Read an original pedigree using its symbols generations and trait key
Read symbols, lines, generations and the trait key before assigning genotypes.
Section B16.3 · Monohybrid inheritance
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must be read before interpreting shading?
What you will learn
- Locate relatives and generations in a pedigree
- Read shading from the key before inferring inheritance
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Shapes in this key | Square: male; circle: female |
| Filled in this key | The stated characteristic is present |
| I-1 and I-2 | Linked parents in generation I |
| II-1 and II-2 | Sibling offspring in generation II |
Locate relatives and generations in a pedigree
A pedigree diagram records relationships and the presence or absence of a supplied characteristic across generations. In this original key a square represents a male, a circle represents a female, and a filled symbol means the stated trait is present. A horizontal partner line joins two parents; a descendant line leads to the sibling line and their offspring. Follow the lines instead of assuming nearby symbols are related.
Generation labels I and II mark the first and second rows in this model. Individuals are numbered within each row: I-1 and I-2 are the parents, while II-1 and II-2 are their offspring. The two offspring are siblings. Roman generation numbers and individual numbers locate records; neither is an allele label or a prediction of which phenotype a future child will have.
Read shading from the key before inferring inheritance
Shading has no universal dominance meaning. Filled means the trait named by the key, which could be a dominant or recessive feature depending on the supplied model. In the introductory pedigree, both parents are unfilled, II-1 is filled and II-2 is unfilled. This tells you which records show the stated characteristic, but no allele symbols or inheritance rule have yet been supplied.
Before assigning genotypes, read the characteristic key, the stated inheritance rule and the known parent-child links. Later lessons use separate declared autosomal recessive and dominant models. These are invented pedigrees for interpretation practice, not private family histories or clinical assessments. A symbol’s shape or shading alone cannot establish a whole genotype or determine the mechanism for an unspecified real trait.
Sources for this lesson
Pause and explain
What relationship do II-1 and II-2 have in the original linked pedigree?
Worked example
Read the generations, parental links and filled status in the original introductory pedigree. Is dominance established?
Show the worked solution
- Read the supplied shapes and IDs: I-1 and I-2 are the unfilled parents, linked to II-1 and II-2.
- Use the filling key to identify II-1 as showing the stated trait. The key supplies no dominance rule, so filled status alone does not establish dominance.
Answer Original introductory pedigree: I-1 square and I-2 circle are unfilled parents; II-1 square is filled and II-2 circle unfilled. The key identifies trait status, not dominance.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Using the supplied key, identify the generation and trait status of filled square II-1.
Give me a hint
Separate the generation label, shape and fill.
Compare my reasoning
- Read II as generation two.
- Read square as male in the key.
- Read filled as the supplied trait present.
II-1 is the first numbered male in generation II and shows the trait named by the key.
Look for these in your work
- Correct generation and position
- Correct shape and trait interpretation
The diagram joins I-1 and I-2 to a sibling line containing II-1 and II-2. Identify the parent pair and the relationship between the two generation-II individuals.
Give me a hint
Follow the descendant lines.
Compare my reasoning
- Locate the generation-I partner line.
- Follow its descendant line to both offspring.
I-1 and I-2 are the parents; II-1 and II-2 are their sibling offspring.
Look for these in your work
- Correct parent pair
- Correct sibling relationship
A student calls the filled trait dominant without an inheritance rule. Explain what shading establishes and what additional information is needed.
Give me a hint
Shading follows the key, not a universal genetic rule.
Compare my reasoning
- Use the key to identify trait presence.
- Distinguish trait presence from dominance.
- Request the stated genetic model or suitable evidence.
Shading establishes the recorded trait status under the key. It does not itself establish dominance; a supplied inheritance rule and appropriate genetic evidence are needed before assigning genotypes or mechanism.
Look for these in your work
- Trait status distinguished from dominance
- Names missing model or genetic evidence
Common mistakes
- Assuming filled always means recessive
- Reading individual numbers as alleles
- Inferring genotype before reading the trait and inheritance keys
What can you explain now?
What relationship do II-1 and II-2 have in the original linked pedigree?
Compare with the explanation
They are siblings
Both descend from the same linked parent pair.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Without your notes, locate relatives and generations in a pedigree and read shading from the key before inferring inheritance. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.