What are Dominant and Recessive Traits?
Dominant and recessive traits describe how two different alleles for the same gene interact to determine an organism's observable characteristics. A dominant allele masks a recessive one when both are present.
A dominant trait is expressed whenever at least one dominant allele is present (e.g., Aa or AA), while a recessive trait only appears when both alleles are recessive (aa).
- •Expressed with just one copy of the allele (Aa or AA)
- •Masks the recessive allele's effect
- •Written with an uppercase letter (e.g., A)
- •Example: brown eyes in humans
- •Only expressed when both alleles are recessive (aa)
- •Effect is hidden when a dominant allele is present
- •Written with a lowercase letter (e.g., a)
- •Example: blue eyes in humans
Step-by-step worked examples
A pea plant has the genotype Aa, where A (purple) is dominant over a (white). What color are its flowers?
Genotype Aa contains one dominant allele (A). A dominant allele is expressed whenever present. Therefore the flowers are purple.
Two brown-eyed parents (Bb × Bb) have a child. What is the probability the child has blue eyes (bb)?
Bb × Bb cross → genotype ratio 1 BB : 2 Bb : 1 bb. Blue eyes require the recessive genotype bb. Probability of bb = 1/4 = 25%.
A rabbit has genotype aa for coat color, where A (black) is dominant over a (white). What is its phenotype?
Genotype aa contains no dominant allele. Only recessive alleles are present. The recessive trait is expressed: white coat.
Flashcards
Quick quiz
Q1.Which genotype(s) express a dominant trait?
Q2.A recessive trait is expressed when the genotype is…
Q3.In pea plants, purple (A) is dominant over white (a). What color is an Aa plant?
Q4.Two heterozygous (Bb) parents have children. What fraction is expected to show the recessive trait?
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Common mistakes
Assuming a recessive trait 'disappears' if not seen in a generation. — Correct: Recessive alleles can hide in heterozygous carriers and reappear in later generations.
Thinking dominant means 'more common' in a population. — Correct: Dominant refers to how an allele is expressed, not its frequency in a population.
Believing Aa shows a blend of both traits. — Correct: Aa shows the fully dominant trait; blending only happens in incomplete dominance, a different pattern.
Assuming only homozygous genotypes exist. — Correct: Heterozygous genotypes (Aa) are common and always show the dominant trait.
FAQ
What are dominant and recessive traits?
Dominant traits appear when at least one dominant allele is present; recessive traits only appear when both alleles are recessive.
What is an example of a dominant and recessive trait?
In pea plants, purple flower color (A) is dominant over white flower color (a).
How do you know if a trait is dominant or recessive?
By observing cross results: dominant traits mask recessive ones in heterozygotes (Aa), and appear in a 3:1 ratio in F2 crosses.
Can recessive traits skip a generation?
Yes — a recessive allele carried by heterozygous (Aa) parents can be hidden, then appear in their children if both parents pass it on.