Pedigree charts for biology class: a student and educator guide

Short version. A pedigree chart is a family diagram that tracks a trait across generations. Four classical inheritance patterns are routinely taught: autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant. Reading a pedigree is a four-question process — is the trait dominant or recessive, is it autosomal or sex-linked, what are the likely genotypes, and is your conclusion consistent with the whole chart? This page gives the working vocabulary, worked examples, and practice questions at AS, A-level, and introductory university level.

The minimum vocabulary you need

Before reading a pedigree, you need a small set of symbols. This is the simplified version used in biology teaching; the full clinical reference is in our pedigree symbols page.

The four classical inheritance patterns

Autosomal dominant (AD). The trait appears in every generation. A single copy of the dominant allele (heterozygous, Aa) is enough to produce the phenotype. Affected parents transmit the trait to approximately half their children. Both males and females are affected in roughly equal numbers. Examples biology students meet: Huntington disease, polydactyly, achondroplasia, some forms of dwarfism.

Autosomal recessive (AR). The trait can skip generations. Two copies of the recessive allele (homozygous, aa) are needed to produce the phenotype. Parents of affected children are often unaffected heterozygous carriers (Aa). Affected individuals cluster in a single sibship. Consanguineous unions raise the probability of two identical recessive alleles meeting. Examples students meet: cystic fibrosis, sickle cell anaemia, phenylketonuria (PKU), Tay-Sachs disease.

X-linked recessive (XR). Affected individuals are overwhelmingly male. Females can be heterozygous carriers (X^AX^a) without showing the trait; males have only one X chromosome (X^aY), so a single recessive allele produces the phenotype. There is no father-to-son transmission, because fathers pass a Y chromosome (not an X) to sons. Examples: haemophilia A and B, red-green colour blindness, Duchenne muscular dystrophy.

X-linked dominant (XD). A single X-linked dominant allele produces the phenotype. Affected fathers transmit to all daughters and no sons (because all daughters receive their father's only X chromosome). Affected mothers transmit to half their children of either sex. Examples: X-linked hypophosphataemia (vitamin-D-resistant rickets), fragile X syndrome (complex because of the CGG repeat mechanism), Alport syndrome in its X-linked form.

Mitochondrial inheritance is occasionally covered at advanced level — strictly maternal, affecting both sexes, with variable expression due to heteroplasmy.

Practice pedigree 1: identifying inheritance patterns

                I-1         I-2
                 ■───────────○
                      │
        ┌─────────────┼────────────┐
      II-1          II-2         II-3
       □             ■            ○
                      │
             ┌────────┼──────┐
           III-1    III-2  III-3
            ○         ■      □

Q1: Is the trait dominant or recessive? The trait appears in every generation (I-1, II-2, III-2) — vertical transmission. That suggests dominant. An affected parent (I-1) has an affected child (II-2) and unaffected children (II-1, II-3), consistent with a heterozygous affected parent transmitting to about half.

Q2: Autosomal or sex-linked? Look for male-to-male transmission. I-1 (affected male) transmits to II-2 (affected male): this is male-to-male transmission. Sex-linked inheritance is ruled out (a father passes Y, not X, to his son). Therefore the trait is autosomal dominant.

Q3: What is I-1's likely genotype? I-1 is affected. If they were homozygous (AA), all their children would inherit one A allele and be affected. But II-1 and II-3 are unaffected. So I-1 must be heterozygous (Aa). Half the offspring carry A (affected), half carry a (unaffected) — consistent with observed 1 in 3 affected, within sampling variation.

Practice pedigree 2: the carrier question

             I-1           I-2
              □─────────────○
                   │
           ┌───────┼───────┐
         II-1    II-2    II-3
          ●       ○       ■
       (carrier) (unaff.) (affected)
          │                  │
          │         ──────── │
         II-1 × unaff. male
              │
          ┌───┴───┐
        III-1   III-2
         ○       ●   ←  proband
      (unaff.) (carrier)

Given: trait is autosomal recessive. II-1 is a known carrier.

Q: What is the probability that III-1 (the daughter of II-1 and an unrelated unaffected male) is a carrier?

Reasoning: II-1 is an obligate carrier (Aa) because her brother II-3 is affected (aa) and their parents (I-1 and I-2) must both be heterozygous to have produced an affected child. II-1's partner is unrelated and unaffected; assume he is AA (not a carrier) in the absence of evidence otherwise.

Cross: Aa × AA. Offspring genotypes: 50% AA (unaffected, not a carrier), 50% Aa (unaffected, carrier). So III-1 has a 50% chance of being a carrier, given the information in the chart.

(If you were told III-1's partner's family history or were told the population carrier frequency, you could compute her children's risk — but that is a further question.)

Practice pedigree 3: X-linked or autosomal?

            I-1             I-2
             □───────────────●
                     │
      ┌──────────────┼──────────────┐
    II-1           II-2           II-3
     ■              ○              □
              │                │
     ┌────────┴───────┐   ┌────┴────┐
   III-1           III-2  III-3    III-4
    □               ■      □        ○

Q: What is the most likely inheritance pattern?

