What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Pedigree Definition and How to Read a Chart

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-19

A pedigree is a standardized diagram of a family that uses geometric symbols to show biological relationships, sex assigned at birth, and the presence or absence of a trait across generations. Reading a pedigree chart means converting those symbols into genotypes, then testing each candidate inheritance pattern against the family structure until only one pattern survives.

That conversion is the core skill of clinical genetics. A well-drawn pedigree can reveal that a condition is recessive before any laboratory work begins, can flag consanguinity that raises the prior probability of a homozygous variant, and can expose a de novo event that changes recurrence risk for the whole family. The National Society of Genetic Counselors (NSGC) has published standardized nomenclature since 1995, and that standard remains the reference used in clinical practice and in the literature [1]. Later revisions clarified how to depict sex assigned at birth separately from gender identity and modified how carrier status is shown [2].

Why the Pedigree Still Matters

Sequencing has not replaced the family history. It has made the family history more valuable, because a variant call only becomes interpretable when it segregates with disease in the right people. In a Chinese pedigree with non-syndromic autosomal recessive deafness 12, exome sequencing found a homozygous CDH23 variant in the proband while both parents were heterozygous carriers, and functional work confirmed that the proband expressed only the truncated protein while the parents expressed both normal and truncated forms [3]. The pedigree told the laboratory which relationships to test. The laboratory confirmed what the pedigree predicted.

The same logic runs in reverse. When a variant appears in an affected person but also in an unaffected relative, the pedigree forces you to consider incomplete penetrance rather than assuming the variant is benign. A GEFS+ pedigree carrying a heterozygous SCN1A missense variant included five affected family members and one asymptomatic carrier, which is exactly the kind of observation that shapes how a variant is classified [4].

Standard Pedigree Symbols

The symbol set is small and stable. Learn these and you can read almost any published pedigree.

Core symbols

Square: male, or more precisely, an individual assigned male at birth. The NSGC revision emphasizes that the square and circle encode sex assigned at birth, not gender identity [2].
Circle: female, assigned female at birth.
Diamond: sex unspecified or unknown, used when a pregnancy loss occurred before sex could be determined.
Horizontal line between two symbols: a mating or union. A line drawn from the midpoint downward is the descent line.
Vertical descent line: connects a mating line to the offspring generation.
Sibship line: the horizontal bar from which each child's vertical line drops.
Filled (solid) symbol: affected by the trait or condition under study.
Empty (open) symbol: unaffected, or not known to carry the trait.
Half-filled symbol: a carrier. The 2022 revision changed how carrier status is depicted, so check the legend of any pedigree drawn after that update [2].
Double horizontal bar between mates: consanguinity, meaning the two partners share a common ancestor.
Arrow pointing to a symbol: the proband, the first affected individual through whom the family came to medical attention.
Diagonal line through a symbol: deceased.
Small triangle: pregnancy loss (miscarriage or termination), with gestational age noted if known.
Number inside a symbol: the number of individuals of that sex and status, used to compress large sibships.

Symbols that are still evolving

Standardization is not complete. Genetic counselors working in assisted reproductive technology reported that current nomenclature does not capture the number of embryos conceived, frozen, and implanted, nor their genetic testing history, and participants in that study created their own symbols to fill the gap [5]. Surveys of genetic counselors likewise found wide variability in symbols for transgender and gender non-conforming patients, with 81% of respondents unaware of any standardized symbol for transgender individuals [6]. Transgender community members interviewed about pedigree nomenclature described the need to represent sex assigned at birth and gender separately so that both clinical risk and identity are respected [7]. When you read a modern pedigree, read the legend first.

A Summary Table of Symbols and Their Meaning

Symbol or line	Meaning	Notes
Square	Male, assigned male at birth	Encodes sex assigned at birth [2]
Circle	Female, assigned female at birth	Encodes sex assigned at birth [2]
Diamond	Sex unspecified	Common for early pregnancy loss
Filled symbol	Affected	Trait under study is present
Empty symbol	Unaffected	Not known to carry the trait
Half-filled symbol	Carrier	Depiction revised in 2022 [2]
Horizontal line between two symbols	Mating or union	Descent line drops from its midpoint
Vertical line below a mating	Descent to offspring	Feeds the sibship line
Double horizontal bar	Consanguinity	Partners share a common ancestor
Arrow	Proband	First affected individual identified
Diagonal line	Deceased	Drawn through the symbol
Triangle	Pregnancy loss	Gestational age noted when known
Number inside symbol	Multiple individuals	Compresses large sibships

How to Read a Pedigree Step by Step

Work in a fixed order. Skipping steps is how students misclassify families.

