1. Determine the genotypes
(letters) of the parents. Bb x Bb
2. Set up the punnet square with one parent on each side.
3. Fill out the Punnet square middle
4. Analyze the number of offspring of each type.
In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are:
RR = round
Rr = round
rr = wrinkled
If you get stuck make a "key". Sometimes the problems won't give you obvious information.
Example: In radishes, a bent root is a dominant trait, though some roots are straight (which is recessive). If a straight rooted plant is crossed with a heterozyous bent root plant, how many of the offspring will have straight roots?
Say what? First of all, this problem doesn't make it easy. Start by assigning genotypes and phenotypes. It doesn't matter what letter you pick, but it may be easiest to pick a letter that represents the dominant trait. In this case, use the letter B for bent.
Bb = bent
bb = straight
Now use the key to figure out your parents. In this case you have a straight root plant (bb) crossed with a heterozyous bent plant (Bb). Once you've figured that out, the cross is simple!
If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.
3/4 round, 1/4 wrinkled