Pedigrees - Online Tutor, Practice Problems & Exam Prep

Understanding pedigrees is essential in genetics, as they illustrate inheritance patterns of traits and disorders. Key types include autosomal recessive, where affected individuals can arise from unaffected parents, and autosomal dominant, where the trait appears in every generation. Sex-linked inheritance, particularly X-linked recessive, often affects more males than females. A flowchart can aid in identifying these patterns by guiding through questions about affected individuals and their parents. Recognizing these distinctions is crucial for analyzing genetic disorders and predicting inheritance in families.

Pedigree Symbols

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.

Autosomal Pedigrees

Okay. So now we're going to go over various autosomal pedigrees. So autosomal means anything that is not on a sex chromosome. So autosomal mutations or autosomal alleles, they're all going to be on chromosomes that are not sex chromosomes. So they're going to be inherited in an autosomal fashion. So the first type of autosomal inheritance that we're going to talk about is autosomal recessive disorders. So autosomal means not on a sex chromosome, and recessive means that in order to have the phenotype, you're going to need 2 recessive alleles.

Here's an example of an autosomal recessive pedigree, and I'll disappear so you can see it. And when you look at this pedigree, I want you to be able to identify that this is autosomal recessive. So the way to do that, I've given you a few hints here. The first thing is you want to look at how many children come from affected or unaffected parents. In autosomal recessive disorders, what you're going to see is that there are affected individuals, so fully recessive individuals that appear in the offspring of unaffected parents. Right? So they at least have one dominant allele. And so, what we see here is that if we look at some unaffected parents, we can see that offspring are produced from unaffected parents. Right? Again, this happens again down here, where we have an affected individual and 2 unaffected parents. That's the first thing that starts suggesting this might be recessive.

The second thing that shows us that it's probably autosomal is that the affected offspring occur in both males and females. If you start seeing more males or more females, we start thinking that this might actually be a sex-linked inheritance. But if it's pretty much males and females equally, then it is likely to be autosomal. So if we look here, we can see that we have both females and males that are affected. And although there's not a lot of affected, so it's hard to tell whether it's equal, we still can see that there are both males and females are affected, so it's likely autosomal.

The third thing that suggests recessive is that there are only a few affected offspring. So looking at all of the people in this one family, there are only 3 out of several that actually have the disease, and so that's suggesting recessive. So autosomal recessive disorders, they're going to have affected individuals that show up in unaffected parents. It's going to be males and females almost equally, and there's only really going to be a few that show up, in an entire family.

Now, let's go and move on to autosomal dominant. Now, autosomal dominant is going to be a disorder that's caused from a mutation, on an autosome, so not a sex chromosome, and it's going to be a dominant allele. So it's going to be an uppercase letter. Now, how we can tell this is through these little cheat sheets here. The first thing is that the phenotype is going to appear in every generation. This suggests dominance. Right? Because anytime you get one dominant allele, that person is going to express the phenotype. It's going to have the disease. Whereas, in autosomal recessive, you actually have to have 2 recessive alleles in order to have that phenotype. So, dominance in a family is going to appear much more often than a recessive.

Now, if we have an affected parent, we're going to have affected children. If you have more than 1 child, you're going to start seeing affected children. So affected parents are going to pass it to their children, and we're also going to see it equally in males and females, which is not written here, but we're saying males are equal to females. This suggests autosomal. And then one really unique part of this is because when we think dominant, we always think common. Right? We say something, oh, that trait must be dominant because everyone has brown hair. It's sort of this ingrained thing that you didn't actually learn, but you just think it's like that. But in autosomal dominant disorders, it's actually different because autosomal dominant disorders are rare.

Now you may ask, how in the world is a dominant disorder rare? Well, it is if you have the uppercase r allele, the uppercase dominant allele, then you're going to express that. But this allele can be really rare in the population. So if you look at one individual family with this allele, you're going to see that a lot of the people in that family are affected because it's a dominant disorder. But, if you look at 20 families or 100 families or an entire population of families, you're going to find that that allele is not present. It's very rare. It's very rarely present in any family. But if it is present, then you're going to see it a lot. So, autosomal dominant disorders are dominant, but they're actually rare in the population as a whole.

So here, if we're looking at an autosomal dominant, what we can see is that affected parents have affected children. We can also see that it's pretty equally distributed among males and females. There are 3 males and 3 females here. We can see, we actually can't see that it's rare in the population because if we look here, we're just looking at one family and it looks really common. But, usually, in questions like this, they'll say, oh, this extremely rare disease shows up in this family. And, if it were really that rare and autosomal recessive, we would only see it in a couple of people in the family. But, if it's rare and dominant, then we see it looks like this.

