Estimated risk of genetic disease IA% + Flashcards Preview

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1

 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

2

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)

 

3

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

4

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.

5

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

Consanguinity

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

6

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.

7

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.

8

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.

9

Founder effect and bottleneck effect (pic)

10

Hardy Weinberg

•Allele frequencies remain constant generation to generation.

Relative proportion of genotype frequencies remain constant generation to generation.