Most of times, a gene is a sequence of nucleotides that stores information about a protein. The cell uses those information to build a protein. Let us use the ABO blood group as an example.

In ABO blood group, there are three variants of the gene, I^A, I^B and i.

The most direct result of gene expression is the production of certain proteins. I^A and I^B each makes a protein that modifies the surface of red blood cells. However, i does not make such protein. Human usually has two copies of every genes and a person usually can have any combination of these genes. It turned out that only having a I^A or I^B is enough for their read blood cells to have certain modifications. So
I^A/I^A
and
I^A/i both are type A,
I^B/I^B
and
I^B/i both type B.

People who has I^A/I^B are type AB and those with i/i type O.

Proteins often work together in a pathway where one protein works on the product made by another protein. The expression of gene can have a greater impact than the function of the related protein alone. There is a rare blood type called the Bombay blood group. The red blood cell from people with this blood group is similar to type O. However, people with this blood group may still has I^A and I^B gene. It turned out there is another gene H that makes the molecule I^A and I^B modify. Without the H gene, I^A and I^B do not have the source material they need.

The patterns of gene expression are often passed from parents onto their offspring and can result in offspring having the same or similar traits to their parents. For instance, two different families, Family A and Family B, both have the LCT gene, which is the gene that allows the production of a protein called lactase that helps digest milk sugars. Family A's expression pattern expresses the LCT gene. Family A would then have the lactase protein, and as the parents in A pass down the gene expression pattern to their children, both the parents and the children of A would be able to digest milk sugars. Family B, on the other hand, has an expression pattern that does not express the LCT gene. The parents in B would not have the lactase protein, and the pattern would be passed down to their children such that neither the parents nor the children would be able to digest milk sugars. Sometimes, even the number of proteins can depend on the gene expression patterns, and the differences in the number of proteins can be passed from parent to offspring and result in differences in traits between families. Gene expression patterns, therefore, can be hereditary. However, it is very important to note that gene expression patterns can be changed by what we do, and the change in expression patterns is a crucial part of adaptation.

For instance, when we change what we eat, we can change our own gene expression patterns and adapt to our new diet. When we start exercising more, we may also be changing how our genes are expressed in our muscles, lungs, and so on and adapt to the stress we're putting on our bodies. After a while, we find that we can exercise more intensely and for longer periods of time. Because of this adaptive process, we see that expression patterns can result in traits being passed down to offspring, but the passing down does not mean that the patterns have to stay fixed over the offspring's lifetime.

A gene only has any effects if it is being expressed. Thus, if you have an allele that has some effect - say, it would make you stupid - but the gene is not being, expressed, then the allele in question will not have the effect of making you stupid.

The way that genes work is that they encode proteins, and those proteins are what actually do stuff regarding making you you. Most of the mass in your body (apart from the water) is proteins, which were manufactured from the DNA template. The DNA itself, however, does nothing. If a gene is present, but not being expressed, then that means that no proteins are being created from that gene, even if the gene encodes them.