SINGLE GENE DISORDERS - MENDELIAN INHERITANCE

Question 1

What are the three traits that autosomal dominant conditions can result in?

Answer 1

• Variable expressivity - variation in clinical features of disorder from person to person
• Reduced penetrance - a heterozygote showing no clinical signs of it
• Sex limitation - some signs presented in only one sex

These are exceptions to the usual rules of autosomal dominant inheritance

Question 2

What are single gene disorders?

Answer 2

Mutations in single genes which often causes a loss of function

Question 3

What will be seen in a pedigree with an autosomal dominant condition?

Answer 3

• Affects multiple generations
• Affects males and females
• All forms of transmission including male to male

Question 4

What is the main symptom of Marfan syndrome?

Answer 4

Affects aorta and stretches aorta - a connective tissue disorder. This is because of a mutation in the gene for fibrin

Question 5

What is the chance of a parent passing on an autosomal dominant condition?

Answer 5

50%

Question 6

What is age dependant penetrance?

Answer 6

This is where there is a delay in the onset of symptoms for a disorder which can make a person appear to not have the disorder even though they have the genotype

Question 7

Give an example of age dependant penetrance

Answer 7

Huntington disease - a progressive neurological disorder which results in involuntary movements, dementia and psychiatric disturbance. Symptoms don’t often develop until 40 years old. This is also incomplete penetrance as not 100% of the population with the disease show symptoms

Question 8

Give an example of a illness caused by new mutations

Answer 8

Achondroplasia - increase in paternal age increases likelihood of mutations in sperm cell meaning neither of the parents have the condition but the offspring does

Question 9

Who is more likely to be affected by x- linked recessive conditions?

Answer 9

Boys

Question 10

What do pedigrees for X- linked dominant cases look like?

Answer 10

• Females more likely to be affected than males
• No male to male transition

Question 11

What is mitochondrial inheritance?

Answer 11

• Exclusively maternal inheritance
• Mitochondrial DNA inherited from mothers

Question 12

What type of condition is myotonic dystrophy?

Answer 12

Autosomal dominant

Question 13

What is anticipation?

Answer 13

• Unstable expanding trinucleotide repeat mutation
• In successive generations:
  age of onset of genetic condition is lowered and/or severity of phenotype is increased