16.2 The Roles of Genes in Determining Phenotype

A gene is a DNA region; an allele is one version of that gene; a locus is its position on a chromosome. Genotype is the allele combination an organism carries, while phenotype is the observable result of genotype and environment.

Homozygous means two matching alleles; heterozygous means different alleles; dominant describes expression in a heterozygote, not greater biological value; recessive describes an allele masked in that combination.

Separating these terms prevents treating a phenotype as a direct label for one allele. The environment and interactions between genes can alter the observed result.

A heterozygous plant can show the dominant flower colour while still carrying a recessive allele that may appear in offspring.

Dominant does not mean common, stronger or healthier. It refers to the phenotype expressed in a specified genotype.

A genetic diagram uses allele symbols, parental genotypes, gametes and a cross to show possible offspring genotypes and phenotypes. Probabilities describe expected proportions over many births, not a fixed order for one family.

Define allele meaning → write parental genotypes → list possible gametes → combine one gamete from each parent → state genotype and phenotype ratios → check that probabilities sum to 1.

The diagram makes segregation and fertilisation explicit. It also reveals whether a conclusion depends on complete dominance, independent assortment or another stated assumption.

A heterozygote cross Aa × Aa gives expected genotypes 1 AA : 2 Aa : 1 aa and a 3:1 phenotype ratio only if A is completely dominant and the loci behave as assumed.

A 3:1 ratio is not universal. Codominance, linkage, lethal alleles or environmental effects change the expected phenotype pattern.

The chi-squared statistic compares observed and expected counts: χ² = Σ((O−E)²/E). A large value indicates a larger discrepancy, but significance is judged using degrees of freedom and a chosen probability threshold.

State the null hypothesis → calculate expected counts → compute each contribution → sum χ² → find degrees of freedom → compare with a critical value or p-value → accept or reject the null.

Random sampling alone can move counts away from the exact ratio. The test quantifies whether the deviation is compatible with chance under the assumed genetic model.

If χ² is below the critical value, there is insufficient evidence to reject the expected ratio; this does not prove the ratio is true.

Failing to reject the null is not proof of no biological difference. Check sample size, expected-count assumptions and whether the model itself is appropriate.

A gene can affect phenotype by encoding a protein or functional RNA. The protein may act as an enzyme, receptor, transporter or structural component; its activity changes a pathway that contributes to the phenotype.

A DNA change can alter expression, amino-acid sequence, folding or protein amount. The final phenotype depends on cell context, other genes and environmental conditions, not just the allele label.

A mutation in an enzyme gene can reduce pigment production, but temperature or nutrient supply may still modify how much pigment is visible.

One gene does not always map to one simple trait. Explain the molecular link before claiming a direct genotype–phenotype equivalence.

Gibberellin can stimulate stem elongation by promoting cell expansion and, in some tissues, cell division. It changes the expression or activity of proteins that alter cell-wall properties and growth responses.

The hormone links developmental state to the mechanics of elongation. The response depends on concentration, tissue and interaction with other plant hormones.

A dwarf plant lacking sufficient gibberellin may regain stem elongation after treatment, but the response plateaus if another resource or pathway becomes limiting.

Gibberellin is not a universal ‘growth chemical’ that always increases every tissue’s size. Interpret the response in its developmental context.