Pedigree analysis uses family trees to track how traits or genetic disorders pass from one generation to the next. This cheat sheet helps students read symbols, compare inheritance patterns, and assign possible genotypes. It is useful for solving genetics problems involving dominant, recessive, sex-linked, and mitochondrial traits.
Clear pattern clues make it easier to move from a diagram to a genetic explanation.
The most important ideas are recognizing affected individuals, tracing parent-to-child patterns, and using genotype rules consistently. Autosomal dominant traits often appear in every generation, while autosomal recessive traits can skip generations. X-linked traits show different patterns in males and females because males have only one X chromosome.
Genotype assignment depends on both the phenotype and the inheritance pattern being tested.
Key Facts
- In a pedigree, a square represents a male, a circle represents a female, and a shaded symbol represents an affected individual.
- A horizontal line between two individuals represents a mating pair, and a vertical line leading downward connects parents to their children.
- Autosomal dominant traits usually appear in every generation, and an affected child usually has at least one affected parent.
- Autosomal recessive traits can skip generations, and two unaffected carrier parents can have an affected child.
- For an autosomal recessive trait, affected individuals are aa, carriers are Aa, and unaffected noncarriers are AA.
- For an autosomal dominant trait, affected individuals are usually Aa or AA, and unaffected individuals are aa.
- X-linked recessive traits are more common in males, and an affected male passes his X-linked allele to all daughters but no sons.
- Mitochondrial traits are passed from affected mothers to all children, but affected fathers do not pass the trait to their children.
Vocabulary
- Pedigree
- A pedigree is a family diagram that shows how a trait appears across generations.
- Affected
- An affected individual shows the trait or disorder being studied in the pedigree.
- Carrier
- A carrier has one recessive allele for a trait but does not show the recessive phenotype.
- Autosomal
- An autosomal trait is controlled by a gene located on a non-sex chromosome.
- X-linked
- An X-linked trait is controlled by a gene located on the X chromosome.
- Genotype
- A genotype is the allele combination an individual has for a particular gene.
Common Mistakes to Avoid
- Assuming every trait that skips a generation is X-linked is wrong because autosomal recessive traits also commonly skip generations.
- Labeling an unaffected person as AA too quickly is wrong because they may be an Aa carrier in a recessive inheritance pattern.
- Forgetting that males have only one X chromosome is wrong because a male with an X-linked recessive allele will show the trait.
- Treating shaded symbols as genotypes is wrong because shading shows phenotype, and several genotypes may produce the same phenotype.
- Ignoring affected parents when assigning genotypes is wrong because parent and child phenotypes must be consistent with the inheritance pattern.
Practice Questions
- 1 In an autosomal recessive pedigree, two unaffected parents have an affected child. What are the genotypes of the parents and the affected child?
- 2 A woman who is a carrier for an X-linked recessive disorder has children with an unaffected man. What is the probability that each son will be affected?
- 3 In an autosomal dominant pedigree, an affected heterozygous parent mates with an unaffected parent. What is the probability that each child will be affected?
- 4 A trait appears in every generation and affects males and females equally. What inheritance pattern is most likely, and what evidence supports that conclusion?
Understanding Pedigree Analysis Reference
A strong pedigree solution starts by treating each family member as evidence, not as a label to guess. Begin with people whose genotype is certain. For a recessive condition, an affected person gives the clearest starting point because both gene copies must carry the allele.
Then work outward to their parents, siblings, partner, and children. An unaffected parent of an affected child must carry the recessive allele. This conclusion is stronger than a general pattern clue because it follows directly from inheritance.
For dominant conditions, unaffected people are especially useful because their genotype is fixed. Writing known alleles first prevents a common error where students assign every unaffected person the same genotype without enough evidence.
Sex-linked pedigrees require careful attention to which parent gave which chromosome. A son receives his X chromosome from his mother and his Y chromosome from his father. This means a father cannot pass an X-linked allele directly to a son.
A daughter receives one X chromosome from each parent. Therefore, an affected daughter with an X-linked recessive condition must receive the recessive allele from her father and from her mother.
A female can carry an X-linked recessive allele without showing the condition when her other X chromosome has the usual dominant allele. In basic school problems, this is why a trait may seem to move through unaffected females before appearing in male children.
Probability in pedigree questions describes a new birth, not a guarantee for a whole family. If two carrier parents have children, each pregnancy has its own chance of producing each possible genotype. Earlier children do not change the allele combination available in a later pregnancy.
A Punnett square helps organize these possibilities, but only after the parent genotypes are justified from the pedigree. For example, if two carriers have one unaffected child, that child may be a carrier or may not carry the allele.
The pedigree alone may not distinguish those possibilities. Students should write both options when the evidence is incomplete, then calculate risk using the probabilities for each possible genotype.
Real families are often more complicated than classroom pedigrees. A person may have a gene variant but show no visible trait because of incomplete penetrance. A trait can vary in severity among relatives.
New mutations can produce an affected child with no earlier family history. Small families can make a pattern look misleading simply because there are few people to observe. Adoption, unknown biological parentage, and incomplete medical records can limit conclusions too.
In textbook problems, the intended inheritance model is usually stated or implied, so use that model consistently. In real genetic counseling, pedigrees are one source of evidence alongside medical history and DNA testing. The key learning habit is to separate what the diagram proves from what it only suggests.