Inferring Genotypes from Pedigrees — Infer genotypes of pedigree members - Genetics

SECTION 1

Historical Context & Motivation

Long before scientists could read DNA in a lab, they needed a way to track traits through families. If a child was born with a genetic condition, doctors and researchers wanted to know: where did the trait come from, and who else might carry it? The answer came from a simple but powerful tool called a pedigree — a diagram that maps out a family tree and shows who has a particular trait and who does not.

Pedigrees let geneticists work like detectives. By looking at patterns across generations, they can figure out the genotype (the actual allele combination) of each person — even when those alleles are invisible. This skill is still essential today in genetic counseling, medical genetics, and biology classrooms.

1865 • Mendel's Pea Experiments

Gregor Mendel published his work on pea plants, establishing the laws of inheritance and the idea that traits are passed through discrete "factors" (later called genes).

1905 • First Formal Pedigree Charts

Scientists began using standardized family diagrams to trace genetic conditions like hemophilia and color blindness through royal families and medical case studies.

1953 • DNA Structure Discovered

Watson and Crick described the double helix structure of DNA, giving scientists a physical explanation for how alleles are stored and passed on.

1990s–Today • Genetic Counseling Expands

Pedigree analysis became a core tool in genetic counseling, helping families understand their risk for inherited conditions like cystic fibrosis and sickle cell disease.

The central question this lesson addresses is: given a family diagram showing who is affected and who is unaffected by a trait, how can you figure out the genotype of every person in the pedigree? This detective work is one of the most practical skills in genetics.

SECTION 2

Core Principles & Definitions

Before you can read a pedigree, you need to understand a few key ideas. Every person carries two copies of each gene — one inherited from each parent. These copies are called alleles. A dominant allele (written as a capital letter, like A) shows its effect even when only one copy is present. A recessive allele (written as a lowercase letter, like a) only shows its effect when a person has two copies.

Phenotype vs. Genotype

The phenotype is what you can see (affected or unaffected). The genotype is the hidden allele pair (AA, Aa, or aa). Pedigrees show phenotypes; your job is to infer the genotypes.

Carriers

A carrier is someone with one dominant and one recessive allele (Aa). They look unaffected but can pass the recessive allele to their children.

Pedigree Symbols

Squares represent males; circles represent females. Filled (shaded) shapes mean the person is affected. A horizontal line between two shapes means they are mated (parents). A vertical line leads down to their children.

Autosomal vs. X-linked

An autosomal trait is carried on one of the 22 non-sex chromosomes. An X-linked trait is carried on the X chromosome and follows different patterns in males and females.

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KEY TAKEAWAY

Think of a pedigree like a mystery novel. The phenotypes (shaded or unshaded shapes) are the clues you can see. The genotypes are the hidden secrets you must figure out. Just like a detective uses clues to solve a case, you use the pattern of affected and unaffected individuals to deduce each person's allele combination.

SECTION 3

Visual Explanation — Reading a Pedigree

The diagram below shows a three-generation pedigree for an autosomal recessive trait. Notice how two unaffected parents in Generation I produce an affected child in Generation II. This is the classic clue that both parents must be carriers.

A three-generation autosomal recessive pedigree. Filled shapes are affected (genotype aa). Unaffected parents of affected children must be carriers (Aa). The question mark (?) means we know the person has at least one A but cannot yet determine the second allele without more information.

In the diagram, notice the filled square in Generation II. Because the trait is recessive, that individual must have two recessive alleles (aa). His parents in Generation I are both unaffected, so they must each carry at least one dominant allele (A). But since they produced an aa child, each parent must also carry a recessive allele. That makes them both carriers with the genotype Aa.

SECTION 4

Step-by-Step Method — How to Infer Genotypes

Here is a reliable step-by-step method you can follow every time you see a pedigree. Think of these as the detective's rules for cracking the case.

The Four-Step Strategy

Step 1 — Determine the inheritance pattern. Is the trait autosomal dominant, autosomal recessive, X-linked dominant, or X-linked recessive? Look for clues: does the trait skip generations (likely recessive), does it affect mostly males (likely X-linked), or does every affected person have at least one affected parent (likely dominant)?
Step 2 — Assign definite genotypes first. Start with the individuals whose genotypes you know for sure. Affected individuals in a recessive disorder are always aa. Affected individuals in a dominant disorder have at least one A allele.
Step 3 — Work outward from affected individuals. Each affected child received one recessive allele from each parent. This means both parents must carry at least one recessive allele. Use this logic to fill in the parents' genotypes.
Step 4 — Determine remaining unknowns using probability. Sometimes you cannot pin down a genotype with certainty. In that case, note the possible genotypes and, if needed, calculate the probability using a Punnett square.

Punnett Square Probabilities

CARRIER × CARRIER CROSS

Aa × Aa → ¼ AA : ² ⁄₄ Aa : ¼ aa

When two carriers mate, each child has a 25% chance of being affected (aa), a 50% chance of being a carrier (Aa), and a 25% chance of being homozygous dominant (AA).

