Count alleles without confusing a census and a sampling model

1. Orient: Count alleles without confusing a census and a sampling model

Evolutionary population change concerns inherited variation across generations, not an individual changing its genotype because conditions demand it. A population must be defined in space, time and reproductive context before a frequency has a useful denominator. Samples from different life stages or locations may represent different populations rather than a temporal evolutionary trajectory.

2. Explain the population mechanism

For a diploid autosomal two-allele locus, each AA individual contributes two A copies and each Aa individual contributes one. Thus p(A)=(2NAA+NAa)/(2N). Genotype frequencies concern individuals; allele frequencies concern gene copies. Missing genotypes, duplicated loci and sex-linked copy counts require different accounting, not an automatic factor of two.

Random union of gametes with allele frequencies p and q predicts p²,2pq,q² for the next zygote pool. This is a conditional reference distribution, not a claim that the observed population must have those counts. Stable allele frequencies additionally require absence of relevant evolutionary forces under the ideal Hardy–Weinberg model; random mating alone does not eliminate selection, mutation or migration.

The displayed sampling standard deviation sqrt[p(1−p)/200] treats 200 independent allele draws. A census of 100 related diploid individuals is not automatically 200 independent draws. Family clusters, spatial structure and genotype-dependent detection alter the sampling process. Retain original counts and sampling units before comparing proportions.

3. Work a complete evidence calculation

Among100 individuals with30 AA,40 Aa and30 aa, A copies=60+40=100 of200, so p=0.5. Observed heterozygosity0.4 differs from random-union expectation0.5. Independent200-copy sampling SD is sqrt(0.25/200)≈0.03536, but that independence is an additional model assumption.

4. Predict and investigate

Predict first: If heterozygotes decrease while the two homozygote classes increase equally, must allele frequency change?

Compare AA30/Aa40 with AA40/Aa20. Reconstruct every allele count and compare observed with expected heterozygosity. Identify a biological sampling unit and explain why pooled siblings cannot automatically justify the independent-copy sampling SD.

Retain the original population definition, first prediction, controls and full numerical ledger, including conflicting or null results. These are original bounded teaching calculations, not measured populations.

5. Transfer and explain

Apply the mechanism to a changed case

A sample has20 AA,60 Aa and20 aa. Calculate p(A), observed heterozygosity and random-union expected heterozygosity.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

p=(40+60)/200=0.5; observed heterozygosity0.6 and reference0.5. The excess does not uniquely identify selection: sampling, structure, mating and scoring require investigation.

6. Reflect and retain evidence

State a competing mechanism, an observation that would distinguish it, and a limitation of the supplied sampling or life-cycle model. Preserve first reasoning and a criterion-linked human review in the General Biology II workspace. Public practice feedback and simulated evidence do not certify an independently demonstrated comparative argument, field investigation or practical skill.

Supporting scientific reference, checked 30 October 2026. All prose, numerical packets and diagrams are original. No organism collection, culture, breeding, release, personal genetic inference or clinical procedure is requested.