### Description

If a single relative has an inherited condition, then a patient may be concerned about his or her risk of developing that condition. Edwards developed a quick estimate for the risk to the patient that uses the population frequency of the condition and the relationship of the person to the affected relative. It is based on the assumption that each degree of relationship halves the correlation between relatives.

Relationship

Degree of Relationship

parent

1

child

1

sibling

1

uncle or aunt

2

nephew or niece

2

grandparent

2

grandchild

2

half-sibling

2

first cousin

3

great grandparent

3

great grandchild

3

half uncle or aunt

3

half nephew or niece

3

Degree of Relationship

Fraction of Genes Shared

1

0.5 (1/2)

2

0.25 (1/4)

3

0.125 ( 1/8)

maximum risk to a first degree relative =

= (population frequency) ^ (0.5)

maximum risk to a second degree relative =

= (population frequency) ^ (0.75)

maximum risk to a third degree relative =

= (population frequency) ^ (7/8) =

= (population frequency) ^ (0.875)

Limitations:

• This is the maximal risk.

• The calculation should not be used if more than one relative is affected.