Lecture 4 Flashcards
What is Penetrance?
The proportion of individual organisms having a particular genotype that express the expected phenotype - variation in the population
What is Expressivity?
The degree to which a phenotype is expressed (mild to severe); variation in the individual
The genotype does not have a consistent phenotype.
What is incomplete penetrance?
Identical known genotypes yield less than 100% expected phenotypes.
What is an example of incomplete penetrance?
Polydactyly (extra fingers and toes) is not fully penetrant. If only half the genotype for polydactyl have extra digits, you would say that it shoes 50% penetrance.
How do you deal with problems that have incomplete penetrance in the?
Multiple the probability of the parents having the disease by the percent given of the penetrance.
Example: 1/4 x 80% penetrance = 20%
How does the disease Piebaldism show Variable Expressivity?
It is a rare autosomal dominant disorder that involves the absence of cells called melanocytes (skin pigment) in certain areas of the skin and hair
Variable as patch could be anywhere on body (variable)
How does Huntington’s Disease show variable exppressivity?
It is a rare autosomal dominant disorder that causes cognitive decline and dementia
Variable as it can begin at different ages or people can have different symptoms.
What environmental conditions can cause phenotypic variation?
- Age
- Sex
- Temperature
- Chemicals
What is the norm of reaction?
The range of phenotypes expressed by a single genotype under different environmental conditions.
How does the Himalayan allele of rabbits demonstrate an environmental effect on phenotype?
The dark pigment only develops at low temperatures. The enzyme necessary for pigment production is inactivated at higher temperatures.
What is phenocopy?
A change in phenotype arising from environmental factors that mimic the effects of a mutation in a gene but is not found in the genotype
What is an example of phenocopy?
The chemical thalidomide (helps pregnant women from feeling nauseous) can produce a phenocopy of a rare dominant trait called phocomelia in which limb development is disrupted.
What does Mendel’s Law of Independent Assortment state?
The inheritance of one trait will not affect the inheritance pattern of another trait.
What is a Genetic Interaction?
Different combinations of alleles from two or more genes can result in different phenotypes, because of interactions at the cellular or biochemical level.
What is Complementation?
Occurs when two strains of an organism with different homozygous recessive mutations that produce the same phenotype, produce offspring of the wild type phenotype when mated or crossed.
When will complementation occur?
Only occur if the mutations are in different genes - not in the same gene.
What does the other genome do in complementation?
Supplies the wild-type allele to “complement” the mutated allele
What is a heterogenous trait?
A mutation in any one of a number of genes that can give rise to the same phenotype.
What happens if two parents are deaf for different genetic reasons?
Their children will not be deaf
What happens if the children are deaf for the same genetic reasons?
The children will all be deaf.
What is Epistasis?
The masking of the expression of one gene produced by another. No new phenotypes are produced.
What does the “epistatic gene” do?
The masking
What does the “hypostatic gene” do?
The one being masked.
What is the F2 phenotypic ratio for Recessive Epistasis?
9:3:4
- Homozygous recessives at one gene pair mask expression from the other gene
What is the F2 phenotypic ration for Dominant Epistasis?
12:3:1
- One dominant allele at one gene masks expression from the other gene.
What is the Molecular Mechanism of Recessive Epistasis in Mice?
c/c: no pigment is synthesized, therefore mice are white regardless of genotype at the A locus; known as recessive epistasis.
(The genotype at A determines how the pigment is deposited)
What Ratio does a Recessive Lethal Allele produce?
2:1
What Ratio does complementation produce?
9:7
What is Pleiotropy?
A single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects
How is pleiotropy expressed in sickle cell disease?
Respiratory problems, sickled cells, chronic infections, joint pain, enlarged spleen, stroke
How is pleiotropy expressed in cystic fibrosis?
Mucus clogs the and leads to infections
Mucus obstructs the pancreatic ducts creates digestion problems.
What is an example of complementation?
If two deaf parents who are deaf for different reasons have children, the children will never be deaf.
A/A b/b x a/a B/B = A/a B/b (wild type dominant allele)
What is epistasis?
Epistasis is when the expression of one gene is masked by the other gene.
What is the epistatic gene?
The gene that does the masking.
What is the hypostatic gene?
The one getting masked
What is recessive epistasis?
Homozygous recessives at one gene pair mask the expression of the other gene
Why do farmers use consanguinous mating for plants?
To keep the plants genetically uniform
However, this can cause an entire population to be wiped out by a pest, due to them all being genetically identical
What is Heteosis?
When you are more genetically “fit” than your parents.
Happens when two inbred lines are crossed, all hybrids will be heterozygous and display hybrid visor (more fit)
What does inbreeding cause?
Increases the frequency of homozygotes and decreases the frequency of heterozygotes.
What is the Hardy Weinberg Principle?
Predicting genotypes through allele frequencies in a population.
What is the equation for Hardyweinburg for proportion?
p2 (AA) + 2pq (Aa) + q2 (aa) = 1
*This is for proportion (two probabilities)
What is the equation for Hardweinburg for frequency?
p + q = 1
*This is for frequency
When is the Hardy Weinberg equation presumed to be wrong?
- Nonrandom mating’
2.Unequal Survival - Migration
- Population Subdivision
What is Dosage Compensation?
A way of equalizing gene expression in the face of different gene dosage.
Where are the X and Y chromosomes homologous?
Only homologous at pseudoautosomal regions which are essential for X-Y chromosome pairing
What is the XX-XO system?
Females: XX
Males: XO
What is the XX-XY System?
Females: XX
Males: XY
Females: XO
What is X-inactivation?
If a cell contains more then 2 X chromosomes, all but one of them are inactive (only one remains active)
What is a Barr body?
Densely staining inactivated condensed X chromosome that is present in each somatic cell of most female mammals
How many Barr bodies do XO females have?
None
How many Barr Bodies do XXY males have?
1 Barr body
How many Barr bodies do XXX females have?
They would have two Barr bodies
Genes on the X-Chromosome for Calico Cats?
The orange gene is on the X-Chromosome
O = orange
o = black
What happens if a calico cat is heterozygous?
genotype: Oo
Phenotype:
Some cells are orange
Some cells are black
What does a black dot represent for calico cats?
That the allele is present, but not contributing to the phenotype.
What happens early in development for X-inactivation?
Early in development, one X-chromosome is inactivated by each cell (random process)
What are genetic mosaics?
Females that are heterozygous for X-linked traits
What are females that have been reported to be colour blind in only one eye?
Heterozygotes and Mosaics.
X in one eye
Xcb in the other different eye
*Heterozygous for X-linked traits
