Test cross and family trees sim
Cross a purple pea plant of hidden genotype with a white tester, grow the offspring one at a time and decide PP or Pp, then tag every person in a family tree with their genotype.
Using a test cross with a homozygous recessive plant to find whether a dominant-phenotype pea plant is homozygous or heterozygous, and reading a family tree (pedigree) for a recessive condition: carriers, genotypes that cannot be told, and the chance for the next child · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Test cross and family trees. Part 1: a purple pea plant is PP or Pp, and you cannot tell by looking. Cross it with a white plant, which must be pp: if any offspring is white, the purple plant is Pp; if many offspring are all purple, it is very likely PP, because a Pp plant gives a purple offspring only half the time, and n purple in a row has a chance of (1/2)ⁿ. Part 2: in a family tree, two parents without a condition who have a child with it show that the allele is recessive and that both parents are carriers (Ff). Simplified on purpose: illustrative: a teaching model of pea plants and a made-up family, not real data; one gene with two alleles in each part: P (purple) and p (white) in peas, F and f for cystic fibrosis; complete dominance: a Pp plant is as purple as a PP plant; flower colour depends only on this gene here; seeded chance: a fixed list of random numbers, so the same steps always grow the same plants; Start again picks new hidden genotypes; each offspring gets one allele from each parent, each equally likely, and every seed grows; the hand pollination (removing the anthers and brushing on pollen) is shown only as the paintbrush on the bench; 1 in 20 is used as the line for "very likely": it is a choice, not a law; real family trees are confidential and are read with a genetic counsellor; the bench models are teaching models, drawn smaller than real plants.
- Glow: the part of the bench for the step you are on
- Close-up: the test cross, what each answer predicts, the offspring and the family tree
- Dashed outline: an empty place to drop something
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