Estimated risk of genetic disease IA% + Flashcards Preview

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 Alleles and fitness

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


Sources of genetic variation

•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)



Assumptions underlying HWE

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


Mutation and Migration (Gene flow)

  • 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.


Non-random mating

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


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


Natural selection

•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.


Small population size

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.


Genetic drift

  • 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.


Founder effect and bottleneck effect (pic)


Hardy Weinberg

•Allele frequencies remain constant generation to generation.

Relative proportion of genotype frequencies remain constant generation to generation.