G2 +3- Single gene disorders Flashcards Preview

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Flashcards in G2 +3- Single gene disorders Deck (20)
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1
Q

what are the classifications of genetic disease?

A
• Chromosomal abnormalities
• Single gene disorders :
- Autosomal dominant
-Autosomal recessive
-X-linked (recessive and dominant) 
-Mitochondrial
•  Multifactorial traits (polygenic)
2
Q

What are autosomal dominant traits?

A
  • affected heterozygotes
  • 50% risk for offspring of affected individuals
  • males and females equally affected
  • variable expression
  • variable penetrance
3
Q

what is variable expression?

A

• Many genetic conditions have a range of clinical features
• Many affected people:
– only have some features
OR
– show some features to a varying degree

4
Q

what is variable penetrance?

A

The proportion of people with a genotype who show the associated phenotype

5
Q

what is age dependent penetrance?

A

The proportion of people with the predisposing genotype that show the associated phenotype increases with age

6
Q

what is type 1 osteogenesis imperfecta?

A

Mutation reduces rate of production of one of collagen subunits

7
Q

what are characteristics of osteogenesis imperfecta?

A
  • Brittle bones
  • Frequent fractures
  • Osteopenia (undermineralised bones)
  • Blue sclerae
  • Deafness
  • Abnormal dentition in some forms
8
Q

what is Type II osteogenesis imperfecta?

A
  • Mutation results in production of structurally abnormal collagen
  • New mutations because affected pregnancies do not survive
  • Low recurrence risk
9
Q

what condition is cystic fibrosis ?

A

autosomal recessive condition

10
Q

what are characteristics of autosomal recessive condition- cystic fibrosis?

A
  • Chronic obstructive lung disease (thick mucus)
  • Pancreatic insufficiency (viscous pancreatic juices)
  • Meconium ileus (bowel obstructed by thick viscous contents)
  • High sweat electrolytes (Na and Cl)
  • Infertility in males
  • Life expectancy reduced. Heart and lung transplant often needed in later life
11
Q

what is the risk of CF in children of carrier couple?

A

1/4

12
Q

Describe genetic testing for cystic fibrosis.

A

• over 500 different mutations described in CFTR gene
• p.Phe508del mutation accounts for 70-80% of CF mutation in UK population
• ~ 85% of CF carriers can be detected by testing
for 30 of the most common mutations found in the UK population

13
Q

Describe cascade screening for CF carriers.

A

relatives of affected children

14
Q

Describe population screening for CF carriers.

A

• pre-pregnancy
• early in pregnancy:
-couple screening
- mother first and father if mother is a carrier

15
Q

what are high risk populations in haemoglobinopathies (thalassaemia /sickle cell disease)?

A
  • Mediterranean
  • Africa
  • Indian subcontinent
16
Q

What are autosomal recessive traits?

A
  • homozygotes affected
  • heterozygous carriers usually asymptomatic
  • 25% recurrence risk for sibling of affected individuals
  • much lower recurrence risk for other relatives
  • high risk populations
  • consanguinity
17
Q

Describe Duchenne muscular dystrophy.

A
  • X-linked recessive
  • Incidence in males: 1/3000
  • Delay in motor development
  • Onset of weakness in pelvic and shoulder girdles
  • Wheelchair bound by 10-12 years
  • Death from involvement of respiratory and cardiac muscles
18
Q

Describe X-linked recessive traits?

A
  • male hemizygotes affected
  • female heterozygotes usually unaffected
  • 50% risk for sons of carrier females
  • 50% risk that daughters of carrier females will be carriers
  • all daughters of affected males are carriers
  • no male to male transmission
19
Q

Who is affected most by X-linked dominant trait?

A

males more severely affected than females

20
Q

Describe X-linked dominant traits.

A
  • hemizygous males and heterozygous females affected
  • more severe in males
  • X-linked dominant traits lethal in males
  • often “patchy” in affected females (random X inactivation)
  • 50% risk for sons of carrier females
  • 50% risk for daughters of carrier females
  • all daughters of affected males are affected
  • no male to male transmission