Genes Determine Traits

Simply put, a gene is a portion of DNA that contains information for the physical expression of a trait or characteristic. For example, let's say that gene A determines the trait of hairline. Gene A will make a protein A, which will physically express a certain kind of hairline. If you see a person with a widow's peak hairline, you can conclude that it is because a certain gene on a certain chromosome has expressed itself. 

The physical traits we can see, such as hair color, eye color, height, skin color, and hairline are called phenotypes. The section of DNA associated with a certain trait or phenotype is called a genotype. While we cannot see genotypes, we can see phenotypes, and every phenotype has a genotype that caused it to appear. 

A Shuffled Deck

Genes are like a deck of playing cards: each deck contains many cards, and each card has a symbol that signifies a meaning. Like cards, genes can be shuffled and passed on. In card games, different combinations of cards change hands, but the individual cards do not change--the symbols on the cards stay the same. Genes too can be passed on from generation to generation without being altered.

As an example, a woman who has red hair has gene B expressed in her body. Her child can inherit gene B without any alteration, and therefore have red hair.

Alternate Versions

You now know that for every trait you see there is a gene behind it. But traits are not of a single type. Each trait has two variants. For example, a widow's peak is not the only type of hairline--some people have hairlines that go straight across in a horizontal line. If your mother has a widow's peak due to gene A, she also had another gene a that could have expressed a different hairline. She has two alternate versions of the gene that determines hairline. One is gene A and the other is gene a. The scientific term for these alternate versions of a gene is alleles.