Dihybrid Crosses and the 9:3:3:1 Ratio
Two gene pairs on different chromosomes assort independently, so a cross between two double heterozygotes gives four phenotypes in a 9:3:3:1 ratio. The grid is four by four and the ratio is a prediction, not a law.
What a learner can do afterwards
- Sets out the four gamete types from a doubly heterozygous parent.
- Completes a four by four grid and reads the phenotype ratio off it.
- Names the stage of meiosis at which independent assortment happens.
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The Mendelian dihybrid cross tracks two genes at once, using parents heterozygous for both, such as RrYy crossed with RrYy. Two laws run the show. Segregation says the two alleles of one gene part into different gametes. Independent assortment adds that genes for different traits part independently, so each gamete gets a random mix of the two genes. That independence happens in metaphase I, when each homologous pair lines up facing one pole or the other. Those alleles arise by mutation.
A doubly heterozygous parent therefore makes four gamete types in equal numbers: RY, Ry, rY and ry, one allele of each gene per gamete. Memorize that set, because every box of the grid is built from it. Dominant alleles mask recessive ones in any heterozygote, so uppercase letters win wherever they appear.
The four by four grid fills in mechanically and holds 16 boxes. List the four gamete types of each parent along the top and side, then combine them box by box. Count phenotypes, not genotypes: 9 boxes show both dominant traits, 3 show one mixed combo, 3 show the other, and 1 shows both recessives. That gives the predicted ratio 9:3:3:1, which real counts wobble around by chance. Linked genes on one chromosome travel together instead and spoil the pattern.
Pin the biology behind the numbers to metaphase I, when each homologous pair lines up facing one pole or the other and each pair chooses independently. That random lineup shuffles which alleles travel together. Two warnings: linked genes on one chromosome travel together and spoil the ratio, and the ratio is a prediction for real counts to wobble around, not a law.
Four gamete types fill a 16 box grid, giving a predicted 9:3:3:1 phenotype split.
Two readings written for this topic, plus two readings that summarise Khan Academy articles. Videos by Khan Academy on YouTube; viewing follows platform terms.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.