Estimating risk of inheritance

Question 1

Define an allele

Answer 1

Each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

Question 2

Define fitness

Answer 2

“Fitness” means the relative ability of organisms to survive and pass on genes.

Question 3

How can alleles affect fitness?

Answer 3

Alleles can affect fitness:

• Not at all in most cases (neutral allele)
• Sometimes decrease (deleterious allele)
• Rarely increase (advantageous allele)

Question 4

How does allele frequency change with autosomal dominant inherited disorders typically in a population over many generations?

Answer 4

• Relative frequencies remain constant

* Dominant conditions (alleles) do not become more common at the expense of recessive ones

Question 5

What is the Hardy-Weinberg principle?

Answer 5

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.

Question 6

What factors influence allele frequency in a population, which can skew observed values from predicted values?

Answer 6

These influences include

• mate choice
• mutation
• selection
• genetic drift
• gene flow
• meiotic drive

Question 7

What assumptions underly the Hardy-Weinberg principle?

Answer 7

• 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

Question 8

What is gene flow?

Answer 8

Introduction of new alleles as a result of migration or intermarriage, leads to new gene frequency in hybrid population.

Question 9

What is meiotic drive?

Answer 9

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.

Question 10

How does non-random mating affect allele frequencies?

Answer 10

Non-random mating leads to increased mutant alleles, thereby increasing proportion of affected homozygotes.

Question 11

What is assortative mating?

Answer 11

Choosing of partners due to shared characteristics.

Question 12

What is consanguinity?

Answer 12

Marriage between close blood relatives.

Question 13

Describe natural selection

Answer 13

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.

Question 14

Describe negative selection

Answer 14

o Reduces reproductive fitness.
o Decreases the prevalence of traits.
o Leads to gradual reduction of mutant allele.

Question 15

Describe positive selection

Answer 15

o Increases reproductive fitness.
o Increases the prevalence of adaptive traits.
o Heterozygote advantage.

Question 16

What is the heterozygous advantage?

Answer 16

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.

Question 17

How can population size play a part in allele frequency?

Answer 17

Large populations can balance out fluctuations, but small populations can exhibit “genetic drift” and cause “founder effect”, where particular alleles can be unusually prevalent

Question 18

Define the founder effect

Answer 18

Founder effect: the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony.

Question 19

Define genetic drift

Answer 19

Genetic drift: random fluctuation of one allele transmitted to high proportion of offspring by chance.

Question 20

What is the bottleneck effect?

Answer 20

Bottleneck effect: reduced genetic diversity due to some natural phenomenon e.g. natural disasters, epidemics.

Question 21

Give an example of a disease prevalent due to the founder effect

Answer 21

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)

Question 22

What is Ellis-van Creveld syndrome?

Answer 22

Nasal teeth and polydactyl, prevalent in Amish of Pennsylvania

Question 23

What are the applications of the Hardy-Weinberg principle?

Answer 23

• Useful for calculating risk in genetic counselling

* Useful for planning population based carrier screening programmes