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Introducing alleles as different versions of a gene and how they determine specific traits.
Have you ever wondered why you might have your mother's curly hair but your father's eye color—or even a trait, like a hitchhiker's thumb, that neither of them seems to have at all?
Think of a gene as a basic recipe for a trait, like 'hair texture.' An allele is a specific version of that recipe, such as 'curly' or 'straight.' Because you inherit one set of chromosomes from your biological mother and one from your biological father, you possess exactly two alleles for every gene. These two alleles live at the same location on your paired chromosomes. If you inherit the same version from both parents, your alleles are identical. If you inherit different versions, you have a mixture that determines how the trait actually appears in your body.
Quick Check
If a gene determines 'seed color' in a pea plant, what would the different 'flavors' (like green or yellow) be called?
Answer
These different versions are called alleles.
Not all alleles are created equal. In many cases, one allele acts as the 'boss' and masks the presence of the other. This is called a dominant allele. If you have even one dominant allele, that is the trait that will show up physically. On the other hand, a recessive allele is 'shy.' Its trait will only appear if you have two copies of it (one from each parent). If a dominant allele is present, the recessive trait stays hidden in your code, even though you still carry it.
In Gregor Mendel's famous experiments, he looked at plant height. 1. The allele for 'Tall' is dominant. 2. The allele for 'Short' is recessive. 3. If a plant has one 'Tall' allele and one 'Short' allele, the plant will grow up to be Tall because the dominant allele masks the recessive one.
Quick Check
What is the only way a recessive trait, like blue eyes or short height, can actually be seen in an organism?
Answer
The organism must inherit two recessive alleles (one from each parent) and have no dominant allele present.
To make genetics easier to track, scientists use a simple letter code. We use a capital letter for dominant alleles and the lowercase version of the same letter for recessive alleles. For example, if we use the letter for brown eyes (dominant) and for blue eyes (recessive), the possible combinations are: - : Two dominant alleles (Brown eyes) - : One dominant, one recessive (Brown eyes) - : Two recessive alleles (Blue eyes) Notice that in the combination, the capital letter is always written first!
Let's use the letter for 'Freckles' (dominant) and for 'No Freckles' (recessive). 1. Person A has the code . Since they have one , they will have freckles. 2. Person B has the code . Since they have no capital , they will not have freckles. 3. Person C has the code . They will have freckles.
Imagine a species of alien where Purple skin () is dominant over Green skin (). 1. You see a Green alien. 2. Because Green is recessive, you immediately know its code must be . 3. If that Green alien had even one , it would have been Purple. Therefore, to be Green, it must have received a from both its mother and its father.
Which of the following represents a dominant allele in genetic notation?
If stands for yellow seeds (dominant) and stands for green seeds (recessive), what color will a plant with the code be?
An organism can have three different alleles for a single gene inherited from their parents.
Review Tomorrow
In 24 hours, try to explain to a friend the difference between a 'gene' and an 'allele' using the 'recipe' analogy.
Practice Activity
Look at your own traits (like whether your earlobes are attached or detached). If detached earlobes are dominant (), write down the two possible letter codes you might have.