So what is a pedigree? This is a map of human matings. Generally, geneticists create pedigrees because someone comes to them with a disorder, and they want to understand what is happening. They question if it is genetic. The person with the concern is referred to with a specialized term, Propositus. I hope my pronunciation is correct; please forgive me if it's not. They are the first family member who approaches the geneticist to examine their family because they suspect something might be wrong. A pedigree is a map, and to understand it, one must know the symbols used.
I've provided an example or a cheat sheet here with different symbols. We have symbols for males, females, matings, parents, and children, including twins. These twins are non-identical because they are male and female. Identical twins are usually indicated with a special notation, which you typically won't deal with in most cases. Notice that an affected male is marked in black, the same applies to females. Sometimes, you might see half-moon symbols; these represent heterozygotes for recessive genes, meaning they are carriers. A cross through a symbol indicates death, and generally, an arrow points to the propositus, the first person who came with the concern.
When analyzing pedigrees, it's vital to differentiate between mating and affected versus non-affected individuals. Most of the pedigrees you'll encounter are relatively simple. I'm going to show you some examples. You will need to understand and memorize these symbols because, in genetics, during your tests and quizzes, you will likely be given a pedigree and asked to identify the inheritance pattern.
What do I mean by inheritance pattern? It involves determining if the genetic trait is sex-linked, dominant, or recessive. I will go through an example of each to hopefully clarify this for you. You will be presented with a pedigree and asked about the type of inheritance it represents—whether it's dominant, recessive, or sex-linked. I'm here to help you figure that out. Let's turn the page.