Rule #1: A gene is a code for a message that instructs a cell in your body to make something that your body needs.

• The DNA molecule (DNA stands for Deoxyribo-Nucleic Acid) is the molecule that contains the genetic message. The DNA molecule looks like a ladder with two rails and many rungs. The rails are the backbone of the molecule. The rungs are actually composed of two sets of molecules that fit together. The sequence of molecules down one rail makes up the message; the complimentary sequence on the opposite rail allows the message to be faithfully duplicated.
• The molecule Adenine (abbreviated by the letter "A") fits with the molecule Thymine (abbreviated by the letter "T"). The molecule Guanine (abbreviated by the letter "G") fits with the molecule Cytosine (abbreviated by the letter "C"). These four molecules; A, T, G, and C make up the code for the genetic message. (View a PPT that demonstrates DNA replication here)

Rule #2: Genes are strung together on a chromosome, like beads on a string. A chromosome may contain hundreds to thousands of genes strung together. Chromosomes insure that all the genes get passed on when a cell divides.

• Chromosomes come in pairs called homologs. This complicates matters because each genetic message is essentially duplicated, available in two copies on each corresponding or linked (homologous) chromosome.
  
• These pairs of chromosomes make heredity possible: you get one half of your chromosome pairs from each of your parents; you give one half of your chromosome pairs to each of your children, and your spouse/partner also gives one half of their chromosome pairs to each of your children.
  
• It is possible that the gene messages on the two homologous chromosomes may have slightly different messages [genetic scientists call these different messages for the same corresponding gene alleles]. Genetic scientists also call the change in a message a mutation.
  
• The relationship between these messages determines a dominant or recessive relationship between the alleles. If the gene message is always expressed in the cell, that allele is said to be dominant. If the gene message is masked by the dominant gene message, the allele that is masked is said to be recessive. In order for the recessive trait or message to become evident, both copies on both chromosomes must be recessive.

Rule #3: Every human has 23 pairs of chromosomes, 46 in all. Twenty-two (22) pairs of chromosomes are called the autosomes and have a corresponding number of 1-22. One pair of chromosomes is called the sex chromosomes. The sex chromosomes determine your gender: Males have one X and one Y chromosome; Females have two X chromosomes. Virtually every cell in your body has these 46 chromosomes in the nucleus of the cell.

• It is possible for a person to survive with all or part of a chromosome missing or with an extra chromosome. The net result is that a person may have hundreds, if not thousands of chromosomes missing, extra or both. Genetic scientists call the presence or absence of a whole chromosome in a person a chromosomal syndrome (Down syndrome, also called Trisomy 21, is the most common chromosomal syndrome, with an extra chromosome #21). Sometimes parts of chromosomes are missing, extra or both. Genetic scientists call these deletion, duplication, and deletion-duplication syndromes, respectively.
  
• Sometimes different genes work together to cause genetic hearing loss. Genetic scientists call this phenomenon polygenic inheritance. Sometimes a gene may only be expressed if exposed to specific environmental conditions. For example, when exposed to loud noises, some people may be more susceptible to developing a noise-induced hearing loss than others. Part of this may be genetic. Genetic scientists call this multifactoral inheritance.
  
• Other parts of a cell, specifically the mitochondria, also have DNA. Changes in mitochondrial DNA can also affect humans. Genetic scientists call these forms extra chromosomal inheritance.
  
• Mitochondria share a symbiotic relationship with cells. This means neither can exist without the other. The mitochondria provide energy to the cell. The cell provides some of the chemicals the mitochondria need to survive. Mitochondria were once bacteria, so their DNA is similar to bacterial DNA. This similarity to bacterial DNA is the basis of a condition that causes sudden and massive hearing loss in some people who are given antibiotics.