Dihybrid cross: independent assortment, gene linkage and crossing over
BiologyGeneticsAges 17–18
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Genetics
A two-gene Punnett square shows each parent's gametes and their frequencies, cells colour-coded by phenotype, and genotype–phenotype ratios computed automatically (9:3:3:1 under independent assortment). Switch to gene linkage with a recombination frequency f: parental gametes each (1−f)/2, recombinant gametes each f/2, and the offspring ratio shifts with f and with the cis/trans arrangement. Students can change the parental genotypes, drag f, and cross 100 random individuals to compare experimental results with theory.
Lesson: Mendel's law of independent assortment; gene linkage and crossing over
What it shows
A dihybrid cross follows two genes at once. If the genes lie on different chromosome pairs they assort independently, so an AaBb parent makes four gamete types in equal proportions and AaBb × AaBb gives a 9 : 3 : 3 : 1 phenotype ratio, illustrated with pea seed colour and shape. If the genes are linked on one chromosome, illustrated with fruit-fly body colour and wing length, each parental gamete makes up (1 − f)/2 and each recombinant gamete f/2, so the offspring ratio depends on the recombination frequency and on the cis or trans arrangement. At f = 50% the result matches independent assortment.
How to use
Choose the Parent 1 genotype and Parent 2 genotype and read the gametes, the colour-coded Punnett square and the ratios. Press Cross 100 several times to compare random offspring with the theoretical outline, and Clear results between trials. Then press Gene linkage, drag the Recombination frequency f (when linked) and switch between Cis heterozygote AB/ab and Trans heterozygote Ab/aB to see how the ratio shifts.
Parameters you can change
- Parent 1 genotype AaBb, AABB, AABb, AaBB, AAbb, aaBB, Aabb, aaBb, aabb
- Parent 2 genotype AaBb, AABB, AABb, AaBB, AAbb, aaBB, Aabb, aaBb, aabb
- Location of the two genes On two different chromosome pairs (independent assortment), On the same chromosome (linked, with crossing over)
- Recombination frequency f (when linked) 0–50 %
- Heterozygote arrangement (when linked) Cis heterozygote AB/ab, Trans heterozygote Ab/aB
Questions to explore
- What phenotype ratio does AaBb × aabb give when the two genes assort independently?
- With linked genes and f = 0, how does crossing two cis heterozygotes AB/ab differ from 9 : 3 : 3 : 1?
- Why do 100 random offspring rarely match the theoretical ratio exactly?
