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Genetic Inheritance

Genetic Terminology

Mendelean Genetics Terminology

  • DominantTrait that is expressed in both the homozygous or heterozygous

  • Recessive: Trait that is expressed only in the homozygous state (requires 2 recessive alleles for expression)

  • Codominance: Both trait are expressed in the heterozygous state

    • common in blood group inheritance like AB, Fy(a+b+)


Different levels of expression affect reaction to the antisera

Double dose: antigen expression is stronger in homozygous state

  • Ex: SS

Single dose: Weaker reactions are seen with heterozygous

  • Ex: Ss

  • Common in Kidd, Duffy, MNSs blood groups

Pedigree Chart
pedigree chart 2.jpg

What is a Pedigree Chart?

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How to read pedigree chart:

  • Square: male & Circle: female

    • Empty Square/Circle: Unaffected individual- the trait is not expressed

    • Filled in Square/Circle:  Affected individual​​- the trait is expressed

    • Half-filled in Square/Circle:  Carrier​​ for trait but individual is unaffected

    • Crossed out Square/Circle: Deceased individual

Other Pedigree Chart Symbols 

Consanguinous- related

  • ie. siblings or cousins

Pedigree Symbols.jpg

Monozygotic- identical twins

Pedigree Symbols.jpg

Proband- (propositus)

  • the patient of interest

Pedigree Symbols.jpg

Dizygotic- fraternal twins


Non-sex chromosomes (Chromosome 1-22 in humans)

Autosomal Dominant

pedigree-analysisPedigree inheritance pa

Autosomal Recessive

pedigree-analysisPedigree inheritance pa

Sex chromosome (23rd chromosome; XY in males XX in females)

X-linked Dominant

pedigree-analysisPedigree inheritance pa

X-linked Recessive

pedigree-analysisPedigree inheritance pa


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