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What Is a Recessive Allele? — Reading Comprehension

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MS-LS3-1
MS-LS3-2

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This middle school reading passage helps students understand recessive alleles, one of the key concepts in genetics and heredity. Using clear examples such as flower color and eye color, the passage explains how recessive traits are inherited only when both copies of a gene are recessive. Students also learn what it means to be a carrier and why certain traits may remain hidden for generations. Aligned with NGSS standard MS-LS3-2, this resource supports science literacy and comprehension while reinforcing vocabulary related to alleles, inheritance, and genetic variation. A perfect addition to any NGSS-aligned science curriculum focused on heredity and traits.

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In genetics, traits are controlled by genes, and each gene has different versions called alleles. A recessive allele is a type of allele that only shows its trait when both copies of the gene are recessive. This means the trait will only appear if an individual inherits the recessive allele from both parents.

For example, imagine a gene that controls flower color. A dominant allele (represented by a capital letter, like R) might produce red flowers, while a recessive allele (represented by a lowercase letter, like r) produces white flowers. If a plant has one of each allele (Rr), the dominant allele will mask the recessive one, and the plant will have red flowers. Only when the plant has rr, two recessive alleles, will it have white flowers.

Recessive traits can be hidden for generations. This happens because someone can carry a recessive allele without showing the trait. These individuals are called carriers. For example, two parents who each carry the recessive allele for blue eyes (but have brown eyes) can have a child with blue eyes if the child inherits the recessive allele from both of them.

Recessive alleles are a key reason why genetic variation exists. They allow traits to pass silently from generation to generation and reappear under the right genetic conditions. This is also important in understanding some genetic disorders, which can only appear when both alleles are recessive.

Fun Fact: Blue eyes are caused by a recessive allele—and scientists believe all people with blue eyes share a single common ancestor who first had that mutation!

What is an allele?

A type of cellA kind of proteinA version of a geneA plant pigment

When does a recessive allele show its trait?

When paired with a dominantWhen aloneOnly when both alleles are recessiveIn all offspring

What does “Rr” mean in genetics?

Two recessive allelesOne dominant and one recessiveIdentical genesA type of plant

What is a carrier?

A person with a dominant geneA plant with two traitsSomeone who has a hidden recessive alleleSomeone who changes alleles

What eye color is mentioned as recessive in the passage?

BrownGreenGrayBlue

Why can recessive traits skip generations?

They are weak traitsThey are always erasedThey are hidden by dominant allelesThey are passed only to animals