And that's an important distinction to make because there's a third type of pedigree I want you to know about, and this is called autosomal polymorphism. Now, polymorphism means that there are 2 or more common phenotypes of a trait. So a good example of this is tasters or supertasters. There are people in the world that have more sensitive taste buds than others, and it's actually fairly evenly divided. And you can test this through this little test where you put a piece of paper on your tongue, and if you taste it, you're a supertaster, and if you don't taste it, then you're just a normal taster. Now, polymorphisms, it's not a mutation, it's not a disease. It's just a phenotype. Some people taste it, some people don't, and that's okay. And these traits are inherited in a normal Mendelian manner, and there's a lot of different ones. There's attached and free earlobes, there are widow's peaks, and this is the super tasters. They're just really common traits that you can have or you don't have it. It's not a big deal. It's not a disease. But autosomal polymorphisms are really common and they look really similar to an autosomal dominant pedigree.
So if we are looking here, this is a pedigree for supertasters, where if you don't taste it, you're recessive, recessive, so and you're black. And if you do taste it, you're in white, and you're dominant. And we can see that if we look here, we have an equal, about equal number of males and females. We see that it's in every generation. So, automatically looking at this, I'm thinking autosomal dominant. But the way that I can tell the difference between autosomal polymorphism and autosomal dominant is that in the question you're given, an autosomal dominant disease is going to be rare in the population. So autosomal dominant equals rare in population, but common in a family. Because here we see it's very common in this family. It's pretty much in every generation, but it's rare in the population as a whole. An autosomal polymorphism is going to be different because it's going to be common in the population and common in the family.

Now, most of the time, you won't get a question about telling the difference between an autosomal dominant and an autosomal polymorphism. This would be a fairly advanced question, but I did want to include it just in case your professor decides to go over it. But most of the time, what you'll be comparing is is this an autosomal recessive or is this an autosomal dominant. And the way that you can tell that is looking through those cheat sheets that I provided above and will provide a really great flowchart at the end of this so that you can easily tell apart which pedigree is which. So those are the autosomal pedigrees. With that, let's move on.

Sex-Linked Pedigrees

Okay. So now we're going to talk about sex-linked inheritance. Sex-linked inheritance means that the disease allele, the mutant allele, is going to be on a sex chromosome, so an X or Y. And that means that when you're looking at a sex-linked inheritance, the first cue that it's a sex-linked inheritance pedigree is that males and females are not represented equally. There's either more males affected or more females affected. Usually, it's more males affected. The reason for that is because males only get one X chromosome. So, let me show you why.

The first one we are going to talk about is an X-linked recessive. So, X-linked means that the mutation or the allele is going to be on the X chromosome, and recessive means that you either need two X chromosomes with a recessive allele to see it, or one X chromosome and a Y chromosome for a male to see it. Okay? Now, X-linked recessive disorders are going to be much more common in males than females. Why? Because the male only needs one allele. Right? It only needs that one X chromosome allele to have the disease. Whereas a female to actually express it has to have two mutant X chromosomes. Right? So, it's much more common for a male to get it because they only need it.

Let's look at a pedigree and look at some of the cues that will tell you that this is an X-linked recessive disorder. The first thing is more males are affected, which I just explained why. It's because males only need the one X, whereas females need two X's. The second one is that an affected male parent will usually not have affected offspring. The reason is that if you have a mutant male, most of the time, females hardly ever get this. So, most of the time in these crosses, you're mating with a homozygous recessive female. And what's going to happen is that all the sons will get the X from their mother, and that's not going to be affected. And all the daughters will get one X from their mother and one X from their father. But because it's recessive, they're also not going to be affected because they're going to be heterozygous. So, an affected male parent will not have affected offspring if they're mating with a homozygous recessive mother. This is most often the case.

Now, this statement does not hold true if you're doing this cross. Right? If you're crossing an affected male parent with a carrier mother, right, with the one recessive allele, because there is a chance to produce this offspring and this offspring. Okay. But this is very rare. Most of the time, this doesn't happen. So, this is going to be the case most of the time, which is why I said an affected male parent will not have affected offspring, but all the daughters will be carriers. So, what you'll see is that when the daughters are carriers, then in the second generation what you see is this. Because we have an affected daughter mating with another affected male, and that produces affected offspring. So, I'm going to show you an example of this.

Another clue is that the sons of affected males do not pass it to their offspring. An example of this is that the mutant allele here. Now, most of the time you're going to mate with a homozygous mother, who is not a carrier. Carriers are fairly rare.

An affected male is going to pass it to all daughters, but no sons, which is what we see. A heterozygous female is going to pass it to half of the offspring. So by looking at the generations and taking males and female differently, we can easily break apart an X-linked dominant disorder. Now, X-linked disorders are more complicated, but stay with me and stay with me to that especially to that flowchart, because the flowchart is really going to help you be able to narrow down what these things look like. And then the last one is a Y-linked disorder, so this is going to be a mutation on the Y chromosome. And what you can see is that only males inherit it. So if you see a pedigree where only males are having the disease, no females at all, and it seems to be fairly common, every male gets it essentially. Right? It's going to be passed on with every male in the family getting it. That's going to be a Y-linked disorder. This is very rare. It's going to be unlikely for you to get a question on a Y-linked disorder on a quiz or a test. Usually, they are overlooked because they are so extremely rare. You are really never going to see it.

