DNA: Basic Genetics

DNA: Basic Genetics

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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