Identify the proband. The arrow marks the entry point. Everything else is organized around that person.
Count generations. Use Roman numerals (I, II, III) for generations and Arabic numerals for individuals within a generation. Individual II-3 is the third person in the second generation.
Mark the mating lines and consanguinity. A double bar between partners is the single most informative structural feature in a recessive pedigree.
Shade the symbols. Separate affected from unaffected from carrier. Note anyone whose status is unknown.
Check the sex distribution. Ask whether affected individuals are predominantly male, predominantly female, or evenly mixed.
Look for skipped generations. An affected child with two unaffected parents points toward recessiveness or a new dominant mutation.
Test each inheritance pattern. Run autosomal recessive, autosomal dominant, X-linked recessive, and mitochondrial hypotheses against the structure. Eliminate the ones that require an impossible genotype.
Note what the pedigree cannot tell you. Small families, incomplete penetrance, and de novo events all generate patterns that mimic other modes of inheritance.

Worked Example 1: Autosomal Recessive

Consider a pedigree with two unaffected parents who have three children. One daughter is affected, one son is affected, and one daughter is unaffected. The parents are not related.

Step 1. The proband is the first affected daughter. She is II-1.

Step 2. Both parents are unaffected but have affected children. This is the classic "skipped generation" signature.

Step 3. Because both parents are unaffected and both have an affected child, each parent must carry one copy of the disease allele. Write both parents as heterozygous, Aa.

Step 4. Each affected child inherited the a allele from each parent, so each affected child is aa. Each unaffected child could be AA or Aa, and the pedigree alone cannot distinguish these.

Step 5. The expected ratio for each pregnancy is 25% affected (aa), 50% carrier (Aa), and 25% homozygous unaffected (AA). The observed 2 of 3 affected is well within the range expected from small numbers.

Step 6. Males and females are affected in roughly equal proportion, which fits autosomal inheritance.

Step 7. Consanguinity would raise the prior probability of this pattern because it increases the chance that both parents carry the same rare allele. Consanguineous families are a productive setting for recessive diagnoses, and studies in consanguineous cohorts have identified homozygous pathogenic variants in genes such as CLN5 and ITPA in affected siblings [8].

A real example follows this structure closely. In the CDH23 deafness pedigree, the proband was homozygous for a frameshift variant while both parents were heterozygous carriers, and the parents expressed both normal and truncated protein while the proband expressed only the truncated form [3]. That is the molecular confirmation of an autosomal recessive pedigree.

Worked Example 2: Autosomal Dominant

Now consider a pedigree in which a father and his mother (the paternal grandmother) are both affected, and the father has two affected children and one unaffected child with an unaffected partner.

Step 1. The trait appears in every generation. This is vertical transmission.

Step 2. Both males and females are affected, and affected males transmit the trait to both sons and daughters. That last point rules out X-linked inheritance, because a father passes his X chromosome only to daughters.

Step 3. Each affected person has an affected parent, so assign a single dominant allele, D. Affected individuals are Dd, assuming the condition is rare and the unaffected partners are dd.

Step 4. Each child of an affected parent and an unaffected parent has a 50% chance of inheriting the trait. The observed 2 of 3 affected children is consistent with that expectation.

Step 5. The unaffected child must be dd, which is informative. An unaffected child of an affected parent proves that the affected parent is heterozygous rather than homozygous, unless penetrance is incomplete.

Step 6. Check for male-to-male transmission. If you find it, X-linked dominant is eliminated and autosomal dominant is confirmed.

The SCN1A GEFS+ pedigree shows why dominant pedigrees need careful reading. A heterozygous missense variant was found in five affected family members and one asymptomatic carrier [4]. That single asymptomatic carrier is the reason the pedigree cannot be read as a simple dominant with full penetrance.

Worked Example 3: X-Linked Recessive

Consider a pedigree with unaffected parents who have one affected son, one unaffected son, and one unaffected daughter. The mother's brother is also affected.

Step 1. Only males are affected. This is the first clue.

Step 2. The affected son has unaffected parents, so the allele is recessive and the mother must be a carrier. Write her as X^A X^a.

Step 3. The father is unaffected, so he is X^A Y. He contributes his Y to his sons and his X^A to his daughters.

Step 4. The affected son inherited his X^a from his carrier mother and his Y from his father. The unaffected son inherited X^A from his mother. The unaffected daughter inherited X^A from her father and either X^A or X^a from her mother, so she is either homozygous unaffected or a carrier.

Step 5. The mother's affected brother confirms that the mother's own mother was a carrier. This is the classic "affected males connected through unaffected females" pattern.

Step 6. No male-to-male transmission should appear. If an affected father has an affected son, X-linked recessive is eliminated.

Step 7. Daughters of an affected male are obligate carriers, because they must inherit his X^a. This is the single most useful shortcut in X-linked recessive pedigrees.

