4.2 Polygenic Inheritance and Phenotypic Plasticity

A characteristic controlled by two or more genes shows polygenic inheritance. If alleles at several unlinked loci contribute small additive effects, many combinations produce intermediate totals. More contributing loci create more classes and a distribution that appears continuous. Human height and skin pigmentation are familiar examples, though both are more complex than a simple additive model.

Discrete variation forms distinguishable categories, as in ABO groups. Continuous variation spans a measured range and often approximates a normal distribution because many genetic and environmental contributions combine. Both can be genetic; the distinction describes phenotype distribution.

Nutrition, illness and development affect height and mass. UV exposure stimulates melanin production without changing pigment alleles. Genotypes therefore generate overlapping phenotype ranges. Heritability does not mean immutability, and environmental influence does not make genes irrelevant.

Phenotypic plasticity is one genotype's capacity to produce different phenotypes in different environments. Discrete alternatives are called polyphenism. Developmental temperature or humidity creates seasonal butterfly forms; nutrition and signaling allow social insects with similar genomes to become queens or workers.

In several reptiles, incubation temperature during a critical window changes offspring sex ratio. Response curves differ by species: some produce males at intermediate temperatures, others at a high pivotal temperature. Temperature alters developmental regulation, not genome sequence, so climate shifts can distort population sex ratios.

Other environmental sex cues include photoperiod, nutrients and location. Common slipper limpets stack together; position and social context influence sex change and help balance reproductive roles. Chromosomal sex determination at fertilization is therefore not a universal rule.

A simple additive pigmentation model with three heterozygous gene pairs produces seven contribution classes, from zero to six contributing alleles. The extreme classes are rare because only one gamete combination produces each, while many combinations produce intermediate totals. Plotting class frequency gives a peaked distribution. Real pigmentation involves more loci and environmental effects, creating more finely divided and overlapping values than the classroom model.

A normal-looking distribution does not prove polygenic inheritance by itself. Measurement error and environmental variation can smooth a trait, while a small number of loci can sometimes create many categories. Evidence strengthens when relatedness predicts similarity, particular loci associate with the trait, and controlled environments still retain genetic variation. Conversely, a wide environmental response does not erase the possibility that genotypes differ in both baseline and responsiveness.

The term reaction norm describes the range of phenotypes a genotype produces across environments. Two genotypes may have parallel reaction norms, preserving their difference as conditions change, or crossing reaction norms, where their ranking reverses. This is why claims that one genotype is simply 'taller' or 'better adapted' require an environmental context.

Temperature-dependent sex determination has a sensitive period rather than responding equally throughout incubation. A few degrees around a pivotal range can cause a large sex-ratio change. Pond slider turtles, alligator snapping turtles and American alligators show different response curves, so a rule learned for one cannot be transferred to all reptiles. Ecological predictions require the species-specific curve plus nest temperatures and maternal nesting behavior.

In slipper limpets, young individuals begin male and may later become female depending on their position in a stack. Being attached above a female favors remaining male, whereas removal or a shortage of females can promote transition. This sequential system makes functional sense because small mobile individuals can supply sperm, while larger settled individuals can invest in egg production. The environmental cue coordinates phenotype with local reproductive opportunity.

Plasticity can be adaptive only if the cue reliably predicts conditions in which the alternative phenotype performs well. Producing a wet-season butterfly form after rainfall can reduce predation, but responding to a misleading cue may be costly. Natural selection therefore acts not only on trait values but on sensitivity, threshold and timing of the response.