Pedigree project

What we're doing and why

Gregor Mendel did not know about DNA. He did not know about chromosomes. He did not know what genes were. He grew thousands of pea plants. He grew them for many years. He looked at every generation. Slowly, he found a pattern in the data.

One pea plant could not show him the pattern. One generation could not show him the pattern. He only saw the pattern because he had so much data.

We will do something like Mendel did. But each of you has only one family. One family is a small amount of data. Your pedigree alone might not show a pattern. The whole class together has much more data. You will compare with a partner. Some patterns will match. Some will not. That is normal. That is how science works.

We will also try a guess. The guess is about how hair color passes from parents to kids. We will use a tool called a Punnett square to make predictions. The guess will work for some families. It will not work for others. Real hair color is more complicated than our guess. When a guess does not work, we learn something new. That is how science grows.

Pick a trait that varies in your family

Before you start, look at your family. Pick a trait where you see different versions. If everyone in your family has the same hair color, hair color will not show a pattern for you — pick something else.

Here are four traits you can choose from. Pick one:

These groups are not perfect. Mendel did the same thing. He sorted pea seeds as "yellow" or "green." He knew some seeds were in between. He picked one group anyway. Rough groups help you find patterns. Rough groups also hide some real biology. Both things are true.

Pedigree symbols

Color, shade, or label each shape with the right group for your trait. If you do not know someone's trait, leave the shape dashed and empty.

How to do this

1. List your relatives. Pick people you know well. Start with yourself, your siblings, and your parents. Then add more people. Try to get at least 8 people in 2 generations. (You can use Throckmorton's family instead.)
2. Pick a group for each relative based on your trait. Unknown is real data, not a mistake.
3. Draw your pedigree on paper. Put yourself at the bottom. Parents above you. Grandparents above them. Use squares for males. Use circles for females. Color or shade each shape based on the group.
4. Look at your own pedigree first. Does any color appear more on one side? Does any color skip a generation? What do you notice?
5. Turn and talk with a partner who picked the same trait as you. Show each other your pedigrees. Do you see the same patterns? Different patterns? If different, why? Write down what you both find. Do not try to make your families match each other.
6. Test the guess with a Punnett square. Pick one cross from your pedigree. (Usually your parents work well.) Use the guess to predict what their kids should look like. Compare your prediction to the real kids. Note where the guess works. Note where it does not.

Worked example: Morgan's family

Here is what a finished pedigree looks like. Morgan picked hair color. She tracked her family across three generations. (Your trait might be different — that is fine. The same kind of work applies.) Notice how she handles unknowns. Notice the pattern of colors.

What Morgan noticed

• Red hair shows up on Mom's side: Grandma Lisa, Mom, and brother Theo all have red.
• Blond shows up on both sides: Grandpa Robert, Aunt Beth, and Morgan herself.
• Brown shows up on Dad's side: both grandparents, Dad, Uncle Tom, and sister Lila.
• The kids in Morgan's generation (Theo, Morgan, Lila) are all different colors — even though they have the same parents.
• Morgan herself is blond, but neither Mom (red) nor Dad (brown) is blond. Her blond came from somewhere — both parents must be carrying a hidden blond allele.
• Two relatives are marked unknown — Mom's brother (Morgan has never met him) and Aunt Pat (deceased before Morgan was born). The dashed shape says "we don't have data," not "we did it wrong." Real pedigrees almost always have gaps like these.

This is the kind of thinking you will do for your own family. Look at who has what. Then ask: "Where could that allele come from?"

If you can't use your own family: Throckmorton's data

Some people cannot use their own family for this project. There are many reasons. Maybe you do not know your biological relatives. Maybe your family history is private. Maybe the people who could tell you are not around. Use Throckmorton's family below instead. The work is the same.

Throckmorton's data has missing pieces — like a real student's data would. Not every relative is known. That is normal.

Punnett squares — making a prediction

A pedigree shows you what really happened. A Punnett square shows you what a model says should happen. If they match, the model is working. If they do not match, the model might be wrong. Or the data might be missing pieces. Or both.

Here is the guess we will try: maybe hair color comes from one gene with a few versions. Each version is called an allele. Each person has two alleles — one from their mom, one from their dad. We use letters to label them.

This guess is probably too simple. Real hair color comes from many genes, not just one. But it is a starting point. Mendel started with simple guesses too. Let us see how well our guess works.

If the guess is right, the rules look like this:

Black wins over everything. If you have even one B allele, your hair looks black. Someone with black hair could be B/B, B/Br, B/R, or B/b. All four look black on the outside.

