On the right is a live Punnett square. Each parent contributes the two alleles shown in the coloured boxes. Click a parent's allele boxes to flip between T (tall, dominant) and t (dwarf, recessive). The 2×2 grid combines the gametes and the phenotype ratio below it updates instantly. Start with the default Tt × Tt and read the answer.
Keep the cross at Tt × Tt. Press ▶ Play and the four boxes fill one by one: TT, Tt, Tt, tt. Three of the four show the dominant tall character and one is dwarf — the classic 3 : 1 phenotype ratio (genotype 1 : 2 : 1). Use ⏭ Step to fill one box at a time, or 🔀 New to try a fresh cross such as TT × tt or a test cross Tt × tt.
Switch to the Dihybrid tab. Now we cross RrYy × RrYy (round-yellow). Each parent makes four kinds of gamete — RY, Ry, rY, ry — by independent assortment, so the grid is 4 × 4 = 16 boxes. Press ▶ Play to fill them and watch the colour-coded tally build toward the famous ratio.
When all 16 boxes are full, count the colours: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. The two recessive combinations (round-green, wrinkled-yellow) are the recombinants — new mixes in neither parent. Note each character on its own is still 3 : 1. ↻ Reset and ⏭ Step to count it yourself.
Open the Sex linkage tab. A carrier mother (XᴴXʰ) marries a normal father (XᴴY) — the trait is haemophilia, carried on the X chromosome. Press ▶ Play to fill the square. The mother's eggs carry Xᴴ or Xʰ; the father's sperm carry Xᴴ or Y. Watch how the affected allele lands.
The finished square reads: daughters ½ normal : ½ carrier — none affected — but sons ½ normal : ½ haemophiliac. A son inherits his single X from his mother; with no second X to mask it, an Xʰ allele shows. That is why colour blindness and haemophilia strike mostly males. ↻ Reset to replay, then revisit the Lecture.