Phenotypic ratio in a heterozygous dihybrid cross w/ recessive epistasis
9:3:4
Key Terms
Phenotypic ratio in a heterozygous dihybrid cross w/ recessive epistasis
9:3:4
Bivalent
Homologous pair of chromosomes
Chiasmata
Point representing where homologous touch and exchange genetic info
Gene pool
Total no.of genes and their alleles in a particular population
Assumptions of the Hardy-Weinberg Principle
Pop is v. large (reduced effect of genetic drift)
Mating within pop. is random - no selective breeding
No selective advantage for any g...
Hardy Weinberg principle
A is dominant, p = freq. of A
a is recessive, q = freq. of a
p + q = 1
p^2 + 2pq + q^2 = 1
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| Term | Definition |
|---|---|
Phenotypic ratio in a heterozygous dihybrid cross w/ recessive epistasis | 9:3:4 |
Bivalent | Homologous pair of chromosomes |
Chiasmata | Point representing where homologous touch and exchange genetic info |
Gene pool | Total no.of genes and their alleles in a particular population |
Assumptions of the Hardy-Weinberg Principle | Pop is v. large (reduced effect of genetic drift) Mating within pop. is random - no selective breeding No selective advantage for any genotype coded for by that allele No mutation No migration Gene pool is stable |
Hardy Weinberg principle | A is dominant, p = freq. of A a is recessive, q = freq. of a p + q = 1 p^2 + 2pq + q^2 = 1 |
When to use p + q = 1 | When given allele frequency |
When to use p^2 + 2pq +q^2 | When given phenotypes |
Evolution | Changes in allele frequencies over time leading to changes in species |
What can affect allele frequencies | Mutations - new advantageous alleles will remain in pop Natural selection Effects of small population Genetic drift Artificial selection and selective breeding |
Selection | Increase in allele frequency |
Stabilising selection | Selection pressure toward the centre increases no. of individuals at the modal values Extreme values are selected against and lost |
Types of selection | Stabilising Directional Disruptive |
Directional selection | Selection pressure towards one extreme moves the mode in this direction Extreme value is advantageous; more likely to survive and reproduce |
Disruptive selection | Selection pressure toward the extremes creates two modal values Intermediae values selcted against - lose those alleles Creates two distnct populations e.g. Darwin's finches |
Genetic drift | Random events causing changes in allele frequencies Effects are greatly increased in small pop or small gene pools Alleles in new generation will therefore be the genes of the 'lucky' individuals and not necessarily healthier individuals |
Polymorphic | Genes w/ > 1 allele |
Effects of small populations | Founder effect and genetic bottleneck reduce genetic diversity by creating small populations |
Founder effect | Occurs when a small group of migrants that aren't genetically representative of the pop. from which they came from, establish in a new area New population is v. small w/ an increase in inbreeding and relatively low genetic variation |
Why does inbreeding cause genetic diseases | Increases impact of recessive alleles and most genetic diseases are caused by recessive alleles |