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What Does It Mean to Be Heterozygous? — Reading Comprehension

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MS-LS3-1
MS-LS3-2

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This NGSS-aligned reading passage introduces middle school students to the concept of heterozygous traits in genetics. It explains that being heterozygous means having two different alleles for a gene, typically one dominant and one recessive. Students learn how dominant traits are expressed and how Punnett squares are used to model inheritance. The passage also covers real-world applications, such as health advantages like resistance to malaria and agricultural benefits through hybrid vigor. Important vocabulary such as alleles, genotype, and heredity are clearly defined, supporting both science comprehension and NGSS MS-LS3-1 standards for heredity and trait variation.

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In genetics, a person is called heterozygous when they inherit two different alleles for a specific gene—one from each parent. Alleles are different versions of a gene. Genes control our traits, like eye color, hair type, or blood type. If the two alleles are not the same, the person is heterozygous for that trait.

In a heterozygous pair, one allele is usually dominant and the other is recessive. The dominant allele masks the effect of the recessive allele, so the dominant trait is the one that shows up in the person's appearance. For example, the gene for brown eyes (B) is dominant over the gene for blue eyes (b). A person with the genotype Bb is heterozygous and will have brown eyes, even though they carry the gene for blue eyes.

Being heterozygous is common in many traits and is important in understanding heredity. Scientists often use a Punnett square to show how genes are passed from parents to offspring. If one parent is homozygous dominant (BB) and the other is homozygous recessive (bb), all their children will be heterozygous (Bb) and show the dominant trait.

In real life, heterozygosity plays a role in both genetic diversity and health. For some diseases, like sickle cell anemia, being heterozygous can actually offer a survival advantage. People with one normal gene and one sickle cell gene usually do not get the disease, and they may have better protection against malaria. In agriculture, heterozygous plants often grow better and are more resistant to disease, a benefit known as hybrid vigor.

Understanding heterozygous traits helps scientists and doctors predict inherited conditions, breed better crops, and learn more about how genes work in everyday life.

Fun Fact: People who are heterozygous for the sickle cell gene are less likely to get malaria—this is called a “genetic advantage”!

What does heterozygous mean?

Two identical allelesNo allelesTwo different allelesOnly recessive alleles

Which pair of alleles shows a heterozygous genotype?

BBbbBbBBbb

What kind of trait shows up in a heterozygous person?

The weaker traitThe recessive traitThe learned traitThe dominant trait

What tool helps predict gene inheritance?

MicroscopeChartPunnett squareGraph

Why do children of BB and bb parents become Bb?

Genes cancel each otherOnly recessive genes are passedOne dominant and one recessive geneGenes are random

What is one benefit of being heterozygous for sickle cell disease?