Pedigrees And Risk Flashcards
What is standard notation on a pedigree diagram
Males: square
Females: circle
Affected:solid
Unaffected: empty
Carrier: half coloured
Carrier x linked: like a bulls eye (only women)
Deceased: crossed through
Proband (seeks medical help): arrow
How are relationships shown in diagrams?
Couples by a horizontal line
Offspring by vertical lines
Identical twins, like an A coming down
Non identical twins, like an upside down V
Consanguineous couples, two horizontal lines
What percentage of DNA is shared between generations?
Siblings share 50% of their DNA
Parents and offspring 50%
Children and grandparents 25%
Aunts and uncles of children 25%
First cousins 12.5%
Why do we draw pedigree diagrams?
Spot patterns
Predict carriers
Calculate risk
Explain to patient
How do you draw pedigree diagrams?
Start with proband and siblings
Choose a parent as aks about their siblings and parents
Do the other parent
Don’t forget possible second marriages…
What are some potential difficulties with drawing pedigree diagrams?
Incomplete information or misremembered
Family history may not be correct
What are the characteristic of an autosomal dominant diagram?
Vertical transmission
Not all offspring affected
Males and females affected equally
At least one affected parent (m/f)
50% risk of inheritance
What does an X linked (recessive) diagram look like?
Not all generation affected
Not all offspring affected
Only males are affected
Males 50% chance of being affected by carrier mother. Daughters 50% chance of being a Carrier
Affected fathers don’t have affected sons. But all their daughters will be carriers
What does a mitochondrial pattern diagram look like?
Vertical transmission
All generations affected
Males and females affected
INHERITED FROM AFFECTED MOTHER as mitchondrial DNA inherited from mother
What is risk?
Calculation of the predicted chance of having the disease or being a carrier
Work from the closest relative with a known phenotype to the proband
Multiply the risks together to get a final risk
Phenotype and disease characteristics should be taken into account. Also sometimes ethnic background
