In Mendel’s experiments, tall pea plants (T) are dominant over dwarf pea plants (t).
What is a Dihybrid Cross?
Topic: Dihybrid cross
Answer
A DIHYBRID CROSS is a cross between two organisms in which TWO PAIRS of contrasting characters are studied together at the same time.
The key word is TWO PAIRS of characters. A MONOHYBRID cross follows only ONE.
The two are best learnt as a contrasting pair: MONOHYBRID — one character — F2 phenotypic ratio 3 : 1 DIHYBRID — two characters — F2 phenotypic ratio 9 : 3 : 3 : 1
Since each parent in a dihybrid cross produces FOUR kinds of gamete, the Punnett square has 4 × 4 = 16 boxes — which is where the numbers 9, 3, 3 and 1 come from.
The dihybrid cross is what demonstrates the LAW OF INDEPENDENT ASSORTMENT, because new combinations appear in F2 that were in neither parent.
Example: crossing a pea plant that is tall with round seeds (TTRR) with one that is dwarf with wrinkled seeds (ttrr). The F2 generation then shows the phenotypic ratio 9 : 3 : 3 : 1.