HOW TO SOLVE DIHYBRID PROBLEMS: A STEP-BY-STEP GUIDE
Example problem: In roses, red is dominant over white. Short stems are dominant
over long stems. If you cross two heterozygous red, short-stemmed roses, what
are the chances of producing the desirable trait of long-stemmed red roses?
Step 1: Write out your key in terms of what’s dominant and recessive.
R = red, r = white
S = short stem, s = long stem
Step 2: Determine the genotypes of the parents and write out the cross.
Parent 1: RrSs
Parent 2: RrSs
RrSs X RrSs
Step 3: Figure out what kinds of gametes each parent can produce.
Parent 1: RS, Rs, rS, rs
Parent 2: RS, Rs, rS, rs
Step 4: Set up a Punnett square for your mating.
RS
Rs
rS
RS
Rs
rS
rs
OVER
rs
Step 5: Fill in the Punnett square.
RS
Rs
rS
rs
RS
RRSS
RRSs
RsSS
RrSs
Rs
RRSs
RRss
RrSs
Rrss
RrSS
RrSs
rrSS
rrSs
RrSs
Rrss
rrSs
rrss
rS
rs
Step 6: Answer the question(s) you’ve been asked.
3/16 chance of producing a red, long-stemmed rose.