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Pedigree Analysis and Mendelian Disorders

Pedigree analysis is the study of inheritance of a trait across generations using standard symbols to infer whether a disorder is dominant, recessive, autosomal, or sex-linked.

In one line: Pedigree solving moves from pattern to genotype: generation pattern first, sex pattern next, carrier logic last.

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Understand it

  • Pedigree charts help trace a trait when controlled human crosses are not possible.
  • Squares represent males, circles represent females, shaded symbols show affected individuals, and carriers may be marked for recessive traits.
  • Autosomal recessive disorders may appear in offspring of unaffected carrier parents.
  • Autosomal dominant traits usually appear in every generation.
  • Sex-linked recessive disorders such as haemophilia are more common in males because they have only one X chromosome.
  • Mendelian disorders such as sickle-cell anaemia, thalassaemia, phenylketonuria, and haemophilia are tested through inheritance pattern, genotype inference, and carrier identification.

Example

If unaffected parents have an affected child, the disorder is likely recessive, and both parents may be carriers. In haemophilia, a carrier mother can pass the affected X chromosome to sons.

Rule to remember

Pedigree rules: unaffected parents with affected child suggests recessive inheritance; affected individuals in every generation suggests dominant inheritance; more affected males with no father-to-son transmission suggests X-linked recessive inheritance.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

In a pedigree, if two unaffected parents have an affected daughter, an autosomal recessive disorder is strongly suggested because both parents can be carriers and the daughter receives two recessive alleles.

Method you can reuse

  1. 1Identify affected and unaffected individuals.
  2. 2Check whether the trait skips generations.
  3. 3Compare male and female occurrence.
  4. 4Look for father-to-son transmission.
  5. 5Infer dominant or recessive pattern.
  6. 6Assign possible genotypes and identify carriers.

Visual tip

Pedigree diagrams must label square as male, circle as female, shaded as affected, half-shaded or marked as carrier where used, horizontal line as mating, and vertical line as offspring.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why are males more often affected in X-linked recessive disorders such as haemophilia?

Think first, then show answer
Males are more often affected because they have only one X chromosome. If that X carries the recessive disorder allele, there is no second normal allele on another X to mask it.

Common wrong answer

Writing that a carrier male exists for an X-linked recessive disorder is usually wrong; a male with the allele on his single X is affected, not merely a carrier.

Don't mix this up

Students often assume every disorder seen mostly in males is Y-linked. Many such disorders are X-linked recessive, not Y-linked.

Exam tip

First decide dominant or recessive from generation pattern, then decide autosomal or sex-linked from male-female distribution and father-to-son transmission.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

This concept is asked through pedigree interpretation, symbol identification, carrier prediction, and differences among haemophilia, sickle-cell anaemia, thalassaemia, and phenylketonuria.

Answer plan

State the pedigree symbols, observe the generation pattern, infer the inheritance type, assign genotypes to key individuals, and support the conclusion with one pedigree rule.
  1. 1

    Enough for 1 mark

    Pedigree analysis is the study of inheritance of a trait across generations using standard symbols to infer whether a disorder is dominant, recessive, autosomal, or sex-linked.

  2. 2

    Add for 2 marks

    Example: If unaffected parents have an affected child, the disorder is likely recessive, and both parents may be carriers. In haemophilia, a carrier mother can pass the affected X chromosome to sons.

  3. 3

    Add for 3 marks

    Pedigree charts help trace a trait when controlled human crosses are not possible.

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