6.1 Basis of germline inheritance and investigation of inherited disease Flashcards
Describe the organisation of DNA.
- DNA is found in uncoiled strands
- During cell division (mitosis and meiosis) the nuclear membrane disappears and chromosomes condense and become visible
What is a karotype?
Describes an individuals complete set of chromosomes using stain and other techniques to organise and visualise the chromosomes.
Chromosomes numbered in size from 1 (largest) to 22 (smallest).
What is an autosome?
All chromosomes are called autosomes except for the sex chromosomes.
What is the centromere?
The constricted region of a chromosome. The place where the mitotic machinery grabs hold of the 2 daughter chromatids.
What are the telomeres?
The DNA rich ends of the chromosome.
What are the 2 daughter chromatids?
A chromatid is one of 2 identical halves of a replicated chromosome.
What are the 4 DNA bases?
Pyrimidines: thymine and cytosine
Purines: adenine and guanine
Describe DNA transcription and translation.
Transcription: synthesis of mRNA from DNA in the nucleus
Translation: codons translated to an amino acid by a ribosome to form polypeptides
Which codon and subsequent amino acid start the translation of all proteins?
AUG -> methionine
Describe DNA mutations, what 2 types exist?
A mutation is a change in the DNA base pair sequence.
2 types:
- Polymorphisms: common DNA sequence changes (may alter protein function but not common)
- Disease associated mutations: tend to alter protein function
Name the main DNA mutation types.
- Silent mutation
- Missense mutation
- Nonsense mutation
- Frameshift mutation
- Point mutation
- Mutations in regulatory regions
- Large deletions or duplications (millions of bases)
- Translocations and inversions
- Extra or missing chromosome
What are silent mutations?
A base pair change that does not change the amino acid seuqence. Often not of functional significance. Also called synonymous mutations.
What are missense mutations?
Change in 1st or 2nd letter of codon, changes the amino acid.
Can be deleterious, neutral or (rarely) advantageous.
What are nonsense mutations?
Change of amino acid to a stop codon- produces a truncated protein.
What are frameshift mutations?
Insertion or deletion of base pairs, usually producing a stop codon downstream, usually creating a shortened protein.
What are point mutations?
Refers to any change in a single base pair (can be missense, nonsense, frameshift, deletion or insertion).
What are alleles?
Different versions of a gene, which are different due to variation (mutation) e.g. blood type, hair pigment.
What is the difference between homozygous and heterozygous?
Homozygous: carrying 2 of the same allele.
Heterozygous: 2 alleles are different.
What are the 2 main differentiations when considering patterns of single-gene inheritance?
- Dominant vs recessive
- Autosomal vs X-linked
Describe autosomal recessive inheritance.
- Males and females equally likely to be affected
- Affected individuals are homozygous for disease alleles
- Parents of an affected child are carriers, they are heterozygous for 1 disease allele and 1 normal allele, they are usually healthy
- E.g. cystic fibrosis
What is consanguinity?
In autosomal recessive inheritance, the 2 parents are disantly related making recessive disease more likely in their offspring.
Describe autosomal dominant inheritance.
- Males and females equally likely to be affected
- Child of an affected parent has a 50% risk of inheriting the disease
- E.g. Huntington’s
Autosomal dominant disease usually appears in multiple generations, except for in which 3 cases?
- De novo = new mutation
- Variable expressivity = the degree to which trait expression differs between individuals
- Regulated penetrance = some individuals will inherit the disease variant but will never show symptoms
Describe co-dominant inheritance.
- Where 2 different alleles are present and are both expressed
- E.g. ABO blood system: people can have AB blood type
Describe X-linked inheritance.
- Conditions caused by genes on the X chromosome
- Males more likely to be affected
- An affected man will pass the mutation to all of his daughters but none of his sons
- For female carriers, 50% of sons are affecetd and 50% of daughters will be carriers
What is the germline?
The cells in an organsim that have the potential to pass genetic material onto future generations.
Lineage of cells passing from zygote to testes/ovaries.
Opposite of germline = somatic.
What is the difference between genomics and epigenome?
Genomics = genetics on a greater scale, sum of phenotype, gene variants, SNPs (individual points of variation) and environment.
Epigenome = extra layer above the genome, subtle signals that instruct the genome.
Why are genetic tests carried out?
- Diagnostic testing (confirmation of diagnosis)
- Mutation detection (diagnosis known but underlying mutation unknown)
- Predictive testing (e.g. if pt is at risk of inheriting a gene due to family member having disease)
- Prenatal testing
Name 3 cytogenic testing techniques.
- Karotype
- FISH
- Array-CGH
Name 3 molecular genetic testing techniques.
- MLPA (being phased out)
- DNA sequencing
- NGS
Describe the FISH technique.
- Fluorescent in situ hybridisation
- Looks at sub-microscopic regions of chromosome using visible light
Describe the array-CGH technique.
- Microarray-based comparative genomic hybridisation
- Running many FISH probes at once
- Using flurorescence to measure the ratio of reference DNA to test DNA bound to each probe spot
Describe DNA sequencing.
- Technique used to sequence bases
- Used for the human genome project
- Used in clinical practice to diagnose single gene disorders e.g. ectodermal dysplasia, osteogenesis imperfecta
Describe next generation sequencing (NGS).
- Uses nanopore sequencing techniques
- Looks at overlaps
- Very complex and wide scale
What are the clinical consequences of DNA variants identified through genetic testing?
- Uncertain significance
- May be pathogenic or non-pathogenic
- Can cause worry for pt and relatives
- Risk of inappropriate use
- Tests have limited sensitivity and cannot find all disease-causing mutations, a negative result will generally not exclude a diagnosis
- A mutation may only be present in some of the patient’s cells (mosaicism)
What are SNPs?
Single nucleotide polymorphisms- a single letter change in DNA
Where is SNP testing used?
- Consumer testing e.g. 23andme, ancestry.com
- BRCA1 and 2 gene testing in the NHS
