A pedigree is a diagram representing a family tree used to examine the existence and frequency of particular phenotypes over many generations. By looking at information from previous generations, it should become evident what type of mode of inheritance is governing how the phenotype is being inherited. For VCE Biology the main focus is on X linked recessive and dominant and autosomal dominant and recessive inheritance.
Notation of a pedigree
When drawing a pedigree it is always important to include a legend, to demonstrate what your symbols mean. There are standardized symbols that are regularly used as indicated below. However, sometimes in VCAA exams they are not so nice to let you know which individuals are carriers!
Note also that roman numerals are used to indicate the generations and the regular numbers to indicate the individuals. It is also important to note that whenever providing evidence to support a mode of inheritance always refer to individuals by their generation (Roman numeral) and individual number.
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Using pedigrees to analyse mode of inheritance
- Males and females are equally affected.
- All affected individuals must have at least one affected parent.
- Two heterozygous parents can have an unaffected child.
- Transmission of the trait is possible from fathers to daughters and sons or from mothers to daughters and sons.
- Examples of diseases that follow autosomal dominant inheritance include Huntington’s disease and Alzheimer’s disease.
- Both males and females are equally affected.
- Two unaffected heterozygous parents can have an affected child.
- Two recessive parents can only have a recessive offspring.
- An affected daughter can have an unaffected heterozygous father.
- Examples of diseases that follow autosomal recessive inheritance include Albinism and Cystic Fibrosis.
X linked dominant
- An affected male will pass on the trait to his daughters that all must be affected.
- An affected father cannot pass trait to sons.
- An affected female may pass the trait to daughters or sons.
- An affected person must have at least one affected parent.
- Examples of diseases that follow X-linked dominant inheritance include Rett’s syndrome and Fragile X syndrome.
X linked recessive
- All sons of an affected mother have the disease.
- All offspring, from a mating of two individuals with the trait will display the trait phenotype.
- The father of an affected female must show the trait.
- An affected son can have an unaffected mother.
- Examples of diseases that follow X- linked recessive inheritance include Red /green colour blindness and haemophila.
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