Create a set of baggies for the class, using pipecleaners and beads. You'll need lots of white pipe cleaners to represent the autosomes, then pink and blue to match the sex chromosomes. Bead colors can vary, though I tried to get them to match the trait. It's a little work to put together, but I use the same bags year after year. You can mix and match beads by what you can find in the store. I had trouble finding brown beads, which I would have preferred to use for the eye genes (instead of blue and grey). I store the pipe cleaners in plastic ziplock bags to be used the next year.
Each male bag will have
1 shorter white chromosome -- ivory bead (blonde hair recessive)
1 Long white chromosome -- blue bead (blue eye recessive)
1 pink chromosome (pipecleaner)-- purple bead (normal blood)
Genotype: Dd Bb H
Each female bag will have:
1 Long white chromosome -- ivory bead (blond hair)
1 shorter white chromosome -- blue bead (blue eye recessive)
1 pink chromosome -- purple bead (normal)
Genotype: dd Bb Hh
1. What do the pipe cleaners represent? _______chromosomes___________________
2. What do the beads represent? __alleles or genes______
3. Humans have ____23_____ pairs of chromosomes.
4. If you have two pink pipecleaners, you are playing the role of ____female__________
5. The blue pipecleaner represents the ___Y__ chromosome.
What color eyes does the mom have __brown_ What is her genotype? Bb
What color hair does mom have ? blonde What is her genotype? dd
Hemophilia (sex chromosomes, colored pipe cleaners)
The purple bead represents the dominant gene -- normal
The clear bead represents the recessive gene -- hemophiliac
In girls: HH = normal | Hh = normal (carrier) | hh = hemophiliac
In boys: H = normal | h = hemophiliac
What is mom's genotype? __Hh_____Is she a carrier? ____yes____
What is dad's genotype? __h__ Why doesn't dad get two alleles for this trait?_Y chromosome doesn't have allele
Each cross will be different. The first cross is just to illustrate how to do the crosses. Once students understand how to do that, the data table is easy to fill out and will go fast. You should circulate in class to help students figure out this part.
Male Genotype: Dd Bb H
Dark hair, brown eyes, normal blood
Female Genotype: dd Bb Hh
Blond hair, brown eyes, carrier for hemophilia
Using punnet squares for each cross above, it should be noted that you will never see a female child with hemophilia. If this is seen, chances are, students got their bags mixed up during the crosses. This is the most common error, students will not put the proper chromosomes back to the correct parent, thus making the data inaccurate. The percentages you see (at compilation of data) should be close to:
50% dark hair, 50% blond hair
75% brown eyes, 25% blue eyes
0% hemophiliac girls
50% hemophiliac boys
Watch this video of how you would sort and then choose from each set of chromosomes. You may want to demonstrate this to the class beforehand.
---On a separate page, answer the following.
1. Create a punnet square for each of the crosses, using your parents. (You'll have a square for hair color, eye color, and hemophilia)
Bb x Bb, offspring will be 3/4 brown eyes, and 1/4 blue eyes
Dd x dd, ofspring will be 1/2 dark hair, 1/2 blonde hair
Hh x H
Female offspring will be 1/2 normal, 1/2 carriers
Male offspring will be 1/2 normal, 1/2 hemophiliacs
2. Explain why women are carrier's for the disease hemophilia. Why do their sons, but not their daughters get the disease? Females can be carriers and will donate one of the alleles for blood proteins to their sons. Males will either receive the normal allele or the abnormal (hemophilia). Females will also receive one of these alleles but will receive another X chromosome from their father which will be normal (assuming dad does not have hemophilia).
3. Describe the difference between how normal traits are inherited and how sex linked traits are inherited. Sex linked traits are inherited on the X chromosomes. Males will only receive one allele (which could be abnormal) but females will receive two alleles and have an opportunity to inherit a normal allele from their fathers.
4. The data table where all the data is combined, shows how many ACTUAL offspring would have each of the traits. The punnet squares (from #1) show the PREDICTED ratios. Compare the actual to predicted ratios for all three traits. The results will be similar but probably not exact.
5. Notice on the data table that no female has the disease hemophilia. Explain why. Females inherit a normal allele from their fathers.
6. If you knew you were a carrier for hemophilia (or your wife was), would you choose to have children. Explain your reasons. Answers vary