Pattern-to-Clue Mapping

Pattern	Key clues	Typical mode
Skips generations, both sexes affected equally	Unaffected parents with affected child, consanguinity raises probability	Autosomal recessive
Every generation, both sexes affected, male-to-male transmission	Vertical transmission, each child of affected parent has 50% risk	Autosomal dominant
Males only, no male-to-male transmission	Affected males linked through unaffected carrier females	X-linked recessive
Affected mother transmits to all children	No paternal transmission, both sexes affected	Mitochondrial
Apparent dominant in a recessive condition	Multiple affected siblings in two generations, unaffected carriers in the middle	Pseudo-dominant

Pseudo-dominant inheritance deserves a note because it fools readers. In a Chinese primary ciliary dyskinesia pedigree, four individuals across two generations were affected, which looked dominant. Sequencing showed two DNAAF1 variants co-segregating in a pseudo-dominant pattern, with the proband, her brother, and her two sons all affected while her parents and husband were unaffected carriers [9]. The apparent vertical transmission came from a recessive allele meeting a carrier partner, not from a dominant allele.

Common Mistakes and Limitations

Assuming a filled symbol means the person has a pathogenic variant. A filled symbol means the person has the phenotype. Phenotype and genotype diverge whenever penetrance is incomplete or expressivity varies.

Treating an unaffected carrier as evidence against a variant. The GEFS+ pedigree included one asymptomatic carrier of a heterozygous SCN1A variant alongside five affected carriers [4]. Asymptomatic carriers are expected in many dominant conditions.

Forgetting that de novo variants occur in recessive disease. The standard teaching is that autosomal recessive conditions require one variant from each parent. That is usually true, but not always. A cohort of 15 cases of autosomal recessive conditions found that one allele was inherited and the other arose de novo, with the de novo variant arising on the paternal allele in 14 of 15 cases [10]. A pedigree that appears to show only one carrier parent may reflect a de novo event rather than a genotyping error or non-paternity.

Reading small pedigrees as if they were large ones. A family with two affected brothers and no other affected relatives is consistent with autosomal recessive, X-linked recessive, and even autosomal dominant with incomplete penetrance. The pedigree alone cannot separate these. Small numbers produce ratios that deviate widely from expectation by chance.

Ignoring variable expressivity. Three siblings with the same homozygous ENPP1 variant presented with markedly different features: one with classic rickets and limb deformities, one with limited elbow extension and hearing loss, and one with early biochemical abnormalities before skeletal changes appeared [11]. Same genotype, different phenotype. A pedigree that records only "affected" loses this information.

Overlooking consanguinity. A double bar between partners changes the prior probability of a recessive diagnosis substantially. Consanguineous cohorts have yielded homozygous pathogenic variants in genes such as GRM1 in a large Pakistani family with severe ataxia and intellectual disability [12].

Assuming a gene has one inheritance mode. GDAP1 variants cause Charcot-Marie-Tooth disease in both autosomal recessive and autosomal dominant forms, and a single study of 11 unrelated families found that most patients had recessive disease while two had dominant disease [13]. The pedigree, not the gene name, determines the mode in a given family.

Mixing up sex assigned at birth with gender. The NSGC revision explicitly separates these concepts in pedigree nomenclature [2]. A pedigree that records only gender identity may lose information needed for X-linked risk assessment.

Quick Review

A pedigree is a standardized family diagram, and the NSGC nomenclature is the reference standard [1].
Square is male, circle is female, filled is affected, half-filled is carrier, double bar is consanguinity, arrow is the proband.
Autosomal recessive: skipped generations, both sexes affected, consanguinity raises probability.
Autosomal dominant: every generation, male-to-male transmission possible, 50% risk per child of an affected parent.
X-linked recessive: males only, no male-to-male transmission, daughters of affected males are obligate carriers.
Pseudo-dominant inheritance mimics dominant disease when a recessive allele meets a carrier partner [9].
De novo variants can account for one allele in autosomal recessive disease, so single-carrier parents do not exclude a recessive diagnosis [10].

Frequently Asked Questions

What is a pedigree in genetics?

A pedigree is a standardized diagram of a family that shows biological relationships, sex assigned at birth, and the presence or absence of a trait. It is the primary tool for inferring inheritance patterns before molecular testing.

What does a half-filled symbol mean on a pedigree chart?

A half-filled symbol indicates a carrier, meaning the person has one copy of a recessive allele but does not show the trait. The NSGC revised how carrier status is depicted in its 2022 update, so always read the legend [2].

How do I tell autosomal dominant from autosomal recessive on a pedigree?

Look for skipped generations. An affected child with two unaffected parents points to recessive inheritance. A trait in every generation with male-to-male transmission points to dominant inheritance.

Why does an X-linked recessive pedigree show only affected males?

Males have one X chromosome, so a single recessive allele causes the trait. Females have two X chromosomes and need two copies, which is far less likely for rare alleles.

Can a pedigree show a dominant pattern when the disease is actually recessive?

Yes. Pseudo-dominant inheritance occurs when a recessive allele is common enough, or the family large enough, that an affected person has children with a carrier. A DNAAF1 primary ciliary dyskinesia pedigree showed exactly this, with four affected individuals across two generations [9].

What is the arrow on a pedigree?

The arrow marks the proband, the first affected individual through whom the family was identified. All relationships in the pedigree are typically organized around that person.