Brown and red/orange/ginger are equal. If you have one of each (Br/R), you look like a mix — red brown. Both colors show.

Blond hides behind the others. To look blond, you need two blond alleles (b/b).

Someone with brown hair might be Br/Br (two browns) or Br/b (one brown, one hidden blond). They look the same on the outside. They are different on the inside. If our guess is right.

Our guess is probably not 100% right. Real hair color comes from many genes. Some of you will find your family fits the guess. Others will not. Both answers are real data.

Morgan's worked Punnett square

Morgan looked at her pedigree. She figured out her parents' alleles:

• Mom Sara is red. She must be R/b. Her dad Robert is blond (b/b), so he gave her a b. She got an R from her mom Lisa.
• Dad Marcus is brown. He must be Br/b. Morgan is blond, so Marcus had to give her a b. He looks brown, so his other allele is Br.

So the cross is R/b × Br/b. The Punnett square fills in like this:

Prediction vs reality

The Punnett predicts that each kid Sara and Marcus have will be:

• 50% red / orange / ginger (R/Br or R/b — both show red shades)
• 25% brown (Br/b)
• 25% blond (b/b)

Morgan's parents had three kids: Theo (red), Lila (brown), Morgan (blond). One of each. With three kids, the prediction said about 1.5 red, 0.75 brown, 0.75 blond. The real answer was 1, 1, 1. Close, but not perfect.

This is normal. A Punnett square shows chances, not promises. Mendel grew thousands of pea plants for each cross. Only then did the numbers settle. Three kids is a very small sample.

Why the Punnett works: gametes are random

A Punnett square treats each parent's alleles like a coin flip. There is a 50% chance of passing one allele. A 50% chance of passing the other. Why 50/50? Because that is what your body does when it makes a sperm or egg cell.

You have two copies of every gene. One from your mom. One from your dad. When your body makes a gamete (sperm or egg), only one copy goes in. Which one? Random. Two things make it random:

Independent assortment

Different chromosomes are sorted separately. Like cards being shuffled. The hair-color allele from your mom does not have to go with the eye-color allele from your mom. Each gene's pair splits on its own. Different genes can go in different gametes.

Crossing over

Sometimes alleles on the same chromosome split up too. During meiosis (the cell division that makes gametes), chromosomes swap pieces with each other. So the chromosome you give to your child is part from your mom and part from your dad. A brand new mix, every time.

This is why your siblings do not look exactly like you. Every egg and every sperm is a fresh random combination. From independent assortment alone, one person can make over 8 million different gametes. Crossing over makes that number even bigger.

So when a Punnett square says "50% chance of getting the brown allele," that 50% is real. It is a real coin flip. It happens inside your cells, every time a gamete is made.

Your Punnett square

For your own pedigree, do the same kind of work. Pick one cross from your pedigree. Usually your parents work well. (Or pick one set of grandparents.) Then try to:

1. Figure out each parent's alleles. Use what they look like, and what their kids look like.
2. Build the 2×2 Punnett square. Show the four possible kids.
3. Compare the prediction to the real kids. Note what matches. Note what does not.

If you cannot figure out a parent's alleles exactly, that is OK. We could not figure out Morgan's grandparents Anita and David either. "We don't know" is a real scientific answer. Make your best guess. Explain why you picked that guess.

When patterns don't agree

You and your partner might see different patterns. Maybe one of you sees blond skip a generation. Maybe the other does not. Do not try to "fix" the difference. The disagreement is data too.

Why might you see different patterns? A few reasons:

• The guess we are using might be too simple.
• One of you might have less family data than the other.
• The pattern might be real but rare. One family might show it. Another might not.

Mendel saw this kind of disagreement all the time. That is why he grew thousands of pea plants. One generation alone could not tell him the truth. Many generations together could.

Our class is doing something similar — but smaller. Each of you brings one family. You compare with one partner. Then we bring it all together as a class. The full class data tells us more than any one family can.

What you'll turn in

• Your hand-drawn pedigree (your family or Throckmorton's).
• A short list of patterns you noticed in your own pedigree (3–5 sentences).
• A note on what your partner saw — what matched, what did not (2–3 sentences).
• One Punnett square testing a prediction for one cross in your pedigree.
• A short note on what the guess got right and what it got wrong (2–3 sentences).

You do not need to be right. You need to show your thinking. Be honest about what does not fit. The mismatches are the most important part. That is how real science works.