NIJ Training Course for Forensic DNA Officers of the Court: Child Inheritance Pattern in Nuclear DNA

A normal human set of DNA is made up  of 46 chromosomes arranged in 23 pairs.  When a person's body generates a normal  body cell hair skin white blood cell, etc.,  it copies its entire chromosome set both  chromosomes from each pair into the new cell.

However, when a person's body generates a sex  cell, females produce eggs, males produce sperm,  it copies only one chromosome or the  other not both from each of its pairs.

Because each sex cell receives a copy of only one  chromosome from each of the original 23 pairs,  every sperm or egg contains only 23 chromosomes  23 half pairs rather than the 46 contained in a  normal body cell. At the creation of a sex  cell, the determination of which chromosome  will be copied from any particular pair is random  like flipping a coin for each pair to determine  which of that pairs two chromosomes will be  copied. This means that 2 to the 23rd power  or 8,388,608 different half sets of DNA  are possible from a person's full set.  When a child is conceived, the mother's egg  and the father's sperm each donates its 23  chromosomes, half of that parent's total  set, toward the creation of the child.  The child then has a full set of 46 chromosomes,  23 pairs (half donated by the mother and half by  the father) since the father's body could produce  2 to the 23rd possible different half sets of his  DNA and the mother's body could produce two to the  twenty third possible different half sets of hers  there are 2 to the 23rd times 2 to  the 23rd or over 70,000,000,000,000  different possibilities for the genetic  makeup of a child of any one couple.