How can a family-tree diagram represent inheritance in a fictional organism?

First decide what the diagram records, and keep those things distinct:

1. Relationships: who the fictional parents and offspring are, and which generation each belongs to.
2. Observed trait: a clearly defined feature you assign to each individual, such as “has a blue tail tip.”
3. Model assumptions: any inheritance rule you are choosing for the story or classroom exercise.

A pedigree is a way to display relationships across generations and examine a proposed inheritance pattern. Genotype means an organism’s genetic makeup; phenotype means an observable characteristic. Those are related ideas, but an observed appearance is not itself a genotype label. [S1]

For a simple, explicitly fictional model, you might assume one gene with two versions and say which version produces the visible trait. Treat that as a rule of your example, not a rule for every inherited trait: inheritance can be more complex than simple dominant–recessive examples. [S2]

Yes. Pick a small, consistent key and print it beside the diagram. For this example, use:

• □ = fictional individual labeled male in the story; ○ = fictional individual labeled female. If those labels are irrelevant, use one shape for everyone instead.
• Filled symbol = the defined trait is observed; empty symbol = it is not observed.
• A horizontal line between two symbols = the modeled parent pair; a vertical line down to a branch = their offspring.
• I, II, III = generation labels; numbers identify individuals within a generation.
• Gg or gg = proposed model genotype, shown as a separate label, not encoded by the fill.

These are choices for a readable fictional diagram. Add a note defining every symbol, and do not make the shading carry two meanings at once. The pedigree can organize the example’s relationships and pattern; it does not, by itself, establish the hidden genotype of a real organism. [S1]

Assume a fictional species called moonmoths. The trait is “glow spot present.” For this exercise only, assume g produces a glow spot when an individual has two copies (gg), while Gg and GG do not show one. This is a deliberately simplified recessive model, not a claim about real organisms.

Key: □ or ○ = individual; filled = glow spot observed
     ─ = modeled parent pair; │ and ├ = offspring links

Generation I       I-1 □ Gg ─── I-2 ○ Gg
                              │
Generation II          ┌─────┴─────┐
                       II-1 □ gg   II-2 ○ Gg
                       filled     empty

The example stipulates that both parents are Gg and that the first offspring is gg; the symbols show those stated assignments, not a discovery made from the drawing. Under the chosen rule, gg matches the defined visible trait, while Gg does not. A reader can check whether the stated labels and shading agree with that rule. The diagram alone does not prove the model is true, determine an unshown individual’s genotype, or establish that all traits follow this pattern. [S1] [S2]

Separate what the fictional record says from what the model proposes. For example, label an uncertain appearance unknown rather than shading it as absent. Put inferred or assumed genotypes in brackets, such as [Gg assumed], and explain the basis in a note. If parentage or a relationship is unspecified, leave the connection out or mark it unspecified instead of filling the gap with a guess.

Before sharing, ask: Does every symbol have one stated meaning? Is the trait defined in observable terms? Can a reader tell which details are observations and which are assumptions? Does the caption say this is a fictional model? Those checks make the diagram easier to interpret without claiming more than the example contains. Keep it about invented organisms; this format is not a basis for inferring real people’s traits, ancestry, or health.