Natural Science Forum / Biology / Biology / March 2008

Step 1. Define symbols.

And just as in algebra, it is best to write them down explicitly, so
it is clear.

Let
s = allele for smooth
w = allele for wrinkled.

(There are some conventions for these symbols. Best is to follow what
you instructor/book prefer. Mine are ok.)

One parent is ss. You know that because the problem said it is
pure-breeding -- which in this context means it is homozygous = has
two of the same allele. It makes two "kinds" of gametes, but both are
s in this case.

Similarly the other parent is ww. (You didn’t say it is pure-breeding,
but I suspect that was the intent.) It makes w and w alleles.

Step 2. Now for the Punnett...

It is simply a grid to help you see all the combinations. Since we are
dealing here with only one gene, a 2 x 2 array is all we need. One
parent across the top. Say, ss. So label one column with s and the
other column with s. Each column is one gamete type. (Seems easy with
ss; but remember, Punnett squares will be used later with more
difficult cases, so make sure you understand the easy ones.)

Other parent along the side. Label one row w and one row w.

Now fill in each square of the 2x2 grid with whatever you get from the
headings. If s column intersects with w row, you get sw. What the
Punnett square is doing is to help you visualize all possible
combinations of pairs of gametes, one from each parent.

(This is easier to do with pictures and pointing. Trying to explain it
here in plain text is perhaps not so good. Surely this is in your
textbook?)

In this case, all four squares come out the same, sw. Thus 100% of the
progeny will be heterozygous sw -- one allele of each type.

Step 3. What is the phenotype? Well, I think you are supposed to know
which allele is dominant.

In the real world... it is not particularly predictable which is
dominant (unless one knows some things about the alleles). The way one
finds out -- the way Mendel found out -- is to do precisely this expt.

ok?

bob