Inheritance is the passing of genetic information from one generation to the next. With a handful of terms and a Punnett square you can predict what fraction of the offspring will show a feature. Extended: how a test cross reveals a hidden genotype.
Genotype is the genetic make-up of an organism in terms of its alleles; phenotype is its observable features. A dominant allele is expressed if it is present; a recessive allele only when no dominant allele is present. Crossing two heterozygotes gives a 3 : 1 ratio; crossing a heterozygote with a homozygous recessive gives 1 : 1.
In the sim, tall (T) is dominant to dwarf (t) in pea plants. Choose a genotype for each parent, press Make the cross, then press Grow 200 seeds.
| Term | Meaning | Pea example |
|---|---|---|
| Inheritance | transmission of genetic information from generation to generation | a parent passes T or t to its seeds |
| Genotype | the genetic make-up of an organism, in terms of the alleles present | TT, Tt or tt |
| Phenotype | the observable features of an organism | tall or dwarf |
| Homozygous | two identical alleles of a particular gene | TT or tt |
| Heterozygous | two different alleles of a particular gene | Tt |
| Dominant allele | expressed if it is present in the genotype | T |
| Recessive allele | only expressed when no dominant allele is present | t (dwarf only in tt) |
Each gamete carries only ONE allele of the gene. Write the gametes as T and t, never as Tt, and circle each one.
Press Next generation twice: pure-breeding parents, then F1, then F2. Use the Show genotypes box to hide and reveal the alleles.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the Punnett square sim, what does Tt x Tt give?
You cross Tt with tt in the sim. What phenotype ratio is shown?
Grow 200 seeds from Tt x Tt gives 154 tall and 46 dwarf. Why is it not exactly 150 : 50?
In the F1 and F2 sim, what are all the F1 plants?
Extended. A black guinea pig could be BB or Bb (black, B, is dominant to white, b). Press Breed a litter of 8, decide, then press I think BB or I think Bb. Press New mystery guinea pig to try again.
All-black offspring do not prove the parent is BB: a Bb parent could, by chance, give only black pups. A large number of offspring makes the conclusion much more reliable.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which term describes the observable features of an organism?
In cats, short hair (H) is dominant to long hair (h). Two short-haired cats have a long-haired kitten. What are the parents' genotypes?
Which cross gives offspring that are all identical to the parents?
Extended: a black guinea pig is crossed with a white one (bb). The litter has 5 black and 4 white pups. The black parent's genotype is:
This skill is now recorded on your profile. Keep the chain going.