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How Homozygosity Affects Phenotype — Reading Comprehension

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Dive into the fascinating world of genetics with this engaging reading passage, 'How Homozygosity Affects Phenotype.' This audio-integrated resource for 7th graders breaks down complex concepts like alleles, genotypes, and phenotypes using simple language. Students will learn how specific combinations of genes, particularly homozygous states, determine an organism's observable characteristics. Aligned with NGSS Standard LS3.A: Inheritance of Traits, this passage clarifies how an organism's phenotype is determined by the combinations of alleles it inherits, enhancing understanding of genetic inheritance and its impact on traits. Key terms like dominant and recessive alleles are defined, making the passage accessible and informative.

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Genetics diagram showing DNA, alleles, genotypes, and phenotypes in plants and humans — Wide educational illustration of homozygosity and phenotype inheritance in plants and humans

Imagine looking at a family, and you notice that a child has bright blue eyes, just like both of their parents. Or perhaps a plant in your garden produces only yellow flowers, generation after generation. These observations hint at the powerful principles of genetics, specifically how specific gene combinations lead to observable traits, also known as phenotype.

Every living thing, from humans to plants, has a set of instructions called DNA. Within this DNA are sections called genes. Genes are like recipes for different traits, such as eye color, hair texture, or even whether a plant's seeds are round or wrinkled. For each gene, an organism inherits two copies, one from each parent. These copies are called alleles.

Now, let's talk about how these alleles combine. The specific combination of alleles an organism has for a particular gene is called its genotype. When an organism inherits two identical alleles for a specific gene, it is said to be homozygous for that trait. For example, if a plant inherits two alleles for yellow flowers (let's say 'YY'), it is homozygous for yellow flowers. Similarly, if it inherits two alleles for green flowers ('yy'), it is homozygous for green flowers. In contrast, if it inherits one yellow and one green allele ('Yy'), it is heterozygous.

How does homozygosity affect the phenotype? This depends on whether the alleles are dominant or recessive. A dominant allele expresses its trait even if only one copy is present. A recessive allele, however, only expresses its trait if two copies are present, meaning the organism must be homozygous recessive. For instance, in pea plants, the allele for tallness (T) is dominant over the allele for shortness (t). A plant with a 'TT' genotype is homozygous dominant and will be tall. A plant with a 'tt' genotype is homozygous recessive and will be short. In both these homozygous cases, the phenotype directly reflects the alleles present.

Consider the blue eyes example. Blue eye color is often a recessive trait. So, for a child to have blue eyes, they usually must inherit the blue-eye allele from both parents, making them homozygous recessive for that specific eye color gene. If they inherited even one dominant allele for brown eyes, they would likely have brown eyes instead. Understanding homozygosity is crucial for predicting how traits are passed down through generations in genetics, from simple characteristics like flower color in peas to more complex traits in humans.

Interesting Fact: Some genetic conditions are more likely to appear if an individual is homozygous recessive for a particular gene.

What are the two copies of a gene called?

AllelesPhenotypesGenotypesDNA

What determines an organism's phenotype?

Allele combinationsOnly recessive allelesOnly dominant allelesExternal environment

True or False: A dominant allele always shows its trait.

TrueFalse

If an organism has a homozygous dominant genotype, what does this mean?

Two identical dominant allelesTwo different allelesTwo identical recessive allelesOne dominant, one recessive

Which of these is an example of a phenotype?

Blue eye colorGenotype 'Tt'DNA strandA specific gene

Why might blue eyes be considered a recessive trait?

Needs two blue alleles to showOnly needs one blue alleleIt is always dominantInherited from only one parent

What is the primary role of genes in DNA?

Recipes for traitsStoring energyFighting diseasesBuilding bones

The term 'homozygous' is best defined as:

Having identical allelesHaving different allelesObservable traitsGenetic instructions

If a plant with genotype 'TT' (tall) crosses with a plant with genotype 'tt' (short), and 'T' is dominant, what would be the phenotype of their offspring's height?

All tallAll shortHalf tall, half shortMedium height

A scientist observes a new species of flower where red (R) is dominant over white (r). If a flower shows a white phenotype, what can be concluded about its genotype?

It must be 'rr'It must be 'Rr'It could be 'RR' or 'Rr'It must be 'RR'