The Basics of DNA Types and Inheritance
Introduction to Autosomes, Sex Chromosomes & Mitochondrial DNA
Jul 1, 2009
DNA is the chemical of heredity, and the reason biological relatives share features. While paper records can tell people something about their past, genetics can also play a huge role in genealogy. People compiling family trees may look to genetic testing as part of these projects.
In humans, nuclear DNA refers to the 23 pairs of chromosomes in the nuclei of most cells. It includes the sex chromosomes X and Y, and the non-sex chromosomes, numbered 1–22. These non-sex chromosomes are also called autosomes.
There is also DNA outside the cell nucleus. Many cells have thousands of mitochondria, organelles that give the cell oxygen. These mitochondria contain a small amount of genetic information, called mitochondrial DNA (mtDNA).
The DNA and Inheritance of Autosomes
One set of autosomes are inherited from the biological mother, the other set from the biological father. This parent-child inheritance process goes back through generations.
Before autosomes are passed on, material from each pair is swapped, making them different from the original copies. This shuffling process is called recombination, and it is important because it allows mistakes in the genes to be corrected.
Not only do people have genetic material from their parents, but from their biological grandparents, great grandparents, and so on. Since people share more material with closer relatives, autosomal DNA very is useful in recent ancestry.
The DNA and Inheritance of the X Chromosome
The X chromosome contains some of the genetic material needed to be female. Women have two of these X chromosomes, whereas men have one X and one Y. When a woman passes her X to her children, recombination occurs between her two copies, rather like with pairs of autosomes.
A man has no second X to recombine with. Any daughters he has will inherit a virtually unchanged X chromosome. It does change before being passed, as it is able to recombine a little with the Y. These changes are not as significant as that of autosomes and women’s X chromosomes.
The DNA and Inheritance of the Y Chromosome
The Y chromosome is the sex chromosome that contains genetic material required to make a human male. It is inherited from father to son only. The Y recombines with itself, and a little of the X, meaning the copy a father’s son will receive is very similar to the original.
This pattern of inheritance means that Y-chromosomal DNA can be traced back through male lines to a small number of ancestors, making it particularly useful in deep ancestry.
The Inheritance of Mitochondrial DNA
Like Y chromosome DNA, mitochondrial DNA is useful in exploring deep ancestry. Both males and females have mitochondrial DNA, but it is inherited only from the mother. This means that virtually unbroken female lines can be traced back several thousand years.
In an Ask the Geneticist question on Stanford and The Tech’s Understanding Genetics website, geneticist Jamie Conklin says, “It is still being debated whether mtDNA goes through recombination. Since it doesn’t recombine much at all, it can’t fix mistakes well. So mistakes build up faster here than in the chromosomes.”
For more information, read Basic Cell Biology for Understanding Genetics for the basics of DNA, genes and chromosomes. Visit the Chromosomes section of the Genetics Home Reference website for more details of the different types of DNA.
Sources
The Four Types of DNA, Learn. Genetics – Genetic Science Learning Center website, accessed: 18th June 2009.
What is mitochondrial DNA?, Genetics Home Reference website, published: 12th June 2009, accessed: 18th June 2009.
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