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What Are Alleles? — Reading Comprehension

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MS-LS3-1
MS-LS3-2

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This NGSS-aligned passage introduces middle school readers to alleles—different versions of a gene. It explains how alleles are inherited from parents, and how dominant and recessive alleles determine an organism’s traits. Students learn key concepts like genotype, homozygous, heterozygous, and Punnett squares, and see how these apply to real-world examples like farming and health care. This passage supports NGSS standard MS-LS3-1 (Heredity: Inheritance and Variation of Traits) by helping students understand the relationship between genes, traits, and inheritance through engaging, grade-level-appropriate language.

CONTENT PREVIEW

In genetics, alleles are different versions of the same gene. Every organism has two alleles for most genes—one from each parent. These alleles work together to determine an organism’s traits, such as eye color, blood type, or the ability to roll your tongue.

Each gene controls a specific trait, but the way that trait appears can change depending on which alleles you inherit. Some alleles are dominant, meaning they will show up in your traits even if you only inherit one copy. Others are recessive, which means they only appear if you inherit two copies—one from each parent. For example, the allele for brown eyes is usually dominant, while the allele for blue eyes is recessive.

The combination of alleles you inherit makes up your genotype. If you inherit two of the same alleles (like BB or bb), you are homozygous for that gene. If you inherit two different alleles (like Bb), you are heterozygous. Even though someone who is heterozygous carries a recessive allele, they won’t show the recessive trait unless they pass it on to their children with another recessive allele.

Alleles help explain how traits are passed from one generation to the next. Scientists use tools like Punnett squares to predict how alleles will combine and which traits might appear in offspring.

Understanding alleles helps us explore heredity, genetic variation, and even disease risk. In farming, choosing plants with the right alleles can improve crop growth, color, and flavor. In medicine, knowing which alleles a person carries can help doctors understand their health needs.

Fun Fact: Even a tiny change in a single allele can create a completely different trait—like changing flower color from red to white!

What are alleles?

Types of cellsDifferent versions of a geneParts of a phenotypeUnused DNA

How many alleles does a person usually have for each gene?

OneTwoFourNone

Which allele type shows up with just one copy?

RecessiveMutatedDominantHidden

What does it mean to be homozygous?

Having one dominant alleleHaving two different allelesHaving two identical allelesShowing no traits

What is the genotype of someone who is heterozygous?

BBbbBbbB or bb

Why might a person carry a recessive allele but not show it?