Dihybrid crosses track the inheritance of two traits at the same time, such as seed shape and seed color. This cheat sheet helps students organize parent genotypes, find possible gametes, and build Punnett squares without losing track of alleles. It is especially useful for solving Mendelian genetics problems where both traits follow simple dominance.
Students need this reference because most errors in dihybrid crosses come from setup, not from the final counting.
Key Facts
- A dihybrid cross studies two genes at once, such as AaBb x AaBb.
- Use FOIL to find gametes from a heterozygous dihybrid: AaBb makes AB, Ab, aB, and ab.
- The number of possible gamete types is 2^n, where n is the number of heterozygous gene pairs.
- A standard heterozygous dihybrid cross, AaBb x AaBb, gives the phenotype ratio 9:3:3:1 when the genes assort independently.
- In AaBb x AaBb, the genotype ratio is 1 AABB : 2 AABb : 1 AAbb : 2 AaBB : 4 AaBb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb.
- Dominant phenotypes need at least one dominant allele, so A_ means AA or Aa.
- Recessive phenotypes require two recessive alleles, so aa or bb must be present for the recessive trait to appear.
- The probability rule for independent events is P(A and B) = P(A) x P(B).
Vocabulary
- Dihybrid cross
- A genetic cross that follows the inheritance of two different traits or genes at the same time.
- Gamete
- A sex cell, such as a sperm or egg, that carries one allele for each gene.
- Independent assortment
- The principle that alleles for different genes separate into gametes independently when the genes are not linked.
- Genotype
- The allele combination an organism has, such as AaBb or aabb.
- Phenotype
- The observable trait or trait combination produced by a genotype, such as round yellow seeds.
- Punnett square
- A grid used to predict possible offspring genotypes from the gametes of two parents.
Common Mistakes to Avoid
- Leaving out gametes is wrong because every allele combination from each parent must be included before the Punnett square is built.
- Writing gametes with two alleles from the same gene, such as Aa, is wrong because each gamete receives only one allele per gene.
- Using 3:1 for every dihybrid cross is wrong because 3:1 applies to one trait, while AaBb x AaBb usually gives 9:3:3:1 for two independently assorting traits.
- Counting genotypes as phenotypes is wrong because different genotypes, such as AA and Aa, can produce the same dominant phenotype.
- Assuming 9:3:3:1 always applies is wrong because linked genes, incomplete dominance, codominance, or nonstandard parent genotypes can change the expected ratio.
Practice Questions
- 1 List all possible gametes produced by an organism with genotype AaBb.
- 2 For the cross AaBb x AaBb, how many offspring out of 16 are expected to show both dominant traits?
- 3 For the cross AaBb x aabb, what fraction of offspring are expected to have the genotype aaBb?
- 4 A student gets a result that does not match 9:3:3:1 in a two-trait cross. Explain one biological reason and one setup error that could cause the difference.