Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

17.5Inheritance

Pedigree diagrams and Punnett squares — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 9 1 quiz

📚 Cambridge IGCSE Biology 0610 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

17.5 Pedigree diagrams and Punnett squares — IGCSE BIOLOGY notes: Read pedigree diagrams, deduce genotypes, build Punnett squares and use test crosses.Pedigree diagram — a family tree tracing a characteristic, usually a disease, through generations.Squares — males.Circles — femal…

  • Shaded symbols
  • Horizontal line
  • Vertical line
  • Two unaffected parents with an affected child
  • Two affected parents with an unaffected child
  • Condition in every generation

Part 1 of 7Reading a pedigree diagram

Pedigree diagram
  • Pedigree diagram a family tree tracing a characteristic, usually a disease, through generations.
Squares
  • Squares males.
Circles
  • Circles females.
Shaded symbols
  • Shaded symbols show the characteristic; unshaded symbols do not.
Horizontal line
  • Horizontal line joins a male and female who are parents.
Vertical line
  • Vertical line leads down to their children.
Each row
  • Each row one generation, often labelled I, II, III.

Part 2 of 7Dominant or recessive

Dominant or recessive
Two unaffected parents with an affected child
  • Two unaffected parents with an affected child the condition is recessive and both parents are heterozygous.
Two affected parents with an unaffected child
  • Two affected parents with an unaffected child the condition is dominant.
Condition in every generation
  • Condition in every generation with affected children having an affected parent, it may be dominant.

Part 3 of 7Deducing genotypes

Deducing genotypes
Affected recessive person
  • Affected recessive person homozygous recessive.
Unaffected parent of an affected child
  • Unaffected parent of an affected child heterozygous for a recessive condition.
Unaffected child of an affected parent
  • Unaffected child of an affected parent heterozygous.
Unaffected person with no clues
  • Unaffected person with no clues could be homozygous dominant or heterozygous.
1
Find a key family in the pedigree.
2
Decide whether the allele is dominant or recessive.
3
Give genotypes to affected people first.
4
Work out the parents and children from them.

Part 4 of 7Building a Punnett square

1
Choose a capital letter for the dominant allele and the lower case for the recessive.
2
Write each parent's gametes along one side of the grid.
3
Fill each box with one allele from each side.
4
Count the genotypes, then the phenotypes.

Part 5 of 7Ratios to know

CrossGenotypesPhenotype ratio
Heterozygous × heterozygous1 : 2 : 13 : 1
Heterozygous × homozygous recessive1 : 11 : 1
Homozygous dominant × homozygous recessiveAll heterozygousAll dominant
Same homozygous × same homozygousAll like the parentsPure-breeding
Ratios to know
A 3 : 1 ratio means probabilities of 0.75 dominant and 0.25 recessive, or 75% and 25%.
Each fertilisation is a separate random event, so small families often differ from the ratio.

Part 6 of 7Test cross

Test cross
Unknown genotype
  • Unknown genotype a dominant phenotype could be homozygous dominant or heterozygous.
Method
  • Method cross it with a homozygous recessive individual.
All offspring dominant
  • All offspring dominant the unknown parent is homozygous dominant.
About half recessive offspring
  • About half recessive offspring the unknown parent is heterozygous.
Many offspring
  • Many offspring make the conclusion more reliable.

Part 7 of 7More from the specification

1
Chromosomes, Genes, and Protein Synthesis
  • A chromosome is a condensed thread-like structure of DNA located in cell nuclei that carries linear sequences of genes, each coding for the specific amino acid sequence of a functional protein.
2
Mitosis vs Meiosis in Cellular Division
  • Mitosis produces two genetically identical diploid body cells for growth and tissue repair,
  • whereas meiosis involves two nuclear divisions to yield four genetically unique haploid gamete cells with half the original chromosome number.
3
Punnett Squares and Monohybrid Inheritance
  • Genetic crosses and Punnett squares are used to determine phenotypic and genotypic ratios of offspring, showing how dominant and recessive alleles segregate during gamete formation.
4
Pedigree Diagrams and Genotype Determination
  • Pedigree diagrams track the inheritance of single-gene traits across multiple family generations, helping geneticists determine whether an inherited condition is autosomal dominant or recessive.
    • (such as cystic fibrosis or short limb dysplasia)
5
Specific
  • Able to act on only one type of substrate.
6
Daughter cell
  • Cell produced by cell division.
7
Blood group
  • A phenotype set by alleles I-A, I-B and I-O.
8
Protein synthesis
  • Making proteins from amino acids using the DNA code.
9
Monohybrid inheritance
  • Inheritance of one gene.
10
Chromosome
  • A thread-like structure of DNA carrying genes.
11
Mitosis
  • Cell division producing genetically identical cells.
12
Inheritance
  • Transmission of genetic information from generation to generation.
13
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
14
Meiosis
  • Reduction division giving four genetically different haploid cells.
15
Sequence
  • The order of bases in DNA or amino acids in a protein.

Key terms

TermMeaning
Pedigree diagramA family tree showing how a characteristic is inherited.
Punnett squareA grid combining the gametes of two parents.
Test crossA cross with a homozygous recessive to find an unknown genotype.
CarrierAn unaffected individual with one recessive allele.
GenerationOne row of a pedigree.

Avoid these mistakes

1
Writing both alleles of the pair into each gamete — each gamete has one allele.
2
Assuming shaded means male — shading means affected, and shape shows sex.
3
Saying a 3 : 1 ratio is certain in each family — it is a probability.
4
Assuming every unaffected person is homozygous dominant — they may be carriers.

Last updated:

\n