Lynn's Five Rules of Pedigree Analysis


In gerbils, there is a gene which influences fur color. This gene has two alleles: one produces brown fur, the other produces black fur. Investigation of this gene has revealed that the brown allele (which we will represent as B) is completely dominant to the black allele (b). These are the only known alleles for this gene.

Suppose you are asked the following question:

"Honey and Ritz are a mated pair of gerbils. Both Honey and Ritz are brown. In their first litter of babies, one of the babies was black. If Honey and Ritz were to produce a large number of babies, what fraction of those babies do you predict will be brown?"

In order to answer this question, you must be able to figure out what Honey's and Ritz's genotypes are. You can't figure out what kinds of babies they can have unless you can figure this out.

This is pedigree analysis. A pedigree is like a family tree — a genealogy. Geneticists are often faced with the task of figuring out where there are hidden alleles in a family history. The basic tools for this task are pretty simple. Five rules cover about all contingencies.

The first rule is pretty basic. When a new baby is conceived, it takes one sperm and one egg. Each carries one set of chromosomes, and thus one set of genes. This means that the baby receives one set of alleles from each parent.

Rule #1: Every gamete carries exactly one allele for every gene.

There is an exception to this rule because some genes are carried on the X or Y chromosomes, and sperm get one or the other, but not both:

Exception: Sex linked genes in males.

Now let's take a look at Honey and Ritz's little family tree:

How can we figure out what alleles these gerbils are carrying for this gene? Look at the obvious first. Obviously, Honey and Ritz both have at least one B allele. They wouldn't be brown if this weren't true.

What about the baby? Consider the meaning of dominance. Brown is completely dominant to black. This means that any gerbil who has a brown allele will be brown. Conclusion: This baby has no brown alleles. He has to be homozygous recessive.

Rule #2: Any individual with the recessive phenotype (appearance) must be homozygous recessive.

So now our pedigree looks like this:

Now what about the second allele for Honey and Ritz? Remember from Rule #1 that every egg contains one allele for every gene, as does every sperm. This means that our black baby gerbil inherited one of his b alleles from Honey and the other one from Ritz. If they gave them, they've got to have them, so both Honey and Ritz must be heterozygous.

Rule #3: Any individual with a homozygous recessive offspring must have at least one recessive allele.

And here is our final version of this pedigree:

Now you could go ahead and solve the original genetics question.

Here's another situation. More gerbils. This time, our gerbils are Nutmeg and Satin, another breeding pair. Nutmeg is brown, Satin is black. Their first litter of 5 babies are all brown.

Here they are:

Applying common sense and rule #2, we get:

So what about those babies? Can we figure out their second allele? Of course we can. Once again, we resort to Rule #1. Those babies got one allele from Nutmeg and one allele from Satin. Satin is bb, so he has only the recessive allele to give them.

Rule #4: Any individual with a homozygous recessive parent must have at least one recessive allele.

So here's where we stand:

Now what about Nutmeg? Can we figure out her second allele? The answer is…no. Not with the information we've been given. Sometimes you have to accept that you don't have enough data to determine a genotype.

Rule #5: Don't guess. If the information isn't available, don't guess.

Of course, I happen to know a lot more about Nutmeg than you do ;^) Nutmeg's parents were Smudge and Cinnamon. Smudge was black; Cinnamon was brown. So using rule #4, I can tell you that Nutmeg was a heterozygote. Here's this family tree with this information added:

Of course, now we don't know Cinnamon's second allele :^D

So here are our five rules:

Rule #1:

Every gamete carries exactly one allele for every gene.
Exception: Sex linked genes in males.

Rule #2: Any individual with the recessive phenotype (appearance) must be homozygous recessive.
Rule #3: Any individual with a homozygous recessive offspring must have at least one recessive allele.
Rule #4: Any individual with a homozygous recessive parent must have at least one recessive allele.
Rule #5: Don't guess

Want to know how to solve that original genetics question? Ah! That's another essay ;^)


Copyright © 2000 College of DuPage
Center for Independent Learning (630) 942-2185
fancher2@cdnet.cod.edu

Updated 25 September 2004