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Intro to Genetics - Introduction to Human Genetics

39 important questions on Intro to Genetics - Introduction to Human Genetics

List the 6 genetic origins of disease

Recessive gene, single dominant gene, polygenic inheritance, sex-linked inheritance, chromosomal anomalies and regulator gene defects

What is recessive gene (genetic origins of disease)?

Both parents need to have that recessive gene for you to inherit, so it's quite rare

You need 2 copies of the genetic marker (1 from each parent) for it to show in the phenotype

What is single dominant gene (genetic origins of disease)?

Where you have one particular gene marker (usually on one of the autosomes, though not always)

Dominant in expression — you only need to inherit one copy of the two available gene markers for that disorder

Only one parent needs to have that dominant gene — when you inherit that gene, you will develop the disorder

What is polygenic inheritance (genetic origins of disease)?

Many genes involved that each have a very small effect size.

What is sex-linked inheritance (genetic origins of disease)?

Gene marker for these disorders are either on the X chromosome or the Y chromosome

What is chromosomal abnormalities (genetic origins of disease)?

Have an extra whole chromosome

Example: Down syndrome (extra 21st chromosome)

What is regulator gene defects (genetic origins of disease)?

Defects in regulator genes which are involved in helping the gene express itself in the translation and transcription processes

What is the gene mutation in Tay-Sachs disease?

Caused by mutations of HEXA gene on chromosome 15q23-q24.

What happens in Tay-Sachs disease?

The gene tells the body to stop producing this HEXA enzyme that is needed to breakdown a fatty substance, so these fatty lipid starts to build up in the brain and spinal cord. And the brain starts to breakdown, so by the first year of life this child is developmentally delayed.

Not able to walk and talk, can't move, can't speak, and so just slowly breaks down.

For Tay-Sachs disease, are there some subpopulations within the community that have a higher risk?

Yes, because it is a genetic disorder

How did Tay-Sachs disease arise?

Population studies and pedigree analyses suggest that mutations may have arisen from small founder populations

Pedigree analysis = you look at the family tree.

What is the gene mutation in Huntington's disease? Where is it located?

More than 40 repeats of CAG (cytosine, adenine, guanine) trinucleotide in coding region of gene

It is normal to have these repeats, but if you have greater than 40 of these repeats, the person is going to develop Huntington's disease.

chromosome 4 at 4p16.3

What is the trinucleotide repeat (CAG) that is in healthy adults?

9-35

What happens to people who have 36 - 39 repeats of the trinucleotide?

They show reduced penetrance - only some individuals will develop clinical symptoms.

What is juvenile onset? (Huntington's disease)

Individuals with juvenile onset (Westphal variant) usually have expansions >55 repeats and develop Huntington’s disease before 20 years.

For Huntington's disease, when does onset appear to be earlier?

Onset appears to be earlier when the transmission is from the father.

Symptoms of Huntington's disease and mean onset

Cognitive deterioration, personality change, memory loss and depression, and choreic and slow movement (first indication)

Mean onset ~40 yrs, death 12-15 years afterwards

For Huntington's patients, how are movement characteristics characterised?

Involuntary movements:

Involuntary ‘dance-like’ choreic movement
Abates in advance stages of disease, and they become akinetic

Voluntary movements:

Bradykinetic (e.g., they will be slower if you ask them to walk)
Inconsistent and inefficient movement (e.g., abnormal contractions of their muscles)
Sequential and simultaneous movement difficulties (problem putting 2 sequences of motions together and doing two things at the same time)

Huntington's disease - Broad sub-cortical and cortical damage

Neuronal and astrocyte loss in the basal ganglia

Selective degeneration of GABAergic neurons of in the striatum of the basal ganglia

that explains all the movement difficulties that they're facing, because it's affecting the basal ganglia, which deals with movement control

As disease progresses, greater cortical atrophy occurs

that encompasses their cognitive and memory functioning

What is schizophrenia? Symptoms?

Fundamental and characteristic distortions of thinking, perception and affect that are inappropriate or blunted.

Not due to depression or manic symptoms unless it is clear that schizophrenic symptoms precede the affective disturbance; not due to drug intoxication or withdrawal

Psychopathological thoughts

Hallucinations
Paranoid or bizarre delusions
Disorganised speech and thinking leading to significant social and occupational dysfunction

Typical onset in schizophrenia

Young adulthood

In schizophrenia, are demonstration of symptoms continuous or episodic?

Both - continuous and episodic

Fragile X syndrome (genetic orgins)

Sex-linked inheritance

What is the gene mutation in Fragile-X syndrome?

Expansion mutation of a CGG repeat sequence in the FMR1 gene at the locus Xq27.3

Typically developing adults have around 30 copies of these trinucleotide repeats, but it can expand to over 200 copies in individuals affected with the full mutation.

What happens to people with Fragile X syndrome? (gene)

It does not code for the FMR1 protein product (silencing of the gene)

Absence of the FMR1 protein product - essential for synaptic plasticity, development of the shape of the brain, and cognitive development

Who are more affected by fragile x syndrome?

Boys are typically more severely affected than girls

For some disorders we can make simple changes in the _________ and no symptoms of the disorder will arise

Environment

Phenylketonuria - What is the defect?

Children with phenylketonuria (PKU) are unable to metabolize phenylalanine - amino acid found in high-protein foods and some artificial foods

PKU is a disorder that is related to a defective gene on chromosome 12, in which the body cannot break down phenylalanine . It builds up in the body and can cause problems with brain development

How is phenylketonuria prevented?

With early diagnosis and a properly restricted (phenylalanine-free) diet, however, mental retardation resulting from PKU can be avoided.

Can the environment can have an effect on the expression of the genome?

Yes, external factors can affect the switching on and off of genes. There is a continuous interaction between the environment and the genotype.

What is the definition of 'the range of reaction'?

It refers to all the phenotypes that could theoretically result from a given genotype, given all the environments in which it could survive and develop (three identical cuttings taken from 7 plants experiment)

Relationship between child and environment

The child influences their environment but the environment also influences the child

The child actively creates and explores the environment in which they live.

But by virtue of their nature, they evoke different responses from others.

Can the environment produce persistent alterations in the phenotype?

Yes, through altering gene expression. The environment has a role to play, in that the environment can have an effect on this translation process

What are the three different mechanisms involved in the regulation of genes?

Histone modification
DNA methylation
microRNA

What are autoimmune disorders?

The result of the immune system accidentally attacking your body instead of protecting it

What do monozygotic twin studies show for autoimmune disorders?

Monozygotic twin studies often show discordance of these diseases, suggesting a role for environmental factors contributing to disease development

Interplay between and factors may and autoimmune diseases

Genetic, environment, predispose, progress

Circulating immune cells are in _________ to environmental factors (Autoimmune diseases)

Constant exposure

What is the general risk for schizophrenia?

More likely to be continuous than categorical.

It's about the number of genetic risk markers that we have and the environmental influences that says whether we go on to develop some of these symptoms or not.

Risk is seen to be on a spectrum or continuous scale (low risk/high risk)
We all carry some degree of risk
Categorical = you have it or you don't

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PSYC20008

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