Genotype, phenotype and inheritance part 2 Flashcards
Give an example of X-linked dominant inheritance
Fragile X syndrome - asscoaited with intellectual disability and autism and caused by a change in structure of X chromosome
Who is affected by X-linked dominant inheritance?
Hemizygous males (XY)
Heterozygous and homozygous females - only needed on one X
Can affected males give a trait to their sons in X-linked dominant inheritance?
No, because sons will only inherit Y chromosomes
In heterozygous carrier females, what is the chance of them having affected offspring in X-linked dominant inheritance?
50% chance
How is mitochondrial DNA inherited? What does this mean?
Passed via maternal line
Affected mother will always give rise to affected children, but unaffected mother never does, despite genotype of father
Example of a disease linked to mitochondrial DNA
Kaerns-Sayer syndrome - reduction of heart, eye and muscle movements
What is polygenic inheritance?
Phenotypes are influenced by multiple genes
Give an example of a polygenic trait
Albinism - human skin colour
Will two homozygous recessive albino parents of A gene always give albino offspring?
No, despite being recessive, albinism is polygenic so is controlled by multiple genes
What gene is albinism caused by?
Two paralogues of Agene - A1 and A2 (and alleles a1 a2)
When is albinism caused in the A genes?
When either gene is homozygous recessive - so either a1a1 or a2a2
Does a single gene often cause a phenotype?
No, multiple genes act together- usually a disease is triggered when a threshold is passed (common variants with small effect but come together)
What is genome wide association used to do?
Study complex diseases and find SNP’s that occur more often in disease carriers
What does a genome wide association study graph show us?
Shows the SNP’s that pass the threshold to help trigger a disease - in order to inherit the disease you would have to inherit all variants that pass the threshold
3 limitations of GWAS (genome wide association stud)
- Large sample size needed to reach statistical significance - hard for rare diseases
- Correlation does not equal causation - follow up studies needed
- Not all SNP’s are in coding genes
What does linkage mean?
The tendency of genes that are located close to each other on a chromosome to be inherited together during meiosis
Genes on different chromosomes are not linked
What does linkage increase the chance of?
Alleles are inherited together e.g AD on one chromosome and ad on another means that only AD and ad gametes will be formed
What is an important factor in linkage?
Distance
How does distance affect linkage?
Closer together on a chromosome = more likely to be inherited together
Genes far apart act as though on different chromosomes
What is distance measured in in linkage?
CentiMorgan (cM)
What are haplotype blocks?
Groups of genes found close together on a chromosome form haplotype blocks
What type of linkage is common in psychiatric disorders? Explain this linkage
Linkage disequilibrium - when loci on different chromosomes are inherited together
What is penetrance?
Describes whether there is clinical expression of a phenotype in a person
- percentage of people with a genotype that show a symptom
What is expressivity?
Describes the difference in clinical presentation between two people with the same genotype
- the severity of disease in different people
