Example : Cystic fibrosis (CF)
Where:
- A is the wild type allele
- a is the cystic fibrosis mutation.
| |
Maternal
|
| A (p) |
a (q) |
| Paternal
|
A (p) |
AA (p2) |
Aa (pq) |
| a (q) |
Aa(pq) |
aa (q2) |
The Hardy–Weinberg principle states that the genotype frequencies A2, 2Aa,
and a2 will not change if the allele frequencies remain constant from
generation to generation (they
are in equilibrium).
Expressed as:
A2 + 2Aa+ a2 =1
Hardy-Weinberg equation for the general case:
p²+ 2pq+ q² = 1
The genotype frequencies and allele frequencies are in equilibrium if the
following assumptions of the Hardy–Weinberg principle are met:
- There is no natural selection
- Matings are at random ,all individuals reproduce equally, all offspring survive
- There is no appreciable rate of mutation.
- There is no migration.
- There has been no genetic drift.
For CF the frequency of aa in northern Europeans = 1 in 2500
q2=1/2500
q = 1/50
Since the sum of the alleles q + p = 1 , p
= 1 - q
p= 49/50
Carrier frequency = 2pq= 2*(49/50)(1/50) = 98/2500 =.04
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