CARRIER × AFFECTED CROSS

Aa × aa → ½ Aa : ½ aa

When a carrier mates with an affected individual, each child has a 50% chance of being a carrier and a 50% chance of being affected.

UNAFFECTED PROBABILITY (GIVEN UNAFFECTED PHENOTYPE)

P(AA | unaffected) = ⅓, P(Aa | unaffected) = ⅔

Among the unaffected children of two carriers (Aa × Aa), ⅓ are AA and ⅔ are Aa. This is because once we know a child is unaffected, we remove the aa outcome and consider only the remaining three-quarters of possibilities.

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Pro Tip

Always start with the individuals you are most certain about. Affected individuals in a recessive pedigree are always aa — that's your anchor point. Then work upward to parents and outward to siblings.

SECTION 5

Recognizing Inheritance Patterns

Before you can assign genotypes, you often need to identify the mode of inheritance. Different patterns leave different fingerprints on a pedigree. The diagram below compares the four most common patterns side by side.

Four major inheritance patterns compared. Autosomal recessive traits skip generations and require both parents to be carriers. Autosomal dominant traits appear in every generation with at least one affected parent. X-linked recessive traits primarily affect males and are passed through carrier mothers. X-linked dominant traits affect all daughters of an affected father.

Pattern	Key Clue	Affected Individual Genotype
Autosomal Recessive	Two unaffected parents → affected child	aa
Autosomal Dominant	Every affected person has an affected parent	Aa or AA
X-Linked Recessive	Mostly males affected; no father-to-son	Males: X^aY
X-Linked Dominant	Affected father → all daughters affected	Males: X^AY

Summary of Inheritance Pattern Clues

SECTION 6

Worked Example — Solving a Pedigree

Let's walk through a complete example. Imagine a family where cystic fibrosis (an autosomal recessive condition) appears in the third generation. The grandparents (Generation I) are both unaffected. Their son (Generation II) married an unaffected woman, and they had one affected daughter and one unaffected son.

Inferring Genotypes — Cystic Fibrosis Pedigree

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Step 1 — Identify the Inheritance Pattern

Cystic fibrosis is autosomal recessive. We use F for the normal allele and f for the cystic fibrosis allele. Affected individuals must be ff.

Pattern: autosomal recessive
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Step 2 — Assign Definite Genotypes

The affected daughter in Generation III must be ff. She received one f from her father and one f from her mother.

Affected daughter (Gen III): ff
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Step 3 — Work Upward to Parents (Gen II)

Both parents are unaffected, so each has at least one F allele. But since their daughter is ff, each parent must have donated an f allele. Therefore, both parents must be Ff (carriers).

Father (Gen II): Ff | Mother (Gen II): Ff
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Step 4 — Work Upward to Grandparents (Gen I)

The father in Generation II is Ff. He got his f allele from one of his parents (Generation I). Both grandparents are unaffected, so at least one of them must be a carrier (Ff). Without additional information, we know at least one grandparent is Ff, but the other could be FF or Ff.

Grandparents (Gen I): at least one is Ff
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Step 5 — Determine the Unaffected Son (Gen III)

The unaffected son has at least one F allele. Since both parents are Ff, the son could be FF (probability ⅓) or Ff (probability ⅔). We write his genotype as F_ to show the second allele is uncertain.

Unaffected son (Gen III): F_ (⅓ chance FF, ⅔ chance Ff)

SECTION 7

Strengths & Limitations of Pedigree Analysis

Pedigree analysis is a powerful tool, but like any method, it has strengths and limitations. Understanding both will help you know when you can be confident in your genotype assignments and when you need to be more cautious.

Strengths	Limitations
Works without laboratory equipment — only family history is needed.	Small family sizes make it hard to distinguish patterns (dominant vs. recessive).
Can identify carriers who show no symptoms.	Cannot always determine the exact genotype of unaffected individuals (e.g., AA vs. Aa).
Helps predict the probability of future offspring being affected.	Incomplete penetrance and variable expressivity can produce misleading patterns.
Applicable to any trait that follows Mendelian inheritance.	Does not work well for traits controlled by many genes (polygenic traits) or strongly influenced by the environment.

Strengths vs. Limitations of Pedigree Analysis

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KEY TAKEAWAY

Think of pedigree analysis like watching a sports replay without hearing the commentary. You can see who scored (phenotype) but you have to guess the strategy behind each play (genotype). Sometimes the evidence is clear — a player clearly set up the goal. Other times, you need more replays (more family data) to be sure of the strategy.

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Connection to Advanced Genetics

The pedigree skills you've learned here are the foundation for more advanced genetic analysis. As you move forward in genetics, you'll encounter situations where inheritance is more complex, but the same logical reasoning still applies.

