How are males represented on pedigree diagrams?
square
How are females represented on pedigree diagrams?
circle
How are affected individuals represented on pedigree diagrams?
black square/circle
How are carriers of autosomal disorders represented on pedigree diagrams?
half black half white square/circle
How are (female) carriers of X-linked disorders represented on pedigree diagrams?
filled circle w/in white circle
How are deceased people shown on pedigree diagrams?
line through them
How are probands shown on pedigree diagrams (person consulting you)?
arrow pointing to square/circle
How are couples represented on pedigree diagrams?
linked by horizontal line
How are offspring represented on pedigree diagrams?
linked to parents by vertical line
How are dizygotic and monozygotic twins represented on pedigree diagrams?
dizygotic: two lines in a triangle shape
monozygotic: two lines with a small triangle above
How are consanguineous matings shows on pedigree diagrams?
linked by double line
consanguineous if 2nd cousins or closer
2nd cousins share 3.13% DNA
Why do we draw pedigree diagrams?
- provides a clear, simple summary of info
- able to spot patterns easily
- explain pattern to patient
- calculate risk of passing on disease or being a carrier
- allows informed choice
What are complicating factors with pedigree diagrams?
- age of onset e.g. Huntington’s
- -> someone may die before developing disease–> hole in pedigree
N.B. more repeats means more severe disease
What are complicating factors with mitochondrial inheritance?
- offspring inherit different numbers of mutated mitochondria from mother
- different tissues/cells can contain different numbers and this can change with time
- some diseases develop w/ age due to accumulation of mutated mitochondria
What could a sporadic inheritance pattern indicate?
- non-genetic disorder
- chromosomal abnormality
What is risk?
a calculation of the predicted chance of having the disease or being a carrier
How do we calculate risk?
- work from person with known phenotype
- multiply risks together
- take account of all information: phenotype, disease characteristics (e.g. age of onset), and family distribution (e.g. mitochondrial)
- consider ethnic background e.g. sickle cell
If a couple who are known to both be carriers of a recessive disease have an unaffected child- what is the risk of their child being a carrier?
2/3 as we know child is unaffected so we remove 1 possibility
(Bayesian statistics)
If a woman whose paternal uncle has an X-linked recessive disease, what is the risk of her having an affected son?
population risk bc her father is not affected and no one affected in maternal family
