Genetics Flashcards
When do dominant genes express the phenotype?
They express the phenotype even when seen in a heterozygous state
What’s an example of a disorder caused by dominant genes? What is the onset?
Huntington’s disease
A neurodegenerative disease with an onset of around 40 years
When do recessive genes express the phenotype?
Express the phenotype only when seen in homozygous state
What’s an example of a disorder caused by recessive genes?
Inability to metabolise phenylamine; resulting in buildup of phenylamine in brain
Screening at birth
Controllable by diet
Can skip generations
What are some developmental disorders caused by genes?
Down’s syndrome
Fragile X
Rett syndrome
What is Down’s Syndrome caused by?
Extra copy of chromosome 21
Prevalence of Down’s Syndrome
Prevalence: 1 in 1000 births
Features of Down’s Syndrome
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)
What causes Fragile X?
Lengthening of the FMRI gene on X chromosome
Prevalence of Fragile X
Prevalence: 1 in 3600 males, 1 in 4000-6000 females
Features of Fragile X
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
Life expectancy of Down’s syndrome
60 years
What causes Rett’s syndrome?
A mutation of MECP2 gene on X chromosome
Prevalence of Rett’s syndrome?
1 in 10,000 girls (almost exclusively occur in girls)
Features of Rett’s syndrome?
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
What is epigenetics?
Heritable changes in how genes are expressed
Hallmark study demonstrating how the changes in which genes are expressed are heritable
Michael Meaney’s study of rat mums
Meaney’s study on rats?
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
In terms of sex determination in humans, what happens during the first 7 weeks?
XX and XY look the same.
There are both Mullerian (precursor to F organs) and Wolffian (M) ducts
Can go either way
In terms of sex determination in humans, what happens during weeks 7-8?
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
By ___ weeks we can distinguish sex
12
Examples to disruptions to sex determination?
Androgen insensitivity syndrome (AIS)
- XY genetically, appear female
Congenital Adrenal Hyperplasia (CAH)
- XX genetically, appear male
What is Androgen Insensitivity Syndrome?
- 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
What is congenital adrenal hyperplasia (CAH)?
- 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