Reasoning step 1: Affected individuals — II-1 (male), III-2 (male). Both male. Unaffected carriers or transmitters on the female side (I-2, II-2 must have transmitted).

Step 2: I-2 (apparently affected, circle filled) has an affected son (II-1). II-2 (unaffected daughter of I-2) has an affected son (III-2). This is the hallmark of X-linked recessive: carrier females transmit to their sons.

Step 3: Check for male-to-male transmission. II-1 (affected male) has a son III-1 (unaffected). Not male-to-male transmission, so X-linked not ruled out.

Step 4: Wait — if I-2 is filled, she is affected, and for X-linked recessive to produce an affected female she would need to be homozygous (X^aX^a). If the condition is rare, this is unlikely but possible. Alternatively, the filled circle might be a drawing choice indicating "carrier" rather than "affected"; always consult the chart legend. If I-2 is a carrier (not affected), the pattern is cleanly X-linked recessive: carrier mother → affected son.

Answer: most likely X-linked recessive, with the caveat that the legend determines whether filled circles in this chart mean "affected" or "carrier".

Exam-style questions (with answers)

Q1. In a pedigree where an affected father has four daughters (all affected) and three sons (all unaffected), what inheritance pattern is most consistent?

A1. X-linked dominant. An affected father transmits his single X chromosome to all daughters (all affected), and his Y chromosome to all sons (none affected).

Q2. Two unaffected parents have an affected daughter. The condition is believed to be single-gene. What inheritance patterns are consistent?

A2. Autosomal recessive (both parents heterozygous carriers) is by far the most likely. Autosomal dominant with a new (de novo) mutation is possible but rare. X-linked dominant is excluded because if the father carried it he would be affected. X-linked recessive is excluded because an affected female would require a homozygous genotype, which would mean her father (hemizygous) would be affected — contradicting the pedigree.

Q3. A couple both heterozygous for cystic fibrosis are planning children. What is the probability that any one child will have cystic fibrosis?

A3. Aa × Aa → 1 AA : 2 Aa : 1 aa. The homozygous recessive (aa) genotype gives cystic fibrosis. Probability = 1/4 = 25%.

Q4. In a haemophilia pedigree, a carrier mother (X^HX^h) partners with an unaffected father (X^HY). What proportion of their sons are expected to be affected?

A4. Sons receive Y from father and one of the mother's two X chromosomes, each with 50% probability. Sons inheriting X^h are affected. So 50% of sons are expected to be affected.

Q5. Why can a single small pedigree rarely give a certain diagnosis of inheritance pattern?

A5. Because small families produce results consistent with several patterns by chance. Reduced penetrance, variable expressivity, de novo mutations, and mosaicism also confound pattern recognition. In practice, the pedigree narrows the differential; molecular testing confirms the inheritance pattern by identifying the causative variant.

Free tools for students and teachers

• Evagene (Alpha, free). Browser-based clinical pedigree editor at evagene.com. Automatic NSGC symbol enforcement, 200+ disease catalogue, inheritance-pattern identifier useful as a learning aid. Free during Alpha via waiting list.
• QuickPed. Research pedigree editor from the University of Melbourne. Free, web-based, fast for simple charts.
• ConceptViz. AI-assisted diagram generation including simple pedigrees from text descriptions. Good for rapid sketching.
• Madeline 2.0. Free from the University of Michigan. More oriented to research linkage pedigrees than teaching, but capable.
• Paper and pencil. Never underestimate this. For AS and A-level homework, drawing by hand with a legend builds the symbol vocabulary better than any software.

How Evagene supports this

Evagene is a clinical pedigree product, not a teaching tool — but several of its features work well as learning aids for biology students at AS, A-level, and introductory university level. The inheritance-pattern identifier runs the chart against autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant, and mitochondrial patterns and explains which are compatible and which are ruled out; this is exactly the reasoning exercise a biology exam tests, rendered transparently. The 200+ disease catalogue lets students build a pedigree around a real condition (cystic fibrosis, Huntington disease, haemophilia) rather than an abstract "trait A".

For teachers, Evagene's GEDCOM import means a class worksheet pedigree can be prepared once and reused across cohorts. Exported charts can be saved as PNG or PDF for worksheets and exam papers. The free Alpha tier makes it suitable for classroom use without a budget line. For educators writing A-level or university questions, the Mendelian inheritance calculator (AD/AR/XR) confirms the expected proportions under each pattern — useful for checking answer keys.

For undergraduate students heading into clinical genetics, medical, or allied-health degrees, using a clinical tool for teaching exercises has a real downstream benefit: the symbol vocabulary, the disease-code awareness (ICD-10, OMIM), and the pattern-recognition language carry directly into clinical training.

Frequently asked questions

Related reading

• Complete pedigree symbols reference
• Pedigree chart examples with inheritance patterns
• How to make a pedigree chart in 5 steps
• Mendelian inheritance calculator
• Standard pedigree nomenclature
• Pedigree drawing software