A pure tall pea plant is crossed with a pure dwarf one, and the F1 plants are then self-pollinated. Work through the cross and show how it proves each of the two laws of inheritance in your NIOS lesson.

1. 1 · State the laws.
   • First law: every feature is governed by two factors; when gametes form, the two part, one into each gamete.
   • Second law: of a pair, the dominant factor shows; the recessive one shows only without the dominant one.
2. 2 · Parents and gametes. TT (tall) × tt (dwarf); gametes T and t.
3. 3 · F1 generation. All Tt and all tall: tallness is dominant, dwarfness is hidden.
4. 4 · Draw the checkerboard for F1 × F1. [Checkerboard: T, t × T, t → TT, Tt, Tt, tt]
5. 5 · Write the result. 3 tall : 1 dwarf; factor make-up 1 TT : 2 Tt : 1 tt.
6. 6 · Link each law to the cross.
   • T and t go into different gametes: the first law.
   • Tt plants are tall because T is dominant: the second law.
7. 7 · Close in one line. So features pass on as paired factors, now called genes.

Scoring tip: Write Parents, Gametes, F1 and F2 as labelled lines, and box the 3 : 1 ratio.