A dihybrid cross tracks the inheritance of two different traits at the same time, such as seed shape and seed color in pea plants. It helps students see how alleles from each parent can combine in many possible ways. Dihybrid crosses matter because they connect Mendel’s laws to real patterns of genetic variation.
A 4 by 4 Punnett square is often used when both parents are heterozygous for both traits.
In the classic example RrYy × RrYy, R represents round seeds, r represents wrinkled seeds, Y represents yellow seeds, and y represents green seeds. Each parent can make four types of gametes: RY, Ry, rY, and ry. When these gametes combine in a Punnett square, the offspring phenotypes usually appear in a 9:3:3:1 ratio if the genes assort independently and show complete dominance.
This method is useful for predicting probabilities, not exact outcomes in a small group of offspring.
Key Facts
- A dihybrid cross follows two traits at the same time, such as seed shape and seed color.
- For RrYy, the possible gametes are RY, Ry, rY, and ry.
- A heterozygous dihybrid cross uses 16 boxes because 4 gamete types × 4 gamete types = 16 offspring combinations.
- In RrYy × RrYy with complete dominance, the phenotypic ratio is 9:3:3:1.
- The expected phenotypes are 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green.
- The multiplication rule can be used for independent traits: P(A and B) = P(A) × P(B).
Vocabulary
- Dihybrid cross
- A genetic cross that tracks the inheritance of two traits at the same time.
- Independent assortment
- The principle that alleles of different genes separate into gametes independently when the genes are unlinked.
- Gamete
- A reproductive cell, such as sperm or egg, that carries one allele for each gene.
- Genotype
- The allele combination an organism has for a gene or set of genes.
- Phenotype
- The observable trait or appearance produced by a genotype and the environment.
Common Mistakes to Avoid
- Listing only two gametes for RrYy is wrong because a dihybrid heterozygote can form four gamete types: RY, Ry, rY, and ry.
- Using a 2 by 2 Punnett square for a full dihybrid cross is wrong because two heterozygous traits produce 4 gamete types per parent, requiring 16 boxes.
- Treating the 9:3:3:1 ratio as guaranteed offspring counts is wrong because it predicts probabilities, and real offspring numbers can vary by chance.
- Applying independent assortment to linked genes without checking is wrong because genes close together on the same chromosome may be inherited together more often than expected.
Practice Questions
- 1 In the cross RrYy × RrYy, how many of the 16 Punnett square boxes are expected to show the round yellow phenotype?
- 2 For RrYy × RrYy, what is the probability of an offspring being wrinkled green? Give your answer as a fraction and a percent.
- 3 Explain why the gametes from an RrYy parent are RY, Ry, rY, and ry rather than RR, rr, YY, and yy.