Genetic Inheritance

In order to properly explain the different patterns of inheritance, we need to learn a little bit about genetics. DNA is present in each of our cells, and it contains all of the information that makes us human. Humans have  thousands of genes encoded in their DNA, each of which plays an important role in life. Everyone has two copies of every gene; one inherited from the mother, and one inherited from the father. You might hear these individual copies of the gene referred to as alleles.

Errors in our DNA are called mutations. Mutations can be inherited from our parents, or they can arise spontaneously during development.

What is an autosomal recessive pattern of inheritance?

Autosomal refers to all of the chromosomes except for the sex chromosomes (more about these later).  Recessive is the opposite of dominant; that is, its characteristics can be masked by a dominant gene.  Here is an example.

Let’s take the case of Bob and Mary. Bob and Mary each have one mutated copy of a particular gene (which renders that gene defective) and one normal, working copy of that gene (remember everyone has two copies of each autosomal gene). In the case of many genes, this won’t cause a problem, because the good copy of the gene compensates for the fact that the other copy isn’t working properly; we call this a recessive inheritance pattern, meaning that as long as you have one good copy of the gene, you can be perfectly healthy.

When Bob and Mary have a child, they each pass on one copy of each gene to the child. There are four gene combinations that Bob and Mary could pass onto their child, depending on which gene each adult transfers. We depict these four possibilities in the diagram below. We’ll call the properly working gene “E”, and we’ll call the mutated gene “e.” So remember, Bob and Mary each have one copy of E, and one copy of e. Their genes are shown outside of the box, and the four possibilities for their child are shown in each of the four squares in the box.

In the top left corner of the box, both Bob and Mary have passed on a good copy of the gene to their child; there is a 1 in 4 (25%) chance that this will occur. In this case, the child is perfectly healthy, as he or she has two good copies of the gene.

The top right corner and bottom left corner are similar; in each, one of the parents passes on a good copy of the gene, and the other parent passes on the defective copy. So there is a 2 in 4, or 50% chance, that this will happen. The child will be a “carrier,” meaning that they are carrying one defective copy of this particular gene (like the parents), but it does not generally result in disease.

In the bottom left corner of the box, both Bob and Mary have passed on the defective copy of the gene to their child; there is a 1 in 4 (25% chance) that this will occur. In this case, the child has two defective copies of the gene, and so they have no properly working copy of the gene. This can result in disease.

What is X-linked inheritance?

We talked before about how everyone has two copies of every gene. This is true in most cases, but there is one pair of genes called the sex chromosomes that change the situation slightly. The two sex chromosomes (the X chromosome and the Y chromosome) are responsible for distinguishing men and women. Everyone has two sex chromosomes; however, women have two X chromosomes, while men have one X chromosome and one Y chromosome. So while women have two copies of every gene since they have two X chromosomes (and don’t need the genes on the Y chromosome), men have only one copy of the genes on their X chromosome, since their other sex chromosome is a Y chromosome (which carries a set of male-specific genes).

So let’s take the example of Bob and Mary again, who we discussed before. But let’s pretend that the gene we were talking about was on the X chromosome. What would happen in this case? Here, the X is the gene on the X chromosome that functions properly, while x is the gene on the X chromosome that does not function properly.

Female children are represented by the left half of the box (because they are XX; note that Mary is XX, but Bob only has one X chromosome). Even if one of them receives the improperly functioning gene, they still have one good copy, and so are merely a carrier of the gene. 1/2 (50%) of Bob and Mary’s female children will have two good copies of the gene; 1/2 (50%) will have one mutated copy of the gene, and are therefore “carriers” of the disease.

The right half of the box represents the situation in which the child is a male (because only one X chromosome is received); in this case, Bob is contributing a Y chromosome (not shown), which carries a different set of genes from the X chromosome. Mary will pass on an X chromosome; 1/2 (50%) of Bob and Mary’s male children will have a properly functioning gene. 1/2 (50%) of their male children will have one mutated copy of the gene; since that gene is the son’s only copy (because his other sex chromosome is the Y chromsome), then he will have the disease. Note that if one of Bob and Mary’s sons has children, none of his sons will be affected, but all his daughters will be carriers.

What is Dominant Inheritance?

A dominantly inherited disease is one in which a single defective copy of a gene can cause disease. This generally means that there are not carriers of the disease as there are in a disease that is recessively inherited. In order for a couple to have an affected child, they would both most likely need to have the disease themselves. Let’s again take the example of Bob and Mary, where each of them has the disease. Remember, the E represents the healthy copy of the gene, while the e represents the mutated copy.

As you can see, there is a 25% chance that Bob and Mary’s child will inherit two bad copies of the gene, and a 50% chance that the child will inherit one bad and one healthy copy. With a dominantly inherited disorder, any of these situations will result in disease (50% + 25% = 75%). There is a 25% chance that the child will inherit both good copies of the gene, in which case the child will not have the disease.