Virtually every introduction to biology course will include a lesson on the theory of Mendelian genetics, along with a basic lab on traits and inheritance. A common and fun genetics exercise enables students to assess their own genetic traits.
In these exercises, students will assess various forms of genetic expression, including dimples, earlobes, freckles, tongue rolling and eye color genetics. Provided with a list of traits, biology students can examine their own genetic inheritance for each trait. This exercise requires a working knowledge of genotypes, phenotypes, dominant and recessive traits, alleles and other components of genetics and heredity.
What is an Allele in Genetics?
Before examining dominant and recessive genetic traits, it's important to understand the concept of an allele.
As explained by genetics researcher and former professor Marie George, an allele is a small bit of DNA coding that comprises a gene; each person has two genes for each sex-linked trait, located at a specific location on a specific chromosome. Every healthy human has 46 chromosomes, comprised of 23 chromosome pairs.
What are Phenotypes, Genotypes and Dominant and Recessive Traits in Genetics?
For certain genetic traits, there is one expression — referred to as genetic phenotype — that's dominant.
In Mendelian genetics, a lower case letter indicates a recessive gene, whereas a capital letter indicates a copy of a dominant gene. Every person inherits two copies of each gene – one copy is inherited from the mother and one copy is inherited from the father; therefore, there are three possible genotypes.
- tt – Homozygous for "t" allele (associated with recessive phenotypes);
- TT – Homozygous for "T" allele (associated with dominant phenotypes); and
- Tt – Heterozygous for "T" allele (associated with dominant phenotypes).
Therefore, with a recessive phenotype, it's certain that the individual has the "tt" genotype – two copies of a recessive gene. On the other hand, an individual with a dominant phenotype may exhibit the homozygous "TT" genotype (two copies of the dominant gene) or the heterozygous "Tt" genotype, which is indicative of one dominant gene and one recessive gene. The recessive gene is "overpowered" by the dominant gene, so the individual expresses the dominant trait.
Notably, in some texts, dominant phenotypes are represented with "A–" (instead of "AA or Aa"), since the second allele can be either "A" or "a."
Teachers and homeschool instructors can create a lab using the following information. This info can also serve as the basis of a worksheet, with students asked to determine their likely genotype; the information provided in bullet points can serve as an answer key. More advanced students can be asked to create a pedigree chart for each trait.
Students can also apply the concepts of genetic inheritance to a partners' exercise. Using their traits and likely genotypes, the partners can use punnett squares to assess their chances of having a blue-eyed baby, a baby with dimples, a baby with blue eyes and dimples, etc.
Eye Color Genetics
The alleles for eye color are "E" and "e." Brown eyes are a dominant trait, while blue is considered recessive, therefore:
- Brown eyes genotypes – EE or Ee;
- Blue eyes genotype – ee.
Inheritance of the Chin Cleft Gene
The alleles for chin clefts are "C" and "c." Chin clefts are a dominant phenotype (which, notably, tends to be exhibited in a way that's more prominent and obvious in men), therefore:
- Clefted chin genotypes – CC or Cc;
- Unclefted chin genotype – cc.
Facial Dimples and Genetic Heredity
The alleles for dimples are "D" and "d." Cheek dimples, which are most obvious while smiling, are a dominant trait, therefore:
- Dimple genotypes – DD or Dd;
- Undimpled genotype – dd.
Inheritance of the Freckles Phenotype
The alleles for freckles are "F" and "f." Freckles — present on the face and/or other parts of the body — are a dominant trait, therefore:
- Freckle genotypes – FF or Ff;
- Unfreckled genotype – ff.
Hereditary Genetics for Attached and Unattached Earlobes
The alleles for unattached earlobes are "U" and "u." The term "[un]attached earlobe" refers to the point at which the bottom of the ear — the lobe — connects to the head. If the earlobe "hangs" down and forms a flap of sorts, it's an unattached earlobe (see photos). Unattached earlobes are dominant, whereas attached earlobes are recessive. Therefore:
- Unattached earlobe genotypes – UU or Uu;
- Attached earlobe genotype – uu.
Widow's Peak Hairline Genetics
The allele for a widow's peak hairline is "W" and "w." A widow's peak — present if your hairline forms a "V" near the center of your forehead — is a dominant trait, therefore:
- Genotypes for a widow's peak hairline – WW or Ww;
- Genotype for people without a widow's peak – ww.
Genetic Inheritance and Tongue Rolling
The alleles for tongue rolling are "T" and "t." Some individuals can curl or roll their tongue (see photo), while others cannot. The ability to roll your tongue is linked to the inheritance of a dominant gene, therefore:
- Genotypes linked to the ability for tongue rolling – TT or Tt;
- Genotype linked to an inability to roll your tongue – tt.
Genes for a Bent Little Finger
The alleles for a bent little finger are "B" and "b." If your pinkie finger bends toward your ring finger, this is a dominant trait. Therefore:
- Bent pinkie finger genotypes – BB or Bb;
- Unbent pinkie finger genotype – bb.
Toe Length Genetics
The alleles for toe length are "L" and "l." If your second toe is longer than your big toe, this is a dominant trait. Therefore:
- Genotypes for a long second toe – LL or Ll;
- Genotype for a short second toe – ll.
Simple Science Experiments – PTC Strips and Gene Heredity
A fun and simple heredity experiment for homeschool students or students in a traditional classroom involves PTC strips, which can be ordered online for just a few dollars. The ability to taste the strips is rooted in one's genetics.
Approximately 70% of people — carriers of the dominant gene — will experience a bitter taste when the PTC strip is placed on the tongue, whereas an individual with two recessive copies of the gene will not taste anything.
Cautions Concerning the Over-Simplification of Genetic Inheritance
It's important to note that in many cases, multiple genes govern the expression of a given trait. The field of genetics is ever-expanding, and scientists have yet to unlock the human genetic code, so many genes remain unidentified. Therefore, it's important to emphasize that the aforementioned exercise is a simplification of a very complex process. For instance, the exercise does not account for processes like spontaneous genetic mutations.
In terms of eye color, the aforementioned exercise fails to account for green eyes, hazel eyes and the vast array of variations. According to a February 2007 article titled "Eye colour more complex than we thought," (Cosmos Magazine), "Generations of students have been taught that blue eye colour is a simple recessive genetic trait, but a new study [proves]...that eye colour is far more complicated, and under the control of many genes....now, 'contrary to what used to be thought, we've discovered it is possible for two blue-eyed parents to have a brown-eyed child,' said lead author Rick Sturm..."
Looking for more ideas for a biology labs, experiment or ideas for a science fair project? Read "Is a Dog's Mouth Cleaner Than a Human's?" for information on how to perform an experiment or science project on this topic.
Biology students who learning about the animal kingdom may wish to read "What is a Quadruped?" and "What is a Mammal?"
Sources:
Wood, Sarah, "Eye colour more complex than we thought," in Cosmos Magazine at CosmosMagazine.com. Published February 23, 2007. Accessed September 11, 2010.
September 2010 phone interview with Marie George, Miami, Fla.
Introduction to Biology lecture and lab notes, Simmons College. Fall 2002 semester.
Mia Carter - Mia Carter - Journalist, Producer/Editor & Web Writer. Carter's work has appeared on CBS and CNN.com.
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