Okay. So those are the sex-linked disorders, including X-linked dominant, X-linked recessive, and Y-linked. With that, let's turn the page, get to that flowchart, and help you understand how to differentiate all these pedigrees if you're looking at a pedigree on a quiz or a test. Okay. So with that, let's move on.

Pedigree Flowchart

Okay. So now, I have made this pedigree flowchart, and I want to walk you through it because I think it's going to be helpful. So often, you're going to get a pedigree on a test or quiz, and you're going to ask, "What type of inheritance pattern is this?" Now, I have given you some ways to figure this out, but that can be really difficult to memorize and answer in a test or quiz, not a quest. Even though it can sometimes feel like a quest to get that grade. So, I made this flowchart, and you can just answer these questions about the pedigree you're seeing, and it'll lead you to the appropriate inheritance. So the first question it says, "Do all affected individuals have an affected parent? Yes or no?" If it's yes, you go on to answer this question. If it's no, you go on to answer this question. And you repeat this. So here, we'll follow this one. "Does this trait affect mostly males? Yes or no?" If no, it's autosomal recessive. If yes, you answer this. And the same thing over here. "Do all affected males have an affected mother? Yes or no?" Yes. "Does the affected father produce daughters who are all affected?" Yes. And you keep answering these questions, and eventually, you will get to your answer, what type of pedigree is this? So I suggest, if you have some problems in the back of the book, or if you are studying for a quiz, or a test, or even just pedigrees that you may see in your textbook, or even the pedigrees that I showed you above, just take one and walk through this pedigree flowchart and see if you can figure out what sort of inheritance it is. And that way, if you can't really remember all the individual characteristics of each pedigree, if you can remember this flowchart, then that will really help you get those questions right and understand how to look at a pedigree and know which inheritance pattern it is. So hopefully this flowchart will help you, but with that, let's now move on.

This pedigree exhibits which of the following inheritance patterns?  

• A. Autosomal Dominant
• B. Autosomal Recessive
• C. X-linked Dominant
• D. Autosomal Polymorphism

This pedigree exhibits which of the following inheritance patterns? 

• A. Y-Linked
• B. Autosomal Recessive
• C. X-linked Dominant
• D. Autosomal Polymorphism

This pedigree exhibits which of the following inheritance patterns? 

• A. Autosomal Dominant
• B. X-linked Recessive Recessive
• C. X-linked Dominant
• D. Autosomal Polymorphism

Here’s what students ask on this topic:

What is a pedigree in genetics?

A pedigree in genetics is a diagram that represents the familial relationships and the inheritance of specific traits or disorders. It uses standardized symbols to denote males, females, affected individuals, carriers, and various other familial connections. Geneticists use pedigrees to track the inheritance patterns of traits and disorders across generations, helping to determine whether a condition is autosomal dominant, autosomal recessive, or sex-linked. Understanding pedigrees is crucial for diagnosing genetic conditions and predicting their occurrence in future generations.

How can you identify an autosomal recessive disorder in a pedigree?

To identify an autosomal recessive disorder in a pedigree, look for the following clues: affected individuals often appear in the offspring of unaffected parents, indicating that both parents are carriers. The disorder affects males and females equally. Additionally, the number of affected individuals is usually low within the family. These characteristics suggest that two copies of the recessive allele are necessary for the phenotype to be expressed.

What are the key differences between autosomal dominant and autosomal recessive inheritance?

Autosomal dominant inheritance requires only one copy of the dominant allele for the trait to be expressed, resulting in the phenotype appearing in every generation. Affected individuals have at least one affected parent. In contrast, autosomal recessive inheritance requires two copies of the recessive allele for the trait to be expressed. Affected individuals can appear in the offspring of unaffected parents, and the trait does not necessarily appear in every generation. Both males and females are equally affected in both types of inheritance.

How do you distinguish between X-linked recessive and X-linked dominant disorders in a pedigree?

In X-linked recessive disorders, more males are affected than females because males only need one copy of the mutant allele on the X chromosome to express the trait. Affected males do not pass the disorder to their sons but can pass it to all daughters, who become carriers. In X-linked dominant disorders, affected males pass the condition to all daughters but no sons. Heterozygous females pass the condition to half of their offspring, regardless of sex. The trait appears in every generation in X-linked dominant disorders.

What is the significance of a flowchart in analyzing pedigrees?

A flowchart is a valuable tool for analyzing pedigrees as it provides a step-by-step guide to determine the type of inheritance pattern. By answering a series of questions about the affected individuals and their parents, the flowchart helps narrow down whether the trait is autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive. This systematic approach simplifies the process of interpreting complex pedigrees, making it easier for students to identify the correct inheritance pattern.