Phenotypic Variability Flashcards
What is meant by phenotypic variability?
variation of gene expression in the same disease in different individuals, such that individuals with the same genetic disease may not have the same symptoms or presentations
What are the 5 reasons as to why phenotypic variablity exists? MUMES
Mutations
Unstable
Modifiers
Environment
Sex
What is meant by modifiers?
When the expression of one gene directly affects the presence of other genes
What is meant by the effects of mutations on the phenotype of a disease?
Different subtypes of the same disease have different presentations, due to mutations within the same gene
What is an example of how sex affects the phenotype of a disease?
In hereditary haemochromatosis - women lose blood through mensturation and therefore generally have lower iron levels
What is an example of how mutations affect the phenotype of a disease?
Duchenne vs Becker muscular dystrophy - slightly different mutations of the same gene cause different presentations
What is an example of how modifiers affect the phenotype of a disease?
In CF, the variety of genes present can increase the degree of CF
What are trinucleotide repeat disorders?
Where an increased number of nucleotide repeats (>27) cause more severe phenotypes throughout generations
What affect do trinucleotide repeats have on the proteins produced?
Changes protein function
What does a larger number of trinucleotide repeats result in?
More severe phenotype and earlier onset of disease
If you have less than how many TNR’s is the mutation considered stable?
Less than 27
If individuals are identical, and there is variation in the phenotype, what is likely to have caused the variation?
The environment
In non-identical individuals with the same mutation, what could cause the variation in phenotype?
The presence of other genes as well as the environment
In unrelated individuals, how does phenotypic variability arise?
Due to unstable mutations like TNRs
If there is a binary presentation of a condition eg half a family have a condition and half don’t, what is likely to cause the variation?
The presence of a second gene modifying the disease phenotype
If there are two forms of the same disease, and one is a milder phenotype, what is likely to cause the variation?
Mutations - Different mutations of the same gene results in different diseases
When there is wide variation in a condition but the variation does not apply to monozygotic twins, what is likely to be causing the variation?
A second gene which the twins did not inherit
When there is marked variability and no two members being affected in the same way, what is likely to cause the variation?
Environment
When there is a large difference in the age of onset between family members, what is likely to cause this variation?
Unstable mutations
What cause of variation is more likely to have transgenerational effects?
The expansion of unstable trinucleotide repeats
