So I have a really awesome picture for you. Here it is! Oh it may look like circles and squares to you, but make no mistake, this is no ordinary picture.
This is a pedigree. A pedigree is like a family tree- it can show information about an inherited trait passed across generations. And this one is actually a small pedigree of us!
Well, our human forms. See, that's me. My sister.
My mom. My dad. I’m not arbitrarily picking random shapes to represent us either.
In a pedigree, the circles represent females. Squares represent males. One way you can remember that is that the letter “C” (for circle) comes before the letter “S” (for square) in the alphabet.
Alphabetically, the letter “F” (for female) comes before the letter “M” (for male). I’ve got a thing about letters of the alphabet though- I realize that may not work for everyone. So if we take a look at this pedigree, these roman numerals represent generations.
There's 2 generations here. This between my parents is called a marriage line. This line here connects parents to children so you can see there are two children from this marriage.
Now you may wonder, why are some of these shapes shaded? What does that mean? Well, the shaded shapes represent a trait that is being tracked in the pedigree.
Here are 2 important facts about the particular trait I am choosing to track. Fact #1 about this trait being tracked is that it’s recessive. Recall that in typical Mendelian inheritance, dominant alleles---if present---will express dominant traits.
Recessive alleles only are expressed when the dominant allele is not present. Fact #2 about this trait is that it is an autosomal recessive trait. Just a reminder that autosomal means a chromosome that is not a sex chromosome.
In human body cells, there are 46 chromosomes. The first 44 (22 pairs) are autosomes. The last 2 (1 pair) are sex chromosomes.
So this trait is not sex-linked since it is autosomal and that means it does not need to be written as coefficients (correction: EXPONENTS) on the sex chromosomes. So what is the trait we’re tracking? Attached earlobes!
Yes, you may not realize it, but look around and you'll see that humans may have free or attached earlobes. Although we want to point out that there may be more than just these two categories for ear lobes, and while this example is used often in basic genetics, there’s probably more to this than just one simple gene. For our example, let’s assume a one gene trait and that free earlobes is dominant, meaning at least one dominant allele must be around.
Attached earlobes is recessive, showing no dominant allele is present. So if we were to put the genotypes next to each of these shapes, what would they be? Well the shaded ones would be easy.
Because we just mentioned that attached earlobes is the trait we’re tracking and it is an autosomal recessive trait. So if we use the letter “e” then these shaded shapes must be lowercase e, lowercase e. Any capital (dominant) letter and the individual would have to have free earlobes and not be shaded.
So let’s look at individual #2 in the first generation. That’s our father. He’s not shaded so he can’t be little e little e.
What about big E, big E? Well there’s a problem. See his children?
Us, ha. Each child must get an allele from EACH parent. So if I received a little “e” from my mom, then I had to get my other “e” from my dad.
Therefore he can’t be big E big E or he’d have no little “e” to give! His genotype must be the heterozygote genotype, Ee. He’s what we call a carrier though he still has a phenotype of free earlobes because of that one capital.
But he carries the lowercase allele. Now that’s just one tiny pedigree. Let’s look at a big family reunion!
Um, well an imaginary one, because I have to confess I don’t really know whether our relatives have free or attached earlobes. I thought about sending a survey out to all of them, but. .
. it felt a little awkward. So here we go, big giant imaginary family of relatives!
Ok just to make sure you understand this---how many siblings does my dad have? Well look, here’s my dad in generation 2 (#4). He has three siblings, all brothers, right here.
What is the phenotype of my paternal grandfather? Well look, here’s my dad. Here is my dad’s dad---that would be my paternal grandfather.
And because his square is shaded---that means his phenotype is attached earlobes. Ok, so let’s go ahead and label all these shaded shapes with the genotype little e little e since we know that’s the trait we’re tracking. Now take a look at generation 1, individual 1.
That would be my paternal grandmother. What’s her genotype? We know it’s not little e little e or her shape would be shaded.
But if we went with EE could that still work? Yes, all the offspring could get a big E from her and a little e from my grandfather. But what about Ee.
Would that work too? Yes! Because the children could still get a big E from her and a little e from my grandfather.
It may be less of a probability, but it’s possible and therefore we must list both that she is EE OR Ee, because we don’t know. All the offspring of my paternal grandparents though are going to have to be Ee. Remember they have to get an allele from each parent and that means they're going to have to pick up that little “e” from my grandfather.
They will be heterozygotes and that’s the only option here. Pause this video and try to solve the right side of this pedigree now! Ta Da!
Imaginary family done! So how’d you do? Well, here are some of the tricky ones.
Did you see that generation 1, individual 4 has to be a carrier only (Ee)? Because if not, then the shaded individual children would not be able to get the “ee” that that they have, because they have to get a little “e” from both parents. How about individual 9?
This female married in, but that’s not the reason that she can be either EE or Ee. If you look at the children, they aren’t shaded. So while they will have to get a little “e” from number 8 as that’s all #8 can give…the other capital letter can be obtained from #9 regardless of whether she’s EE or Ee.
Remember, one option may be more likely, but if it’s possible, you need to include both. Now remember we had made a big deal about how this was autosomal pedigree? Well, what if you are dealing with a sex-linked trait and therefore a sex-linked pedigree?
There are a lot of sex-linked recessive traits. Color-blindness and some male patterns of baldness can be sex-linked. Let’s pretend now, we are told this is a sex-linked recessive trait.
I’m going to keep the old labeled pedigree here that showed an autosomal recessive trait just for comparison, but now here is a brand new sex-linked pedigree. First of all, all the females (the circles) should have an XX to indicate two X sex chromosomes by them. Remember females have two X chromosomes.
Males should have an XY to indicate an X and a Y sex chromosome. That's always a good thing to do first. Now remember that we were told this pedigree is tracking sex-linked recessive traits.
So the shaded one here has the sex-linked recessive trait. A reminder from our sex-linked video about how this works. Let's use the letter "R" for an allele.
Now recall that females that do not have the trait can be either this or this. And the heterozygote genotype (this) is a carrier. She doesn’t have the trait herself because of the dominant allele but she’s carrying it.
Only a female that is this will have the sex-linked recessive trait. So if I’m looking at this pedigree, what would the genotype for individual 1 in generation 1 be? Well notice here she has 3 children and one of her sons here is shaded.
Where does her son get his Y sex chromosome from? The father. Where does he get his X sex chromosome from?
His mother! Individual 1 doesn’t have the trait or she would be shaded, but she must be a carrier if her son received a X chromosome with a recessive allele on it. Now what about individual 2 in generation 2?
Well she can be this like her mother. But look, she could also be this because it’s possible to get one of those from each parent. If it’s possible, you must include it.
Now pause the video and try to solve the last female. How’d you do? Remember the key here is to always check and make sure that when you look at a child---they have to be able to get one of their alleles from EACH parent.
Now remember that both of these examples were recessive. It doesn’t have to be that way. On our handout, you can try one out that follows an autosomal dominant trait.
If it’s a dominant allele that you are tracking, remember it would only take ONE dominant allele for a person to have that trait. Another quick thing to point is sometimes you will see pedigrees that are half shaded. Well that’s just awesome because they're basically letting you know that the half-shaded ones are carriers.
If I wanted to turn our first one into that - that half-shading - then it would look like this. Mapping and understanding pedigrees is important, especially as we continue to make advancements in understanding how genetic disorders are inherited. Well that’s it for the Amoeba Sisters, and we remind you to stay curious!
