The Spread of Inherited vs. Infectious Diseases
https://www.illumina.com/science/education/genomics-101/genes-inherited-diseases.html
This article discusses the science of genetically inherited diseases, including the difference between how recessive and dominant diseases are spread. In class, we learned about epidemics from a contagious or infectious perspective, where diseases are spread through contact with infected people. If we model the spread of genetically inherited diseases as a network using the branching process as described in class, we will see that there are both commonalities and differences between the model of infectious diseases.
Using the branching process, we can model the inheritance of diseases as a set of nodes with directed edges from a set of two parents to their biological children. Just like infectious diseases, a person with the disease is not guaranteed to pass it on to their child. In class, we indicated that an infected person will transmit each person they are in contact with with probability p, independently. The spread of genetic diseases is slightly more complicated, because a person does not have a fixed probability p of getting the disease from a diseased person they are linked to. This probability of transmission varies depending on the genes of both parents, so each “family,” or children of a given set of parents, has a unique p value indicating the chance that the parents transmits the disease to each child. As with infectious diseases, a child node inherits the disease independently of its sibling nodes.
As described in the article, inherited diseases can be dominantly or recessively (among others) inherited. In the case of dominantly inherited diseases, a child gets it when they inherit at least one affected gene, from either parent. As a result, a child with no diseased parents has a 0% chance of inheriting an affected gene, so a 0% chance of becoming diseased. This is similar to infectious diseases; a person who does not contract an infectious disease can be “ignored” in the subsequent generations because they cannot transmit it to anyone. Likewise, a person who does not have a disease that is dominantly inherited cannot pass down any affected genes, so their children’s inheritance only depends on the other parent. If a child has at least one diseased parent, their probability of inheritance is greater than 0 and varies depending on the genotype of both of their parents; the probability p is not fixed throughout the whole graph, but it is fixed for each set of parent’s genotypes.
The major difference between the spread of infectious and inherited diseases is that the disease can appear to “skip” generations, when the disease is recessively inherited. A person gets the disease when they inherit one affected gene from each parent; a person who inherits only one affected gene is a “carrier,” who does not have the disease but can pass down their affected gene to their children. Just like dominantly inherited diseases, there are several variations of the probability p of transmission between two parents and each child. For example, if both parents are carriers, meaning they each have one normal and one affected gene, each of their children has a 25% chance of getting the disease, 50% of becoming a carrier, and a 25% of inheriting no affected genes. If both parents have the disease, each of their children has a 100% chance of getting it. Unlike infectious or dominantly inherited diseases, a non-affected person cannot be ignored in the subsequent generations; if a person is a carrier, has two carrier parents, and has children with a carrier co-parent, their children can still get the disease by inheriting two affected genes. Thus, in the branching process model, it will appear as though the disease “disappeared” for two generations. This characteristic is unique to recessively inherited diseases; for infectious diseases, once an entire wave of people are uninfected the disease can no longer be transmitted to anyone.
As shown, the branching process – although a simplified model – is highly effective in highlighting the core similarities and differences between the spread of infectious and inherited diseases throughout generations.