An Introduction to Basic Genetics

Genes are the basic units of heredity.  Genes encode the information needed to make all the other parts of the cells that make up the human body.  Chromosomes are the structures that carry the genes, like beads on a chain.  Each person has 46 chromosomes, 44 autosomes and 2 of the sex chromosomes, the X and Y chromosomes. Females have 44 autosomes and 2 X chromosomes, while males also have 44 autosomes with one X and one Y chromosome.  Twenty three of these chromosomes came from their mothers (i.e., 22 autosomes and one X chromosome) and 23 from their fathers (i.e., 22 autosomes and one that is either an X or a Y chromosome).  Thus genes and the chromosomes that carry them occur in pairs – 22 autosome pairs and the 2 sex chromosomes.

An overview - and admittedly an oversimplified view - of basic genetics starts with the fact that all people have two genes for nearly every hereditary trait in their bodies. They receive one gene for each trait from each parent for a total of two genes.[3]  One gene [4] on a chromosome they got from their mother and the other gene for that trait on the chromosome of that pair that they got from their father.  Their parents, of course, also have two genes for each trait residing on a pair of chromosomes, but when they create a sperm or an egg, the chromosomes bearing those genes separate, such that there is only one chromosome of the pair, and hence, only one gene for each trait per each sperm or egg. At conception, the sperm from the father combines with the egg from the mother, and the fetus acquires his/her own pair of chromosomes with their attached genes.

Genes are generally either dominant or recessive. The dominant gene is exactly that - dominant. You can think of it as the bully on the grammar school playground; when he's around he prevails. As the genes pair up, whichever gene is dominant will determine what traits will prevail (i.e., be expressed) in the new human being. The gene for the recessive "version" of a particular trait may be there, but it is not allowed to express itself if a gene for the dominant version is also there to overpower it.

[3] The two members of each gene pair are called alleles.

[4] allele 

Previous Chapter:  Introduction

Next Chapter:  How Genes are Passed and Traced

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