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Genotype and Phenotype — Reading Comprehension

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MS-LS3-1
MS-LS3-2

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This comprehensive middle school science passage explores the relationship between genotype and phenotype, aligning with NGSS standards MS-LS3-1 and MS-LS3-2. Students will learn that genotype refers to the specific combination of alleles an organism possesses, while phenotype describes the organism’s observable characteristics. The passage investigates how both genetic makeup and environmental factors interact to determine traits, such as eye color or plant height, and introduces key concepts like homozygous and heterozygous alleles. Real-world examples and scientific explanations illustrate why organisms with different genotypes can sometimes have the same phenotype. The resource includes glossary, multiple-choice quiz, writing prompts, and graphic organizers to support understanding. All content is audio-integrated and designed for grades 6-8, providing differentiated and Spanish-translated versions for accessibility. This resource helps students build a foundational understanding of heredity, variation, and the impact of environment on living things.

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Genetics diagram showing genotype, phenotype, homozygous and heterozygous traits with eye color examples.

Genetics explores how living things inherit traits from their parents. In every population, some individuals have brown eyes, while others have blue or green. Scientists discovered that these differences are caused by a combination of genes and environmental factors. Understanding how our genotype and environment shape our phenotype is essential for learning about heredity and variation in living things.

How Genotype Determines Phenotype
An organism's genotype is its complete set of genes, made up of pairs of alleles. Alleles are different forms of a gene. For example, the gene for eye color can have an allele for brown (B) and another for blue (b). The combination of these alleles—such as homozygous (BB or bb) or heterozygous (Bb)—determines the genotype. The phenotype is the visible or measurable trait, like having brown eyes. In humans, the B allele is dominant, so a person with genotype BB or Bb will have brown eyes, while only someone with bb will have blue eyes. This direct connection between genotype and phenotype is called Mendelian inheritance.

Environment's Role in Shaping Phenotype
Although genotype provides the genetic instructions, the environment can influence how these instructions are expressed. For example, a plant may have the genotype to grow tall, but if the soil lacks nutrients, its phenotype might be a short plant. This demonstrates that phenotype results from the interaction between genotype and environment. Studies show that identical twins, who share the same genotype, can have differences in height or weight if they grow up in different environments. Some traits, like blood type, depend almost entirely on genotype, while others, like body weight, are influenced by both genes and lifestyle factors.

Why Different Genotypes Can Have the Same Phenotype
Sometimes, different genotypes produce the same phenotype. For example, both BB and Bb genotypes result in brown eyes. This is due to dominant and recessive alleles, where the dominant allele masks the effect of the recessive allele. Additionally, environmental factors can sometimes override genetic differences. For instance, a plant with a genotype for green leaves may develop yellow leaves if it receives too little sunlight, regardless of its genes. These complexities show that predicting phenotype from genotype alone can be challenging.

Understanding the relationship between genotype, phenotype, and environment helps scientists explain variation in populations and how organisms adapt to their surroundings. This knowledge is important in fields such as agriculture, medicine, and conservation biology, where predicting traits can have significant impacts.

Interesting Fact: A single human cell contains about 25,000 genes, but environmental factors can affect how many of these genes are expressed throughout a person’s life.

What is the main difference between genotype and phenotype?

Genotype is the genetic makeup; phenotype is the observable traits.Genotype is always visible; phenotype is always hidden.Genotype is only found in plants; phenotype is only in animals.Genotype is caused by the environment; phenotype is caused by genes only.

Which of the following is an example of a phenotype?

The Bb combination for eye colorHaving blue or brown eyesThe allele for brown eyesThe DNA sequence in a gene

What do alleles determine in an organism?

The physical environmentThe genetic makeup and possible traitsThe amount of sunlight a plant getsThe number of chromosomes

What does it mean if an organism is homozygous for a gene?

It has two different alleles for a gene.It has two identical alleles for a gene.It has only one allele for a gene.It cannot express any phenotype.

According to the passage, how can the environment affect the phenotype of a plant?

By changing the plant's genotypeBy determining which alleles the plant hasBy influencing how genes are expressed, such as making a tall plant grow short in poor soilBy making all plants the same height

What is meant by Mendelian inheritance?

A pattern where traits are controlled by dominant and recessive allelesA process of environmental changes onlyThe way plants absorb nutrients from soilInheritance that only involves identical twins

If two organisms have the genotypes BB and Bb, what is true about their phenotype for eye color?

They will both have blue eyes.One will have brown eyes and one will have blue eyes.They will both have brown eyes.They will have green eyes.

True or False: The phenotype of an organism is always determined only by its genotype.

TrueFalse

True or False: Environmental factors can sometimes override genetic instructions and change the phenotype.

TrueFalse

What is an example from the passage of a trait determined almost entirely by genotype?

Blood typeBody weightPlant heightLeaf color