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Three most recent questions: 2010-01-22 Although we can't publish all of the questions we receive, we do our best to share our insights on these issues. If you'd like us to consider a question you have about genetics, please fill out the question form.
Ask a Geneticist by Dr. Barry Starr, Stanford University What is the difference between dominant vs. recessive? Can you tell me what I could do to show my class about this without this being long? -A middle school student from New York
The way people write out dominant and recessive traits is the dominant one gets a capital letter and the recessive one a lower case letter. So for eye color, brown is B and blue is b. As I said above, people have two versions of each gene so you can be BB, Bb, or bb--BB and Bb have brown eyes, bb, blue eyes. Versions of genes are often dominant because the recessive version actually does nothing (click here to learn about other ways that gene versions can be dominant). In the eye color example above, the brown version of the gene makes a pigment that turns your eye brown but the blue version does not make a blue pigment. Instead, it makes no pigment and an eye without pigment is blue. As you can probably guess, if the blue version of the eye color gene made a pigment, then you'd get some mix of brown and blue. There are some cases like this for people. One of the easiest to understand is hair. There are two "hair type" genes, curly and straight. If you have two copies of the curly version, you have curly hair and if you have two copies of straight hair version, you have straight hair. What kind of hair do you have if you have a copy of each? Wavy. Each of these versions contributes something so that you get a mixture of the two. You would write this out as CC is curly, SS is straight and CS is wavy. In terms of what to talk about in your class, the hair type example I discussed above is a pretty good one for incomplete dominance. Maybe ask the class what kind of hair they have and what genes that means they have. You can also ask them about their parent's hair type and whether their results fits the model. For pure dominant and recessive traits, I've listed a bunch below that you could discuss with your class. Hope this helps. Good luck! Bent pinky: The dominant version of the gene causes distal segment of pinky finger to bend distinctly inward toward the ring (fourth) finger. Mid-digital hair: People lacking hair in the middle segments of the fingers have two recessive versions of the gene. Tongue rolling: People with a dominant allele can roll their tongues into a tube shape. People with two recessive versions are non-rollers and can not learn to roll their tongues. Ear lobes: Recessives have attached ear lobes. People with a dominant version of the gene have detached ear lobes. Thumb crossing: In a relaxed interlocking of fingers, left thumb over right results from having 1 or 2 copies of the dominant version of the gene. People with 2 recessives place right thumb over left. More Information |
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