Flashcards · 7402 · A Level Topic 7

Genetics, populations, evolution and ecosystems (A Level) flashcards

17 cards on A Level Topic 7, subtopics 3.7.1 to 3.7.4. Pick a subtopic or study them all.

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Genotype and phenotype: Genotype: the genetic constitution of an organism. Phenotype: the expression of this genetic constitution and its interaction with the environment.
Codominance, with the ABO example: Both alleles are expressed in the heterozygote. Iᴬ and Iᴮ are codominant, and i is recessive to both, so IᴬIᴮ is group AB and group O is always ii.
Why are X-linked recessive conditions more common in males?: Males (XY) have only one X chromosome, so a single recessive allele on it is always expressed. Females need two copies.
Autosomal linkage: Genes on the same autosome are inherited together unless crossing over separates them. A test cross gives two large parental classes and two small recombinant classes.
Epistasis, and the ratios it gives from AaBb × AaBb: An allele at one locus masks or suppresses expression of alleles at another locus. Recessive epistasis gives 9 : 3 : 4; dominant epistasis gives 12 : 3 : 1.
Chi-squared test: formula, degrees of freedom and decision: χ² = Σ(O − E)² ÷ E. Degrees of freedom = number of classes − 1. If χ² is below the critical value at p = 0.05 (3.84 for 1 df), the difference is not significant and is probably due to chance.

Hardy–Weinberg equations and conditions: p + q = 1 and p² + 2pq + q² = 1 (p² homozygous dominant, 2pq heterozygous, q² homozygous recessive). Valid for a large population with random mating and no mutation, migration or selection.
A recessive condition affects 81 in 10 000 people. What proportion are carriers?: q² = 0.0081, so q = 0.09 and p = 0.91. Carriers: 2pq = 2 × 0.91 × 0.09 = 0.1638 (1638 per 10 000).
Population, gene pool and allele frequency: Population: all the organisms of one species in one area at the same time, able to interbreed. Gene pool: all the alleles of all the genes in a population. Allele frequency: how often an allele occurs in the gene pool.

Allopatric and sympatric speciation: Allopatric: a geographical barrier separates populations. Sympatric: no geographical barrier; reproductive isolation arises in the same area. Either way gene pools diverge until they cannot interbreed to give fertile offspring.
Genetic drift: Chance change in allele frequency, with no direction; important in small populations.
Disruptive selection: Both extreme phenotypes are favoured over the intermediate, so the population may split into two groups; it can lead to speciation.
How does reproductive isolation lead to a new species?: Separated populations experience different mutations, selection pressures and genetic drift, so their allele frequencies change until they can no longer interbreed to produce fertile offspring.

Mark-release-recapture: 36 animals marked; a second sample of 40 contains 9 marked. Estimate the population.: N = (n₁ × n₂) ÷ m = (36 × 40) ÷ 9 = 160.
Why does one community replace another during succession?: Each stage changes the abiotic environment (soil depth, water, nutrients, light), making it less hostile and suitable for new species, which may out-compete the earlier ones. It ends in a relatively stable climax community.
Community, ecosystem and niche: Community: all the populations of different species in one area at the same time. Ecosystem: a community and the non-living components of its environment. Niche: the role of a species in its habitat; no two species occupy exactly the same niche.
Carrying capacity, and what limits it: The maximum population size an ecosystem can support. It is limited by abiotic factors (such as light and temperature) and biotic factors (interspecific and intraspecific competition, and predation).