Chromosomes, Genes, Genotypes and Phenotypes

If DNA were to be stretched out its full length, it would be approximately 6 feet long and almost every cell of the body has a copy!  Thankfully, DNA is divided into smaller sections called chromosomes.  Humans have 23 pairs of chromosomes - one set came from the ovum (egg) and one set came from the sperm.  Check them out in the karyotype below.

You may have noticed that you have 22 pairs of chromosomes and the last set designated as X/Y.  The 22 pairs of chromosomes are autosomes and contain genes common to both males and females.  The X/Y pair is called sex chromosomes as they dictate the genetic sex of the individual and contain "sex-linked" traits.  You may recall that earlier that humans have over 30,000 genes (recipes for proteins) and you should connect that these 30,000 genes are found within these 23 pairs of chromosomes.  When we take a closer look at a pair of genes, we find that we received one copy from each parent and that these may be slightly different but code for the same characteristic.  Each copy is considered an allele.  An example using the trait of eye color - an individual may receive an allele coding for brown eyes from the mother and an allele for blue eyes from the father - both code for eye color but are slightly different

An individual’s genetic makeup is referred to as their genotype. The phenotype is the characteristic that is expressed by the genes.   We can return to the eye color example above to show this concept.  The individual above had a genotype of one allele for blue eyes and one allele for brown eyes.  When we look at the individual and see that they have brown eyes, this is their phenotype.