measured in Centimorgans
(cM). This is a measure of length and
quality of the segment, not just the length.
For each person, the total of their segment lengths is approximately
6700 to 7040cM.
8.
A segment of 10cM has 99% chance of being “Identical
by Descent (IBD),” which is what we are looking for. Other segments may be Identical by State
(IBS), identical by chance, identical by population, etc. Many people use 7cM
as the cutoff for Identical by Descent.
But there are reasons to look at shorter cM’s too.
9. Humans
are 99.9% genetically identical. All the
differences are in the .1%.
Note: for information
about the different tests, testing companies and recommendations, see DNA:
Tests and Recommendations
Advanced Terms
Chromosome mapping or browser: shows on
which DNA segments you match selected other people, and from there determining
which segments came from which ancestor.
DNA test chip: a microchip that
describes which genes will be tested and can quickly recognize DNA from samples
being tested. A testing company
specifies which genes they want to test and the chip is custom made.
Endogamy: Endogamous populations are
groups who tend to marry within their own culture, religion, or tribe,
resulting in a small gene pool. It means
that the amount of matching DNA may indicate a closer relationship that it
actually is.
Haplogroup: a genetic population group
or ancestral clan, that groups people by Y-DNA and mitochondrial DNA, from
thousands of years ago. Haplogroups are
identified by FamilyTreeDNA and 23andme.
They can be used with autosomal results to find ancestors in the last
5-6 generations.
ICW (in common with): People you match someone you match. You may not match on the same segment.
Phasing is a process that helps
determine whether matching segments are paternal or maternal.
Recombination: Genetic recombination randomly
“chops” up DNA in each successive generation as a child is being conceived. For example: In creating the child’s
chromosome 1, segments are taken from the father’s chromosome 1 and alternated
with segments from the mother’s chromosome 1.
Usually up to a total of 3 segments.
SNP:
Single Nucleotide Polymorphism: For
Y-DNA only. Pronounced “snip”. Most common type of genetic variation. SNP’s have unique names and a person tests
either positive or negative for the particular SNP. Helps determine Haplogroup.
STR: Short tandem repeat: For Y-DNA
only. Pronounced “stir”. A section of the DNA strands that are
repeated. STR’s have individual tests.
Timeframes: Genealogical timeframe means within the last
10 or so generations where there is a possibility of identifying a common
ancestor through documentary records. Also called recent ancestry. As opposed to ancestral, ancient or
historical timeframe which is also called deep ancestry.
Triangulation: identifying three people
with a matching chromosome segment to prove they are related.
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