Recessive — Science definition

Not expressed in the heterozygote’s modeled phenotype under complete dominance.

Where it is taught

In use

Genotype probabilities are 1/4, 1/2, 1/4; under complete dominance the phenotype expectation is 3/4 dominant and 1/4 recessive.

Mendelian Genetics

Questions using recessive

Problem 1

In a cross between two Bb parents (B dominant), what percent of the offspring are expected to show the recessive phenotype?
Answer and explanation

25 %

Bb x Bb gives BB, Bb, Bb, bb — 1 of the 4 boxes (25%) is bb, the recessive phenotype.

Problem 2

For Aa×Aa, find expected recessive aa probability. Show the reasoning, not just the conclusion.
Hints

Probabilities describe many outcomes, not a guaranteed pattern in every small family.

Answer and explanation

1/4=25%, combining a from each parent.

Problem 3

An expected ratio does not dictate each finite familyAn expected ratio does not dictate each finite familyRecessive-category count / 80 offspringTrial family A15Trial family B25Trial family C20Trial family D17Expected mean under p=1/4: 20; actual samples can vary.
Original simulated family counts, not real breeding data or personal reproductive predictions.
Interpret the supplied visual. After three dominant-category outcomes, is a recessive-category fourth outcome guaranteed?
Answer and explanation

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

A probability describes a model of possible outcomes, not a promised number in every small sample. For a recessive-category probability of 1/4 and 80 independent modeled offspring, the expected count is 20. Actual samples vary through chance, and unexpected counts require evaluation of sample size, model conditions and measurement rather than immediate rejection or certainty. Independent births in the model do not acquire a memory that forces the next child to balance earlier outcomes.

No. Each modeled independent outcome retains its probability; earlier outcomes do not force a balancing result.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 4

A cross expected to give a 3:1 ratio produces 140 dominant and 60 recessive offspring out of 200. Evaluate whether the result is consistent with the expectation.
Answer and explanation

The expectation for 200 offspring is 150 dominant and 50 recessive. The observed 140 and 60 differ by 10 in each category, which is a 6.7 percent shortfall in the dominant class. A departure of this size in a sample of 200 is the kind of variation chance produces, so the data are broadly consistent with a 3:1 ratio rather than evidence against it. Deciding properly requires a statistical test of how often chance alone would produce a deviation at least this large; the honest statement is that the result does not contradict the expectation.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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