Science 101: What is a Gene?
Millennia ago, people began noticing generational similarities in families and the concept of traits passed from parents to children firmly took root. But understanding that those traits were the expression of specific molecular mechanisms of inheritance had to wait for our current era’s advanced technologies. In just about the last 150 years, an explosion of information has revealed the existence of DNA and RNA, characterized their genetic roles and structures, confirmed chromosomes' genetic function, mapped more than 30,000 genes and elucidated their role in health and disease, an ongoing endeavor.
Genes are comprised of different lengths of deoxyribonucleic acid (DNA). DNA is made of phosphate, the sugar deoxyribose, and just four amino acids known as bases. The physical structure of DNA resembles as twisted ladder, with rungs of two paired bases linked to external rails comprised of phosphate and sugar. Each amino acid can only pair in certain ways, adenine with guanine and cytosine with thymine. The arrangement of the paired bases literally spells the difference in each gene's job, and alterations in the order can cause changes in gene function.
The double helix of DNA is further twisted into tight coils that form chromosomes, which reside in a cell's nucleus.Humans have 22 pairs of autosomal chromosomes and one pair of sex chromosomes, called X and Y. Mothers pass along, via an egg, one of each of the 22 autosomal chromosomes as well as the X chromosome, while fathers, via a sperm, pass one copy of the autosomal chromosomes as well as either the X or the Y chromosome.
In this manner, a child inherits one half of each parent's genetic information.  The maternal and paternal versions of the same autosomal gene are known as alleles. When alleles are identical, the gene is called homozygous. However, if the alleles differ because of alterations or mutations, the gene is called heterozygous. With heterozygous alleles, one version may dominate, expressing itself so strongly that the other version is called recessive. A dominant gene alone can cause the disease, such as with Hunting's disease. For a recessive gene to cause disease, the alleles must be homozygous, as in cystic fibrosis when both parents must contribute the gene. 
Mothers also pass along another kind of DNA: a short piece within the cell's powerhouse, the mitochondria. This organelle's genes create proteins the mitochondria needs to function correctly. Defects in mitochondrial DNA can result in diseases involving the failure to produce enough energy in muscles, the kidneys or the brain.
Ribonucleic acid (RNA) is similar to DNA, but exists as one strand. RNA also uses the sugar ribose and its four bases are adenine, uracil, cytosine and guanine. RNA comes in different types to perform specific functions within the cell, including messenger RNA, which via a process called transcription carries the DNA base order information from the cell's nucleus into its cytoplasm where ribosomes translate the information into proteins. Ribosomes are made of another kind of RNA and protein.
The observable characteristics that define us – height, hair color, chin clefts, ear shape and the like -  are called phenotypic traits, but they all are expressions of the collection of a person's genetic material is called his or her genotype or genome. Understanding how our genetic code becomes our physical experience is one of the most active areas of scientific research currently underway, thanks to the achievements of those who completed the mapping of the Human Genome in 20013. The map revealed that the human genome has about 3 billion of DNA base pairs on the 23 pairs of chromosomes. Each chromosome carries hundreds to thousands of genes, each of which makes an average of three proteins. That mind-boggling guidebook to human genetics has given rise to a new era in medicine with the power to change lives and improve health for everyone.
Miescher, F., 1871d. Ueber die chemische Zusammensetzung der Eiter- zellen. Med.-Chem. Unters. 4, 441–460. as cited in Dahm R., Friedrich Miescher and the discovery of DNA. Dev Biol. 2005 Feb 15;278(2):274-88.
National Human Genome Research Institute. Human Genome Project Completion: Frequently Asked Questions. Oct. 30, 2010 Accessed at https://www.genome.gov/11006943.
X Chromsome https://www.genome.gov/Glossary/index.cfm?id=208&textonly=true
Human Genome Project. https://www.genome.gov/11006943