Estimated risk of genetic disease IA% +

Question 1

Alleles and fitness

Answer 1

• Fitness means the relative ability of organisms to survive (long enough) to pass on their genes.
• Alleles can affect fitness:
• not at all in most cases (neutral allele)
• sometimes decrease (deleterious allele)
• rarely increase (advantageous allele)

•In humans bringing up the child important too

–Role of grandparents / family / clan / society in food sourcing, defence against invaders, keeping child safe from harm and education

Note: If selective pressures change, importance of different alleles may change

Question 2

Sources of genetic variation

Answer 2

• Mutation in recessive genes rarely affects carriers
• Common recessive diseases include: Sickle cell disease, Thalassaemia

–Where it does often associated with a selective pressure

• E.g. malaria resistance
• plague or cholera resistance

–De novo recessive mutation uncommon as a cause of disease

•Mutation in dominant and X-linked genes can be inherited or de novo (anew)

Question 3

Assumptions underlying HWE

Answer 3

Ideal Population:

• 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 4

Mutation and Migration (Gene flow)

Answer 4

• Mutations increase the proportion of new alleles.
• Introduction of new alleles as a result of migration or intermarriage leads to new gene frequency in hybrid population.

Question 5

Non-random mating

Answer 5

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

Assortative mating

• Choosing of partners due to shared characteristics e.g Deafness & sign language

Consanguinity

• Marriage between close blood relatives. e.g Cultural pressures for inter-marriage within clans / religions etc.

Question 6

Natural selection

Answer 6

•A gradual process by which biological traits become either more or less common in a population.

Negative selection

• Reduces reproductive fitness.
• decreases the prevalence of traits.
• leads to gradual reduction of mutant allele.

Positive selection

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

Question 7

Small population size

Answer 7

Large populations can balance out fluctuations, but small populations can exhibit “genetic drift” and cause “founder effect”.

Genetic drift:

Random fluctuation of one allele transmitted to high proportion of offspring by chance.

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 8

Genetic drift

Answer 8

• Mutations (alleles) are widespread and neutral.
• Statistical drift of gene frequencies due to chance or random events rather than natural selection in the formation of successive generations.

Question 9

Founder effect and bottleneck effect (pic)

Answer 9

Question 10

Hardy Weinberg

Answer 10

•Allele frequencies remain constant generation to generation.

•Relative proportion of genotype frequencies remain constant generation to generation.