What are Dominant and Recessive Traits?
In genetics, traits are controlled by alleles—versions of genes. Dominant alleles mask recessive ones, shaping what traits you see in offspring. This foundation explains how eye colour, blood type, and countless other characteristics pass through families.
A dominant trait appears when at least one dominant allele is present; a recessive trait only shows when two recessive alleles are present (homozygous recessive). Dominant alleles are typically written as capital letters (A), recessive as lowercase (a).
- •Needs only ONE dominant allele to appear
- •Written as capital letter (A)
- •Masks the recessive allele
- •Shows in heterozygous (Aa) genotype
- •Example: brown eyes (often dominant)
- •Needs TWO recessive alleles to appear
- •Written as lowercase letter (a)
- •Hidden by the dominant allele
- •Only shows in homozygous recessive (aa)
- •Example: blue eyes (often recessive)
Step-by-step worked examples
A parent has Aa (brown eyes, dominant) and a parent has aa (blue eyes, recessive). What traits can their children have?
Parent 1: Aa (can pass A or a) Parent 2: aa (can only pass a) Offspring: Aa (brown eyes) or aa (blue eyes) Result: 50% brown, 50% blue
A plant has the genotype AA for tall height (dominant). Can it produce short offspring?
Parent genotype: AA (only passes A allele) All offspring receive A from this parent If the other parent is AA or Aa, offspring are tall Short (aa) requires 'a' from BOTH parents Answer: No, all offspring will be tall
Two parents both have Aa (widow's peak, dominant trait). What fraction of offspring will lack the widow's peak?
Both parents: Aa Possible offspring: AA, Aa, Aa, aa Frequency: 1 AA, 2 Aa, 1 aa Non-widow's peak = aa = 1/4 = 25%
Flashcards
Quick quiz
Q1.If a parent is AA (homozygous dominant), what allele will they definitely pass to offspring?
Q2.Two parents are both Aa. What is the probability their child is aa?
Q3.Which genotype will express a recessive phenotype?
Q4.A trait runs through a family but occasionally 'skips' a generation. What is likely true?
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Common mistakes
Dominant alleles are always more common in the population. — Correct: Dominance describes an allele's effect in a genotype, not its frequency. Recessive alleles can be common.
Homozygous always means dominant. — Correct: Homozygous means two identical alleles (AA or aa). Only AA is homozygous dominant.
If a parent has a recessive trait, both offspring must have it. — Correct: A parent with aa genotype will pass 'a' alleles, but the other parent may pass 'A', making offspring Aa (dominant phenotype).
Heterozygous individuals don't pass on recessive alleles. — Correct: Heterozygous (Aa) individuals can and do pass recessive alleles to offspring—50% chance per allele.
FAQ
What is the difference between genotype and phenotype?
Genotype is the genetic code (AA, Aa, aa). Phenotype is what you observe (e.g., brown eyes or blue eyes). Dominance connects them.
Can a dominant trait skip generations?
No, if an offspring has the dominant phenotype, at least one parent must carry the dominant allele. Recessive traits skip generations because heterozygotes hide them.
Why do I have traits my parents don't seem to have?
You inherited two recessive alleles from heterozygous parents, expressing a recessive trait that both parents carried but did not show.
What are carriers in genetics?
Heterozygous individuals (Aa) who carry a recessive allele but express the dominant phenotype. They can pass the recessive allele to offspring.