Genetics

Question 1

When do dominant genes express the phenotype?

Answer 1

They express the phenotype even when seen in a heterozygous state

Question 2

What’s an example of a disorder caused by dominant genes? What is the onset?

Answer 2

Huntington’s disease

A neurodegenerative disease with an onset of around 40 years

Question 3

When do recessive genes express the phenotype?

Answer 3

Express the phenotype only when seen in homozygous state

Question 4

What’s an example of a disorder caused by recessive genes?

Answer 4

Inability to metabolise phenylamine; resulting in buildup of phenylamine in brain
Screening at birth
Controllable by diet
Can skip generations

Question 5

What are some developmental disorders caused by genes?

Answer 5

Down’s syndrome
Fragile X
Rett syndrome

Question 6

What is Down’s Syndrome caused by?

Answer 6

Extra copy of chromosome 21

Question 7

Prevalence of Down’s Syndrome

Answer 7

Prevalence: 1 in 1000 births

Question 8

Features of Down’s Syndrome

Answer 8

Distinctive facial features
Eye folds
Flat face
Incidence of heart malformation - often need heart surgery early in life
Low tone - difficulty feeding, gross motor delays
Long term health issues - more likely to develop leukaemia before the age of 2
High co-incidence of early onset Alzheimer’s (until 65)

Question 9

What causes Fragile X?

Answer 9

Lengthening of the FMRI gene on X chromosome

Question 10

Prevalence of Fragile X

Answer 10

Prevalence: 1 in 3600 males, 1 in 4000-6000 females

Question 11

Features of Fragile X

Answer 11

Physical features not as distinctive - less likely to be diagnosed at birth
- Long, narrow faces
- Big forehead
- Large ears
- Low tone leading
- Speech milestones delayed
Social emotional characteristics
- Anxious
- Leading to repetitive behaviours/stereotypies
- Children often referred because symptoms resemble autism
- 5% of children diagnosed with autism actually have fragile X

Question 12

Life expectancy of Down’s syndrome

Answer 12

60 years

Question 13

What causes Rett’s syndrome?

Answer 13

A mutation of MECP2 gene on X chromosome

Question 14

Prevalence of Rett’s syndrome?

Answer 14

1 in 10,000 girls (almost exclusively occur in girls)

Question 15

Features of Rett’s syndrome?

Answer 15

Normal development until 6-18 months, then development slows
Regression: 1-4 years
- Loss of purposeful use of hands; wringing/washing motions
- Less eye contact
- Feeding problems because motor abilities starting to regress
- Not diagnosed at birth
- Lose language
- Become gradually less social
- Seizures
From 10, lose movement
Life expectancy about mid-40s

Question 16

What is epigenetics?

Answer 16

Heritable changes in how genes are expressed

Question 17

Hallmark study demonstrating how the changes in which genes are expressed are heritable

Answer 17

Michael Meaney’s study of rat mums

Question 18

Meaney’s study on rats?

Answer 18

Arch back nursing - lifting body so all pups can nurse
How experiences with good mums can impact offspring and their subsequent offsprings by changes in genetic expression
Patterns in good mum behaviour passed down
- Not learned behaviour/modelling, but because experiences as infants changed expression of genes that get passed on
- Licking/grooming - arousal circuits activate - by molecular mechanisms activate serotonin in hippocampus
- expression of genes depend on methyl groups; stick to particular genes to silence them
- extra serotonin - can knock the methyl group ‘blocks’ off which are blocking the genes for ‘maternal behaviour’ - allows gene to be expressed
- pattern of methylation passed on despite no change in DNA

Question 19

In terms of sex determination in humans, what happens during the first 7 weeks?

Answer 19

XX and XY look the same.
There are both Mullerian (precursor to F organs) and Wolffian (M) ducts
Can go either way

Question 20

In terms of sex determination in humans, what happens during weeks 7-8?

Answer 20

Changes happen which determine sex.

A gene on the Y chromosome causes testes to develop

• testosterone produced
• this early exposure makes Wolffian ducts into external male organs, Mullerian absorbs

If XX, in the absence of testosterone Wolffian is absorbed and Mullerian develops into ext and int female genitalia

Question 21

By ___ weeks we can distinguish sex

Answer 21

12

Question 22

Examples to disruptions to sex determination?

Answer 22

Androgen insensitivity syndrome (AIS)
- XY genetically, appear female
Congenital Adrenal Hyperplasia (CAH)
- XX genetically, appear male

Question 23

What is Androgen Insensitivity Syndrome?

Answer 23

• XY genetically but tissues do not respond to testosterone
• Turn out looking like women
• In absence of sensitivity - have female genitalia, but don’t have internal female organs

Question 24

What is congenital adrenal hyperplasia (CAH)?

Answer 24

• XX genetically but exposed to excessive levels of testosterone prenatally (can be due to mother naturally having high T, supplement T, etc)
• Develop external genitalia that appears to be male
  
  • Varies according to level of T