A cross describes possible offspring genotypes. Population data show how common a trait is over time.
List what each parent can transmit
In a simple model with complete dominance, an Ee parent crossed with ee can transmit E or e. The ee parent transmits e. The resulting Ee and ee offspring are each expected half the time, given ordinary segregation. E may affect the phenotype without changing which letters either parent can pass.
The probabilities describe repeated opportunities, not a guarantee about a particular litter. If the model specifies independence, seeing several Ee offspring in a row leaves the next chance at one half.
Ask what happens across generations
A field count increases from 25 striped individuals out of 100 to 30 out of 200. The count increased but the fraction fell from one quarter to 15%. Use the appropriate population denominator at both times.
To support selection, connect the inherited trait to different contributions of surviving offspring under the environment observed. Migration, sampling and chance can also affect counts, so a changed percentage alone is incomplete evidence.
Apply the guide
An Aa parent is crossed with aa under complete dominance. Each offspring independently has a one-half chance to show the dominant feature. How probable is a run of four offspring all showing it?Reveal explanation
One in sixteen. Multiply four independent one-half probabilities. This run is possible even though the expected share showing the feature is one half.
A trait rises from 30/150 to 40/160. What changed and what remains unknown?Reveal explanation
Its frequency rose from 20% to 25%. The two counts alone do not establish selection as the cause.
Turn the idea into practice.
Apply the rule to a new case, then explain your choice.
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Original OpenExam guide
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OpenStax Biology 2e 12.3 Laws of InheritanceOpenStax Biology 2e 19.1 Population Evolution