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Learning the difference between an organism's physical appearance and its genetic makeup.
Why can two brown-eyed parents sometimes have a blue-eyed baby? It seems like a magic trick, but it's actually a secret code hidden inside their cells waiting for the right moment to appear.
In biology, every living thing has two 'identities.' Think of a house: you have the architect's blueprints (the instructions) and the actual building (what you see). In genetics, the Genotype is the 'blueprint'—the actual genetic code inherited from parents, represented by letters. The Phenotype is the 'building'—the physical expression or observable trait, like height, hair color, or leaf shape. While you can see a phenotype just by looking at someone, the genotype is hidden deep within their DNA.
Quick Check
If a scientist observes that a cat has orange fur, is the scientist describing the cat's genotype or phenotype?
Answer
Phenotype
Genes come in different versions called alleles. We represent these with letters. A dominant allele is like a loud voice that drowns out others; we use a capital letter (e.g., ). A recessive allele is quieter and only shows up if the dominant one is missing; we use a lowercase letter (e.g., ). Because you get one allele from each parent, they come in pairs. Homozygous (purebred) means the two alleles are identical ( or ). Heterozygous (hybrid) means the two alleles are different ().
Let's look at pea plants where Tall () is dominant and short () is recessive. 1. A plant has the genotype . 2. Since it has at least one , the phenotype is Tall. 3. Because both letters are the same, this is Homozygous Dominant.
Quick Check
What do we call a genotype that has one capital letter and one lowercase letter (e.g., )?
Answer
Heterozygous (or Hybrid)
To predict what an organism will look like, you only need to look for the dominant allele. If a genotype has at least one capital letter (like or ), the dominant trait will always be the phenotype. The only way a recessive trait (like blue eyes or short height) appears in the phenotype is if the genotype is homozygous recessive (two lowercase letters, like ). This is why recessive traits are rarer; they are easily 'hidden' by dominant ones.
In fruit flies, Long wings () are dominant over vestigial (short) wings (). 1. A fly has the genotype . 2. Even though it carries the 'short wing' gene (), the 'Long wing' gene () is dominant. 3. The phenotype is Long wings. 4. This fly is a 'carrier' for short wings but doesn't show them!
Imagine a specific flower color where Purple () is dominant and white () is recessive. 1. To have a white phenotype, the plant cannot have any alleles. 2. Therefore, the only possible genotype for a white flower is . 3. If you cross two (purple) plants, there is a chance the offspring will be (white).
Which of the following is a phenotype?
If = Brown eyes and = blue eyes, what is the phenotype of an individual with the genotype ?
An organism with the genotype is considered heterozygous.
Review Tomorrow
Tomorrow morning, try to explain to a friend why a person can carry a gene for a trait (like red hair) without actually having that trait themselves.
Practice Activity
Look at your own traits (e.g., detached earlobes vs. attached). If detached is dominant (), write down the two possible genotypes you might have if your earlobes are detached.