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Biology - Population & Community Dynamics
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→ ALLELE FREQUENCY : Khan VidAllele: Variant of a gene (1 from mom, 1 from dad)
→ B: Brown (dom) BB
B: blue (rec) bb
Understanding Frequency
→ Person 1: Bb
Person 2: bb
f(B) / p (dom allele frequency) : ¼ = 0.25 = 25%
→ 1 B out of 4 variants
f(b) / q (rec allele frequency) : ¾ = 0.75 = 75%
→ 3 b out of 4 variants
⭐︎p + q = 100% / p + q = 1
Phenotype: % of brown eyed ppl: 50%
→ phenotypes are physical manifestations (PTC taster, Huntington's disease…)
● Hardy Weinberg Principle
- No selection (natural or unnatural): probability of alleles wont be more or less likely to
reproduce & have viable offspring.
- No mutation: no alleles turning into new alleles or traits
- Large populations
1. p + q = 1 or 100%
2. Square both sides
(p + q)2 = (1)2 → P2 + 2pq + q2 = 1
→ p = B (dom allele)
p x p = BB (homozygous dom)
q x q = bb (homozygous rec)
2pq = p x q + q x p = Bb (heterozygous)
→ “2% of humans on the planet have red hair. What % of humans are heterozygous for this
trait?”
q2 = 0.02 (2% red hair/recessive trait) ⭐︎always convert to decimal value
√q2 = √.02 → q = 0.14
p = 0.86 (you know this because p + q = 1)
Heterozygous = 2pq = 2(.86)(.14) = .24 = 24%
→ “12% of ppl in Ireland have red hair. What % of this population is homozygous for non-red
hair?”
, q2 = 0.12 ⭐︎acknowledge info given to you.
√q2 = √.12 → q = 0.35
p = 0.65
2pq = 2(.65)(.35) = 0.46
Homozygous = p2 = 0.46 + 0.35 = 0.81 ??
→ CHANGES IN POPULATION
Causes of Frequency Changes
⭐︎When HW Principle conditions are not met, gene pool will change in the following ways:
1. Small Population
- Population shrinks → Chance takes over
2. Non-Random Mating
- Choosing partners based on appearance or location
3. Mutation
- New alleles changing the allele frequencies
4. Migration
- If new individuals flow into a new area/immigrate.
- If individuals flow out of an area/emigrate.
→ “Gene Flow”
5. Adaptation/Natural Selection
- Allows certain alleles to increase in frequency.
- Genes that are not adapted to their environment will gradually be replaced by genes better
adapted.
→ Red hair/Fair skin is good in the north/colder climates than in the south/warmer
Climates.
Genetic Drift
The random change in allele frequency from chance.
Small populations can be significantly affected.
● Founder Effect
- A few individuals separate from the original population and create a new population.
- Allele frequency will most likely be different.
→ Seeds carried by bird/wind to a distant island.
Self pollinating plants allow 1 seed to create a new population from that 1 seed.
● Bottleneck Effect
- Drastic reduction in population size
- Due to severe environmental event (usually natural disaster)
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