Genetics

Question 1

Pleiotropy

Answer 1

Multiple phenotypic expressions or disorders caused by a single gene.

E.g. Marfan syndrome causing aortic root weakening, skeletal stretching and lens dislocation.

Question 2

Heteroplasmy

Answer 2

The presence of a mixture of more than one type of mitochondrial DNA within a cell or individual. It is a factor for the severity of mitochondrial diseases.

Question 3

Locus heterogeneity

Answer 3

Variations in completely unrelated gene loci cause a single disorder.

E.g. retinitis pigmentosa can be caused by multiple genes (autosomal dominant, autosomal recessive and X-linked forms)

Question 4

Allelic heterogeneity

Answer 4

Different mutations within a single gene locus (forming multiple alleles of that gene) cause the same phenotypic expression.

E.g. One CFTR gene with multiple different mutations that all result in CF

Question 5

Lyonisation

Answer 5

Lyonisation (X inactivation) is the process by which one of the copies of the X chromosome is inactivated in female mammals. The X inactivation is random, so in the case of a female carrier of a mutation in a gene on the X chromosome, she may have some milder or later onset symptoms of an XLR condition depending on how
much of the normal chromosome is inactivated.

E.g. Fabry disease - X-linked recessive condition causing hypertrophic cardiomyopathy, chronic kidney disease and an increased risk of stroke. Females may develop all manifestations of the disease, although often later in life and with less severity than in men.