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What Are Pedigrees?
What Are Pedigrees?
Pedigrees represent a family tree, but more importantly, they include a particular trait. Every pedigree shows a lineage (family of parents and descendants) and the prevalence of a given trait (which individuals have it).
Another way to look at pedigrees is that they are a quick way to observe patterns in the phenotype of a family in order to assess how that trait is passed on.
In this pedigree, you can see that the males are represented by squares and females by circles. Individuals who have children together are linked by a horizontal line with the children following underneath. The separate rows, therefore, represent different generations of the family.
If an individual is shaded in, that means that they are 'affected', or have the trait in question. So, if for this pedigree, we are looking at a disease, the father from generation had the disease, as well as 2 individuals in both generations II and III. Sometimes individuals will have their names underneath their symbol as well.
Assigning Genotypes to Pedigrees
Assigning Genotypes to Pedigrees
The purpose of a pedigree is to visually represent data about the given family so that an individual can begin to notice some patterns of inheritance. These patterns will allow you to assign genotypes to individuals when all you know is their phenotype. In other words, it allows you to figure out if individuals are heterozygous, homozygous dominant, or homozygous recessive for the given trait. This is incredibly useful for real-world situations because how often do you have the list of genetic alleles for somebody?
When presented with a pedigree, the first thing you should do is make observations about the inheritance patterns. For instance, with this pedigree, I can see that both male and female individuals are affected. I also see that no one was affected in generation I, but the trait showed up in generation II with their offspring.
This should strike you as odd initially... How could a trait show up if it wasn't present in the parents? Well, that's easy, the trait is recessive!
If a trait is recessive, that means that an individual must have two copies of the allele to have the affected phenotype. So, all individuals who are shaded in (assuming it is a recessive trait) will be homozygous recessive. So, they have a genotype that looks like ff, for instance. If the trait was dominant, the only way it could have appeared in generation II is if one of the parents had the allele in generation I. But if they had the allele, they'd have the trait. So this pedigree shows a trait that cannot be dominant. Assigning genotypes to a pedigree is a lot like a logic puzzle game and can actually be fun!
Let's take a look at another pedigree. This time, we know some genotypes, so let's just investigate what we can see to ensure that we understand what is occurring. In this pedigree, the female in generation I is homozygous recessive and is showing the trait. However, none of her offspring show the trait. How is this possible? Well, if the trait is recessive (which we know it is given her genotype of ff), then the offspring all need two recessive alleles to have the trait. Mom will always pass a recessive allele. But dad is not affected by this trait. This means he is not ff. He could be FF or he could be Ff. We actually don't know for sure, so his genotype is labeled as F?. Note that the mother is always going to pass that recessive allele because that is all that she has. It is like she has a coin with both sides containing tails. Because none of her offspring are affected, we know they HAVE to be heterozygotes (Ff) given that mom gave them f.
Now let's take another look at that same pedigree, this time focusing on generation III. In generation III, we can be certain of the male's genotype as ff because he is affected. The other children in this generation, however, are mysteries. They are unaffected, so could be either Ff or FF.
Their parents in generation II MUST be heterozygotes, however. How can you tell? Well, they have a child who is affected (ff). This means that both mom and dad had to have an f to give. So right off the bat, the parents in generation II can only be Ff or ff. But let's look at their phenotypes. They are unaffected, so can only be Ff. They MUST be heterozygotes by the process of elimination.
You don't need to be experts in this right now, we are going to practice quite a bit in class. Sometimes you won't be able to say with 100% certainty what everyone's genotype is, but you should be able to narrow down some options.
An Unusual Pattern...
An Unusual Pattern...
I have one last pedigree for you to look at on this page. Take a look at this monstrosity of a pedigree showing the trait hemophilia (a disease that prevents the clotting of blood, so even a papercut will never heal). What are some patterns you notice in the affected individuals?
. . .
That's right - they are all males! What is going on here? Well, to figure that out, we'll have to go beyond what Mendel could have imagined. It turns out that genetics is far more complicated than he knew, so let's move on to non-Mendelian genetics next.
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