Lecture 11 Way Beyond Mendel Flashcards
Define Epistasis
Epistasis occurs when one gene masks the effects of another gene.
How does dominant epistasis alter the F2 phenotypic ratio
12:3:1 ratio
How does recessive epistasis alter the F2 phenotypic ratio
9:4:3 ratio
Define Pleiotropy
Occurs when a single gene controls multiple phenotypes.
Give two reasons why not all individuals with the same genotype would express the same phenotype
1) incomplete penetrance
2) Variable expressivity
List the two factors responsible for varying phenotypic expression between individuals of a population
- Penetrance
- Expressivity
Define Penetrance
Penetrance is a measure of the percentage of individuals of a csertain genotype express the phenotype
Define Expressivity
Expressivity is a measure of
Explain what is meant by genomic imprinting
This is where one of the maternal or paternal allele is imprinted via an epigenetic tag and silenced. Once silenced, this allele can no longer be expressed.
Define Maternal imprinting
- The maternal allele is imprinted via addition of an epigenetic tag
- Maternal allele is silenced
- The paternal allele is preferentially expressed in the phenotype.
Define Paternal imprinting
- The paternal allele is imprinted via addition of an epigenetic tag
- Paternal allele silenced
- Only the maternal allele is preferentially expressed in the embryo.
Give an example of a maternally imprinted/silenced gene
The insulin growth factor 2 gene
Give an example of Maternal imprinting
- Insulin growth factor 2 gene in mice
- The IGF2 has 2 alleles, a mutant and wildtype.
- The maternal igf2 allele is silenced.
- The paternal allele is expressed only.
- The paternal allele is
Give examples of pleiotropies
Pulmonary ciliary disease that is due to a mutation in cilia - affects multiple systems
Achondroplasia - recessive mutant for dwarfism. Heterozygotes are recessive lethal mutations causing death, heterozygotes have dwarfism.
Yellow coat colour in mice
Sickle cell disease
Explain achondroplasia
Dwarfism due to premature transition of cartiliage to bone.
Give examples of recessive lethal mutationa
Yellow gene in mice (individuals homozygous for dominant yellow gene die)
Achondroplasia (individuals homozygous for dwarf gene will die)
Albinism is an example of ?
Recessive epistasis.
Give the genotype for individuals with albinism
cc
Recessive epistatic mutation. cc masks the effect of all other coat colour controlling genes.
Which allele determines the phenotype always in maternal imprinting of the IGF2 gene
The paternal allele will always determine the phenotype because the materanl allele is silenced and not expressed. If the paternal allele is dominant, the phenotype is normal. If the paternal allele is recessive, the phenotype is dwarf!!!
Why is the albino c allele epistatic to all other coat colour genes
cc mutants prevent melanin synthesis due to mutations in the tyrosinase enzyme. Tyrosinase enzyme is required for melanin production. Hence no color can be produced.
What type of mutation is the c mutant for coat color. Wild type = C (permits color expression)
This is a loss of function mutation
Define variable expressivity
Occurs where all have the same allele/genotype but the effect of that allele can be altered among individuals. Hence, individuals will show a range of phenotypes of varying severity/ signs and syptoms.
Give an example of variable expressivity
Variable agouti in mice. This is caused by a transposon in the promoter region. The transposon alters the expression of the Yellow Agouti gene, causing different shades of this color.
What is retinitis pigmentosa
Degeneration of retina. This is an example of digenic inheritance