What You Learned Here	Where It Leads
Simple dominant/recessive traits (one gene, two alleles)	Codominance, incomplete dominance, and multiple alleles (e.g., ABO blood types)
Autosomal inheritance	X-linked, Y-linked, and mitochondrial inheritance
100% penetrance assumed (all individuals with the genotype show the trait)	Reduced penetrance and variable expressivity (not everyone with the genotype shows the trait equally)
Using Punnett squares for probability	Using Bayesian probability to update genotype predictions as new family data is collected

From Basic to Advanced Genetics

Modern genetic counselors combine pedigree analysis with DNA testing to confirm genotypes directly. However, pedigree reasoning remains critical because it tells the counselor which family members to test and what results to expect. Understanding the logic of inheritance is the first step toward mastering any branch of genetics.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

In a pedigree for an autosomal recessive trait, two unaffected parents have an affected child. What must be true about the genotypes of both parents? Explain your reasoning.

Show answer

Both parents must be carriers with the genotype Aa. The affected child has the genotype aa, meaning the child received one recessive allele from each parent. Since both parents are unaffected (they show the dominant phenotype), they must each have at least one dominant allele (A). Combined with the fact that each donated a recessive allele (a) to the child, both parents must be Aa.

Question 2: BASIC CALCULATION

Two carrier parents (Aa × Aa) have a child. What is the probability that the child is: (a) affected, (b) a carrier, and (c) homozygous dominant?

Show answer

Using a Punnett square for Aa × Aa, the offspring ratios are: ¼ AA, ² ⁄₄ Aa, and ¼ aa. So (a) the probability of being affected (aa) is ¼ or 25%. (b) The probability of being a carrier (Aa) is ² ⁄₄ or 50%. (c) The probability of being homozygous dominant (AA) is ¼ or 25%.

Question 3: INTERMEDIATE

In a pedigree, an unaffected woman whose father had an autosomal recessive condition marries an unaffected man with no family history of the condition. What is the woman's definite genotype? What is the probability that their child will be affected?

Show answer

The woman's father was affected, so his genotype was aa. He must have passed one a allele to the woman. Since she is unaffected, she must also have one A allele, making her Aa — a definite carrier. The unaffected man with no family history is most likely AA, so the cross is Aa × AA. This produces ½ AA and ½ Aa offspring — none will be affected (probability = 0). However, if the man happens to be a carrier (Aa), the probability would be ¼. Without further information, we assume he is AA and the probability of an affected child is 0%.

Question 4: APPLIED

A genetic counselor is meeting with a couple. The woman is unaffected but her brother has sickle cell disease (autosomal recessive). Both of the woman's parents are unaffected. The man is also unaffected and has no family history. What is the probability that the woman is a carrier? If she is a carrier and the man is also a carrier (carrier frequency in his population is 1 in 12), what is the probability their child will have sickle cell disease?

Show answer

Since the woman's brother is affected (ss), both parents must be carriers (Ss). From an Ss × Ss cross, the unaffected offspring are ⅓ SS and ⅔ Ss. So the woman has a ⅔ chance of being a carrier. If the man has a 1/12 chance of being a carrier, the probability that both are carriers is ⅔ × 1/12 = 1/18. If both are carriers, the chance their child is affected is ¼. So the overall probability is 1/18 × ¼ = 1/72, or about 1.4%. This type of combined probability reasoning is exactly what genetic counselors perform.

Question 5: CRITICAL THINKING

A pedigree shows that every affected individual has at least one affected parent, and the trait appears in every generation. However, one affected father has an affected son. Could this trait be X-linked dominant? Explain why or why not, and suggest what inheritance pattern best fits.

Show answer

This trait cannot be X-linked dominant. In X-linked dominant inheritance, an affected father passes his X chromosome to all of his daughters (making them all affected) but passes his Y chromosome to all of his sons. This means an affected father can never pass an X-linked trait to his sons. The fact that an affected father has an affected son rules out X-linked dominant inheritance. The pattern described — trait in every generation, affected individuals always having at least one affected parent — is most consistent with autosomal dominant inheritance. In autosomal dominant inheritance, either parent can pass the affected allele to children of either sex, which explains the father-to-son transmission.

SUMMARY

Lesson Summary

Summary

A pedigree is a family diagram that maps out the phenotypes (visible traits) of individuals across generations. Your job as a genetics detective is to infer the hidden genotypes (allele combinations) from the patterns you observe. Start by identifying the inheritance pattern — is the trait autosomal recessive, autosomal dominant, or X-linked?

Next, assign definite genotypes to affected individuals first (they are your anchor points), then work outward to parents and siblings using logical deduction. Two unaffected parents of an affected child must both be carriers. When genotypes remain uncertain, use Punnett squares and probability to describe the likelihood of each possible genotype. These pedigree-reading skills are the foundation for genetic counseling, medical genetics, and advanced heredity studies.