Biology · Introductory biology · Worked example
Solve a dihybrid cross: 9:3:3:1 phenotypes
In peas, round seeds (R) are dominant to wrinkled (r), and yellow seeds (Y) are dominant to green (y). Two plants heterozygous for both genes, RrYy, are crossed. Find the phenotype ratio and the probability of a wrinkled green seed.
List each parent’s gametes
The two genes are on different chromosomes, so they assort independently. Each parent makes four kinds of gametes in equal numbers: RY, Ry, rY and ry, each with probability ¼.
Treat each gene as its own cross
Rr × Rr gives ¾ round and ¼ wrinkled, and Yy × Yy gives ¾ yellow and ¼ green. Independent assortment lets us multiply these probabilities instead of filling all 16 cells of the square.
Multiply for each phenotype
Round and yellow: ¾ × ¾. Round and green, or wrinkled and yellow: ¾ × ¼. Wrinkled and green: ¼ × ¼.
Write the ratio
The four phenotypes come in sixteenths: 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green. Among 320 seeds, expect about 180, 60, 60 and 20.
State the assumptions
The 9:3:3:1 ratio assumes complete dominance at both genes and independent assortment. Genes close together on one chromosome are linked and break the ratio, which is why offspring counts are often tested against 9:3:3:1 with chi-square.
Result
9:3:3:1: round yellow 9/16, round green 3/16, wrinkled yellow 3/16 and wrinkled green 1/16. A wrinkled green seed has probability 1/16.
Your turn
Cross RrYy with rryy, a test cross. What phenotype ratio do you expect?
Show the answer and explanation
1:1:1:1: round yellow, round green, wrinkled yellow and wrinkled green, each ¼.
The rryy parent passes only r and y, so each offspring shows the alleles in the RrYy parent’s gamete. Its four gametes are equally likely, and each phenotype has probability ½ × ½ = ¼.
Keep exploring
The Punnett square tool opens with both genes and lists the four phenotypes with their fractions.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
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