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Dominant and Recessive Alleles — Reading Comprehension

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This in-depth, NGSS-aligned science passage for grades 6-8 explores the mechanisms of dominant and recessive allele inheritance. Students will learn how alleles, represented by capital and lowercase letters, interact to determine traits, with examples from Mendel’s pea plants and common human characteristics such as widow’s peak and attached earlobes. The text explains scientific thinking behind these discoveries and shows real-world implications for genetics, health, and biotechnology. Activities include a glossary, multiple choice quiz, writing prompts, and graphic organizers for deeper understanding. Audio integration supports accessibility, and Spanish translations are included. This resource supports MS-LS3-2 and is perfect for classroom instruction or independent learning.

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"Broca Eye Color Chart" by Paul Broca / Wikimedia Commons (Public domain).

Genetic inheritance shapes every living organism, from the color of pea plant flowers to the presence of a widow’s peak in humans. Some traits seem to appear more often in families, while others can skip generations. This pattern can be explained by the way alleles, or different forms of a gene, interact in our DNA. Understanding dominant and recessive alleles helps scientists, doctors, and breeders predict how traits are passed on from one generation to the next.

How Dominant and Recessive Alleles Work
Each gene comes in two copies, one from each parent. These copies may be the same or different. A dominant allele is one that will show its effect if at least one copy is present. It is usually represented by a capital letter, such as 'P' for purple flower color in pea plants. A recessive allele will only show its effect if an organism has two copies, one from each parent. This allele is represented by a lowercase letter, like 'p' for white flower color. When a plant has both alleles (Pp), the dominant allele masks the effect of the recessive one, so the flowers appear purple. This mechanism was first described by Gregor Mendel in the 1800s, who carefully counted flower colors over generations and discovered predictable ratios—about 75% of the plants had purple flowers and 25% had white in the second generation.

Real-World Examples and Connections
Dominant and recessive alleles do not only apply to plants. In humans, traits such as a widow’s peak (a V-shaped point in the hairline) are dominant, while a straight hairline is recessive. Attached earlobes are recessive, while free earlobes are dominant. Another example is the hitchhiker’s thumb, a trait controlled by dominant and recessive alleles. The concept of heterozygous individuals—those with one dominant and one recessive allele—explains why some people can carry a recessive trait without showing it. They are known as carriers. This knowledge is important for understanding genetic diseases, many of which are caused by recessive alleles and can be passed silently from one generation to the next.

Implications for Health and Society
Studying dominant and recessive alleles allows scientists to predict the likelihood of certain traits or diseases appearing in offspring. By using tools like the Punnett square, researchers can visualize how alleles combine and calculate probabilities. This information helps genetic counselors advise families about inheritance risks. Advances in biotechnology, such as gene editing, rely on understanding allele interactions to address health challenges and create new solutions. The patterns discovered by Mendel connect to the broader principle that genes control the structure and function of living things, a key idea in life science.

Understanding how dominant and recessive alleles work gives us insight into the amazing variety of life. It helps us make informed choices about health, agriculture, and technology, and reminds us that even simple patterns in nature can lead to powerful scientific discoveries.

Interesting Fact: Some human traits, like eye color, are controlled by more than one gene, which is why blue-eyed parents can sometimes have a brown-eyed child!

What is a dominant allele?

An allele that is expressed when at least one copy is presentAn allele that is only expressed when two copies are presentA gene found only in humansA type of DNA that cannot be inherited

Which scientist first described the patterns of dominant and recessive alleles in pea plants?

Charles DarwinGregor MendelRosalind FranklinIsaac Newton

What is the function of a Punnett square?

To predict possible combinations of alleles in offspringTo measure the length of DNATo count the number of chromosomesTo draw the shape of a gene

According to the passage, what percentage of Mendel’s second generation pea plants had purple flowers?

50%100%75%25%

What does 'heterozygous' mean in the context of genetics?

Having two copies of the same alleleHaving two different alleles for a geneBeing unable to inherit traitsHaving no alleles

In the passage, what is meant by a 'carrier'?

A person who has two dominant allelesA person who has two recessive allelesA person who has one dominant and one recessive allele but does not show the recessive traitA person who cannot pass on any alleles

Which of the following is an example of a dominant human trait mentioned in the passage?

Straight hairlineWidow’s peakAttached earlobesWhite flower color

True or False: Recessive alleles are always visible in an organism's traits.

TrueFalse

True or False: The concept of dominant and recessive alleles only applies to plants.

TrueFalse

What is one real-world application of understanding dominant and recessive alleles?

Predicting inheritance of genetic diseasesBuilding rocketsMeasuring water pollutionCounting tree rings