Penetrance & Expressivity Flashcards
What is expressivity?
-the degree to which a phenotype is expressed (mild to severe) variation in the individual
-not whether or not it is expressed different shades of allele and other genes influence one allele
What is Penetrance?
-the proportion of individual organisms having a particular genotype that express the expected phenotype (whether or not gene is expressed)-variation in the population
What is complete penetrance?
-identical known genotypes yield 100% expected phenotype
What is incomplete penetrance?
-identical known genotypes yield <100% expected phenotype
-example: polydactyly extra fingers and toes
What is constant expressivity?
-identical known genotypes with no expressivity effect yield 100% expected phenotype
What is variable expressivity?
-identical known genotypes with an expressivity effect yield a range of phenotype
What is incomplete penetrance with variable expressivity?
Identical known genotypes produce a broad range of phenotypes due to varying degrees of gene activation and expression
-mixture of colours
What is split-hand-foot syndrome?
-a rare autosomal dominant disorder that shows variable expressivity
-single gene/mutation
-involves the deficiency of absence of one or more central digits of the hand or foot
What is piebaldism?
-a rare autosomal dominant disorder that shows variable expressivity
-absence of cells called melanocytes in certain areas of the skin and hair
-patch of colour which comes in different sizes
What is Huntington disease?
-a rare autosomal dominant disorder that shows variable expressivity in the time onset of the disease
-occurs in different times in life
-a neuro-degenerative disease that causes loss of muscle coordination cognitive decline and dementia
What causes incomplete penetrance and expressivity?
-due to the effects of other genes and to environmental factors that can alter or completely suppress the effect of a particular gene
What are the environmental factors that can affect phenotypic expression?
- Age
- Sex
- Temperature
- Chemicals (exposure or consumed)
What is the norm of reaction?
-the range of phenotypes expressed by a single genotype under different environmental conditions
-all different phenotypes you can get from the same mutation
What are the environmental effects on the phenotype?
All cells express the enzyme
-core body temp causes protein to DE synthesize/unfold
-affects how the protein works and affects the phenotype
What is the temperature-sensitive allele?
-an example is the siamese allele c subscript s in the Tyrosinase gene
-the protein is inactive at the higher temperature near the cat’s core body temp leaving a light brown background
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What is phenocopy?
-a change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene
-occasionally, environmental factors alone produce a phenotype that is the same as the phenotype produced by a genotype
-thalidomide can produce a phenocopy of a rare dominant trait called phocomelia
-causes children to be deformed in the embryo but they don’t actually have the gene for it
How can we control our environment to influence the appearance of mutant phenotype of a disease?
-having a good diet and exercising can bring the disease to be mild rather than deathly
-effects the occurrence (penetrance) and seriousness (expressivity) of the disease
What does genetic interaction mean?
-different combinations of alleles from two or more genes can result in different phenotypes because of interactions between their products at the cellular or biochemical level
-part of the same pathway leading to a phenotype
What happens when multiple genes contribute to a single characteristic?
-interactions between genes
-f1 cross A/a B/b X A/a B/b (dihybrids)
-would both have brown eyes because of A and B gene
-if there is complete dominance at two distinct traits Phenotypes= 9:3:3:1
-9 have at least two dominant alleles 3 dominant at green three dominant B grey eyes and 1 recessive blue eyes
-Functioning A changes blue to green, functioning B changes from green to brown and in absence of green takes blue to make it gray (slightly lighter than brown)
What occurs for Complete dominance?
-two gene pairs contributing to a single trait four distinct phenotypes which helps visualize independent assortment
-3/4 B/-
-also 9:3:3:1
What occurs for complete dominance in cats?
-D/D this cat does not carry the dilute Gene
-D/d this cat is a carrier of the dilute Gene
-d/d this cat carries two copies of the dilute Gene the color is diluted
-black dilutes to Blue chocolate dilutes to Lilac
What is the molecular explanation for the complete dominance of cat colouration?
-the dilute gene affects the distribution of melanin granules of the cat’s hair
-dominant or dense (D) produces dense pigmentation
-recessive (d) dilute allele: results in pigment clumping for me areas in the hair with no granules this causes the cat coat to dilute or lighten
-the dilute effect is autosomal recessive hence a cat requires two copies of the d allele for the coat to dilute
What is complementation?
-occurs when two strands humans with different genotypes of an organism with different homozygous recessive mutations that produce the same phenotype produce offspring of the wild-type phenotype when mated or crossed
-will only occur if the mutations are in different genes
-the other genome supplies the wild type allele to complement the mutated allele
-will not occur if the mutations are in the same gene
-can occur for two different albinos as well
-9:7
-refer to slide for when black progeny are crossed
What is a heterogeneous trait
-a mutation in any one of a number of genes can rise to the same phenotype
-an example is two parents are deaf but all children can hear therefore they are deaf for different genetic reasons (complementation)
-if the two parents are deaf all of their children are deaf for the same genetic reasons (no complementation)
