Patterns of Inheritance

Question 1

Marfan’s sydrome is caused by a defect in the fibrillin 1 gene, found on which chromosome? This syndrome is an example of what type of inheritance?

Answer 1

Chromosome 15

Autosomal dominant

Question 2

In genetics, what are penetrance, expressivity and anticipation?

Answer 2

• Penetrance - the extent to which a particular gene or set of genes is expressed in the phenotype
• Expressivity - variation in the severity of a disorder in individuals with the same mutation
• Anticipation - worsening of disease severity in successive generations - characteristically occurs in triplet repeat disorders

Question 3

An unaffected child has two parents who are both carriers for cystic fibrosis. What is the probability that the child is also a carrier?

Answer 3

2/3

Question 4

What is X inactivation?

Answer 4

X-inactivation - in female early development 1 X chromosome is inactivated in each cell:

• Generally random - each X chromosome inactivated in ~50% of cells
• ~10% of women have skewed X-inactivation - 80:20 ratio or higher
• X-inactivation is a form of mosaicism

Question 5

How many genes are housed in the small, circular mtDNA?

Answer 5

37

Question 6

Why is mtDNA inherited maternally?

Answer 6

Oocytes contain many mitochondria, sperm cells do not

Question 7

What are homoplasmy and heteroplasmy?

Answer 7

• Homoplasmy - one type of mtDNA
• Heteroplasmy - more than one type of mtDNA