Estimating risk of inheritance Flashcards
Define an allele
Each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Define fitness
“Fitness” means the relative ability of organisms to survive and pass on genes.
How can alleles affect fitness?
Alleles can affect fitness:
- Not at all in most cases (neutral allele)
- Sometimes decrease (deleterious allele)
- Rarely increase (advantageous allele)
How does allele frequency change with autosomal dominant inherited disorders typically in a population over many generations?
- Relative frequencies remain constant
* Dominant conditions (alleles) do not become more common at the expense of recessive ones
What is the Hardy-Weinberg principle?
The Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
What factors influence allele frequency in a population, which can skew observed values from predicted values?
These influences include
- mate choice
- mutation
- selection
- genetic drift
- gene flow
- meiotic drive
What assumptions underly the Hardy-Weinberg principle?
- Mutation can be ignored
- Migration is negligible (No gene flow)
- Mating is random
- No selective pressure
- Population size is large
- Allele frequencies are equal in the sexes
What is gene flow?
Introduction of new alleles as a result of migration or intermarriage, leads to new gene frequency in hybrid population.
What is meiotic drive?
Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will affect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotypic expression.
How does non-random mating affect allele frequencies?
Non-random mating leads to increased mutant alleles, thereby increasing proportion of affected homozygotes.
What is assortative mating?
Choosing of partners due to shared characteristics.
What is consanguinity?
Marriage between close blood relatives.
Describe natural selection
A gradual process by which biological traits become either more or less common in a population depending on how it affects an organisms fitness to survive.
Describe negative selection
o Reduces reproductive fitness.
o Decreases the prevalence of traits.
o Leads to gradual reduction of mutant allele.
Describe positive selection
o Increases reproductive fitness.
o Increases the prevalence of adaptive traits.
o Heterozygote advantage.
What is the heterozygous advantage?
A heterozygote advantage (heterozygous advantage) describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype, and can sometimes give a selective advantage or increased protection from a particular disease.
How can population size play a part in allele frequency?
Large populations can balance out fluctuations, but small populations can exhibit “genetic drift” and cause “founder effect”, where particular alleles can be unusually prevalent
Define the founder effect
Founder effect: the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony.
Define genetic drift
Genetic drift: random fluctuation of one allele transmitted to high proportion of offspring by chance.
What is the bottleneck effect?
Bottleneck effect: reduced genetic diversity due to some natural phenomenon e.g. natural disasters, epidemics.
Give an example of a disease prevalent due to the founder effect
Founder effect: Ellis-van Creveld syndrome
Amish of Pennsylvania - a small number of german immigrants
Assortative mating - marry within their own community.
Natal teeth and polydactyly (extra digits)
What is Ellis-van Creveld syndrome?
Nasal teeth and polydactyl, prevalent in Amish of Pennsylvania
What are the applications of the Hardy-Weinberg principle?
- Useful for calculating risk in genetic counselling
* Useful for planning population based carrier screening programmes
