18 Genetic disorders Flashcards
General rules for pedigree diagrams
- Paternal lineage on right
- Maternal lineage on left
- Offspring birth order generally left to right, denoted in Arabic numerals
- Each generation on separate line, denoted in roman numerals
How much DNA do siblings and parents share with you?
50%
How much DNA do grandparents and aunts and uncles share with you?
25%
What is considered consanguineous mating?
If 2nd cousins or closer (3.13% shared DNA)
What complications may arise when using pedigree?
- May not have information on all or many relatives
- Family history may not be correct
Age of onset
Some diseases develop with age due to accumulation of damage
Mitochondrial inheritance
- Mitochondria have multiple copies of genomes, some normal, some mutant
- Offspring inherit different numbers of mutated mitochondria from mother
What is risk in pedigree?
Calculation of the predicted chance of having the disease or being a carrier
What is hereditary haemochromatosis?
An autosomal recessive disease, caused by a mutation in human homeostatic iron regulator protein (HFE)
What does the mutation in hereditary haemochromatosis cause?
- Excess iron absorption
- Humans cannot excrete iron so builds up in tissues
- Causes tiredness
Why draw pedigree?
- Clear, simple summary
- Spot patterns easily
- Explain pattern to patient
Why not test everyone?
- Causative gene often not known
- Many diseases mix familial and sporadic
- Impossible to test everyone
When the disease is exclusively familial why may a patient not want to test?
They may not want absolute certainty e.g. >80% of people with 50% chance of carrying Huntington’s decline test
Where does the pedigree tree start?
At the bottom with the affected person and siblings
