You have a really complicated question here if
you haven't done much genetics yet.
My college students get confused by Punnett
squares too. It's pretty hard to explain them
without showing them step by step.
I'll help you with the flies, though. Once
you answer my questions, if you still need help,
write back and I promise to help you with the
Okay, so you have your first 2 flies and mate
them. The parent flies would be P1 and P2. (You
started with 4, but let's keep it simple.)
The next generation would be the F1 flies.
I'll assume you're just counting the new ones, so
your F1 generation would be 5 red females and 4
red males. If you then bred your F1 generation
together, you got the F2 generation. Breeding
your F2's together gives you your F3 generation.
Here's a table:
Parental: 2 red female : 2 red male
F1: 5 red female : 4 red male
F2: 11 red female : 0 red male: 2 white male
F3: 20 red female : 15 red male: 8 white male
Now here are my questions for you:
If a trait is dominant, can 2 parents
dominant trait have an offspring with the
If a trait is recessive, can 2 parents
recessive trait have an offspring with the
Once you answer these questions you'll be able
to figure out whether red eyes are dominant or
Now you have to ask yourself whether the eye
color gene is found on the X chromosome or some
other chromosome. Male flies (like male humans)
are XY. Females are XX. Males get a Y chromosome
from their dads. The Y has little information
other than telling him to be male. The male gets
his only X chromosome from his mom, so whether
it's the recessive one or the dominant one, that's
the trait he'll have. While his sister will have
two copies of a gene (EE or Ee or ee), he'll only
have one (e or E). So males are much more likely
to show recessive, X-linked traits. So what do
you think, is eye color an X-linked trait?
Once you can tell me which eye color is
dominant and whether the gene for eye color is on
the X chromosome, you'll be ready to do your
squares. Let me know if you need help.
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