Pedigrees and Risk Flashcards
Why would you not draw a pedigree for a disease gene?
- many diseases are a mix of familial and sporadic
- many causative gene isn’t known
- impossible to test everyone for disease genes
- when disease exclusively familial and mutation known many do not want to test
What are potential difficulties of pedigrees?
Incomplete info (may not have info on all or many relatives)
Incorrect info\Family history may not be correct
Important in clinical setting
Why draw a pedigree?
Provides a clear simple summary of information
Able to spot patterns easily
Explain pattern to patient
Identify potential carriers of risk gene
Calculate risk of passing on disease/ being a carrier
Many diseases mix of familial and sporadic
What is a pedigree chart?
A diagram which shows the presence or absence of a trait within a family across generations, used to analyse the pattern of inheritance through a family
What symbol is used to show a male?
Square
What symbol is used to show female?
Circle
What is used to show a person who is affected?
Coloured in circle
How is the difference between X linked Carrier and Autosomal carrier shown on a pedigree chart?
X linked carrier has a dot in the middle, autosomal has half the shape shaded
How is a consanguineous couple shown on a pedigree chart?
Double lines connecting the male and female
What is the difference between monozygotic and dizygotic twins?
Monozygotic are identical, dizygotic are not (50% shared)
If only females are affected, which type of inheritance is it ilkely to be?
Mitochondrial
If only males are affected, which type of inheritance is it likely to be?
Y linked
If unaffected parents can have affected children, is the pattern of inheritance for that condition likely to be dominant or recessive?
Recessive
If affected parents can have unaffected children, is the pattern of inheritance for that condition likely to be dominant or recessive?
Dominant
If the condition is more common in females, which type of inheritance does it tend to be?
X-linked dominant - affected males transmit to all daughters
If the condition is more common in males, which type of inheritance is it likely to be?
X-linked recessive - affected females transmits to all sons
What is risk?
This is a calculation of the predicated chance of having the disease or being carrier
Why is calculating risk useful?
It allows parents to make an informed decision as to whether they want to start a family or not given the risk of certain conditions
Describe, in detail, the social and cultural issues related to
stigma and bias that might impact upon their decision when deciding whether or not to start a family
A recognition that the couple might be hesitant to seek medical advice at this
point, due to a sense of shame/stigma surrounding sickle and a desire not to mark
oneself out as ‘different’ or ‘disadvantaged’ (1 mark).
Recognition of the potential unconscious bias of the medical professional when
carrying out a consultation (1 mark).
Mention of the historical roots of such stigma (1 mark).
Students might cite the scientific racism of the 19th century that argued for
quantifiable biological and anatomical differences between the black and white
What information do you have to take into account for risk?
Phenotype
Disease characteristics
Family distribution for X-linked and mitochondrial disease
Which side of family disease is on
Which parent has the disease
Ethnic background
Founder effect
biological sex
(All info)
What is the Founder effect?
the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population
Which continent is the area with the highest prevalence of sickle cell trait?
Asia
Many diseases have different prevalence in different populations
What is hereditary haemochromatosis?
Hereditary (familial) haemochromatosis autosomal recessive disease
Mutation in human homeostatic iron regulator protein (HFE)
Result is excess iron absorption. Humans cannot excrete excess iron so builds up in tissues
Wide range of symptoms including tiredness
Give examples of risk modifiers. (3)
Which side of the family the disease is on
- x-linked, mitochondrial
Ethnic background
- different prevalence in different populations
- heterozygote advantage
- founder effect
Information you know about the person
- their phenotype
- biological sex
