6.1 Basis of germline inheritance and investigation of inherited disease

Question 1

Describe the organisation of DNA.

Answer 1

• DNA is found in uncoiled strands
• During cell division (mitosis and meiosis) the nuclear membrane disappears and chromosomes condense and become visible

Question 2

What is a karotype?

Answer 2

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).

Question 3

What is an autosome?

Answer 3

All chromosomes are called autosomes except for the sex chromosomes.

Question 4

What is the centromere?

Answer 4

The constricted region of a chromosome. The place where the mitotic machinery grabs hold of the 2 daughter chromatids.

Question 5

What are the telomeres?

Answer 5

The DNA rich ends of the chromosome.

Question 6

What are the 2 daughter chromatids?

Answer 6

A chromatid is one of 2 identical halves of a replicated chromosome.

Question 7

What are the 4 DNA bases?

Answer 7

Pyrimidines: thymine and cytosine

Purines: adenine and guanine

Question 8

Describe DNA transcription and translation.

Answer 8

Transcription: synthesis of mRNA from DNA in the nucleus
Translation: codons translated to an amino acid by a ribosome to form polypeptides

Question 9

Which codon and subsequent amino acid start the translation of all proteins?

Answer 9

AUG -> methionine

Question 10

Describe DNA mutations, what 2 types exist?

Answer 10

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

Question 11

Name the main DNA mutation types.

Answer 11

• 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

Question 12

What are silent mutations?

Answer 12

A base pair change that does not change the amino acid seuqence. Often not of functional significance. Also called synonymous mutations.

Question 13

What are missense mutations?

Answer 13

Change in 1st or 2nd letter of codon, changes the amino acid.
Can be deleterious, neutral or (rarely) advantageous.

Question 14

What are nonsense mutations?

Answer 14

Change of amino acid to a stop codon- produces a truncated protein.

Question 15

What are frameshift mutations?

Answer 15

Insertion or deletion of base pairs, usually producing a stop codon downstream, usually creating a shortened protein.

Question 16

What are point mutations?

Answer 16

Refers to any change in a single base pair (can be missense, nonsense, frameshift, deletion or insertion).

Question 17

What are alleles?

Answer 17

Different versions of a gene, which are different due to variation (mutation) e.g. blood type, hair pigment.

Question 18

What is the difference between homozygous and heterozygous?

Answer 18

Homozygous: carrying 2 of the same allele.
Heterozygous: 2 alleles are different.

Question 19

What are the 2 main differentiations when considering patterns of single-gene inheritance?

Answer 19

• Dominant vs recessive
• Autosomal vs X-linked

Question 20

Describe autosomal recessive inheritance.

Answer 20

• 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

Question 21

What is consanguinity?

Answer 21

In autosomal recessive inheritance, the 2 parents are disantly related making recessive disease more likely in their offspring.

Question 22

Describe autosomal dominant inheritance.

Answer 22

• 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

Question 23

Autosomal dominant disease usually appears in multiple generations, except for in which 3 cases?

Answer 23

• 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

Question 24

Describe co-dominant inheritance.

Answer 24

• Where 2 different alleles are present and are both expressed
• E.g. ABO blood system: people can have AB blood type

Question 25

Describe X-linked inheritance.

Answer 25

• 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

Question 26

What is the germline?

Answer 26

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.

Question 27

What is the difference between genomics and epigenome?

Answer 27

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.

Question 28

Why are genetic tests carried out?

Answer 28

• 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

Question 29

Name 3 cytogenic testing techniques.

Answer 29

• Karotype
• FISH
• Array-CGH

Question 30

Name 3 molecular genetic testing techniques.

Answer 30

• MLPA (being phased out)
• DNA sequencing
• NGS

Question 31

Describe the FISH technique.

Answer 31

• Fluorescent in situ hybridisation
• Looks at sub-microscopic regions of chromosome using visible light

Question 32

Describe the array-CGH technique.

Answer 32

• 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

Question 33

Describe DNA sequencing.

Answer 33

• 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

Question 34

Describe next generation sequencing (NGS).

Answer 34

• Uses nanopore sequencing techniques
• Looks at overlaps
• Very complex and wide scale

Question 35

What are the clinical consequences of DNA variants identified through genetic testing?

Answer 35

• 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)

Question 36

What are SNPs?

Answer 36

Single nucleotide polymorphisms- a single letter change in DNA

Question 37

Where is SNP testing used?

Answer 37

• Consumer testing e.g. 23andme, ancestry.com
• BRCA1 and 2 gene testing in the NHS