L22: Autosomal recessive conditions

Question 1

Is autosomal reccessive conditions heterozygote or homozygous

Answer 1

Homozygous as you need 2 copy of the recessive allele to inherit the condition

Question 2

What are the chances of two carrier parents with an affected child

Answer 2

25%

Question 3

What is the pedigree of an autosomal condition like

Answer 3

• siblings affected
• both genders affected
• may be consanguinity
• not all generations are affected

Question 4

What are the common autosomal recessive conditions

Answer 4

• cystic fibrosis
• recessive mental retardation
• congenital deafness
• spinal muscular dystrophy

Question 5

What is cystic fibrosis

Answer 5

Impacts many organ systems

• lungs
• heart
• sterility in males
• pancreas

Question 6

What is cystic fibrosis caused by

Answer 6

A mutation in the CFTR gene

Question 7

What does the CFTR gene code for

Answer 7

Membrane bound chloride channel in the epithelia

Question 8

How does the CFTR mutation differ in population

Answer 8

Ethnicity groups can have different mutations in the CF gene to cause CF

Question 9

What is spinal muscular atrophy

Answer 9

• muscle wasting

Question 10

What is spinal muscular dystrophy caused by

Answer 10

Mutation in the survival motor neurone gene (SMN gene)

Question 11

What are the exceptions to Mendelian inheritance for autosomal recessive

Answer 11

• genetic heterogeneity

- pseudodominance

Question 12

What is genetic heterogeneity

Answer 12

When defects in more than one gene can cause a disorder

Question 13

Give an example of genetic heterogeneity

Answer 13

Sensorineural deafness. If both parents have different mutations of the deafness the child will be a carrier

Question 14

What is pseudodominance

Answer 14

When a carrier and affect parent mate so chance has 50 percent of being affected not 25

Question 15

How do we determine the frequency of carrier in the population

Answer 15

By hardy Weinberg principle

Question 16

When is the hardy Weinberg principle only true

Answer 16

• random mating
• large population
• no selection of genotypes
• no new alleles

Question 17

How can random mating in the hardy Weinberg principle be disrupted

Answer 17

By consanguinity as the overall risk increases to 5%

Question 18

What is heterozygote advantage

Answer 18

When carrier are resistant to some disease/infection

Question 19

Two example of heterozygote advantage

Answer 19

• sickle cell resistant to malaria

- cystic fibrosis resistant to malaria

Question 20

What are the application of clinical genetics for autosomal recessive ssive conditions

Answer 20

• risk counselling
• prenatal diagnosis
• prenatal risk assessment

Question 21

What is risk counselling

Answer 21

When there is an autosomal recessive affected individual compared to a unaffected person to see if they have they are a carrier by screening

Question 22

What is prenatal diagnosis

Answer 22

1) collect source of foetal tissue: chronic villus sample

2) Analyse source : direct mutation analysis