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Mendelian & Non-Mendelian Traits in Humans: Definition & List

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  • 0:00 Traits
  • 0:58 Mendelian Traits
  • 2:40 Non-Mendelian Traits
  • 4:11 Examples in Humans
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor: Stephanie Matalone

Stephanie taught high school science and math and has a Master's Degree in Secondary Education.

Mendelian traits are physical characteristics passed down from parents based on dominant and recessive alleles. This lesson will go into more detail about what Mendelian and non-Mendelian traits are and how they are passed down through generations with examples of each.

Traits

Ever wondered where those freckles and dimples came from? The presence of freckles and dimples are just a few examples of Mendelian traits that are passed down from our parents. On the other hand, your hair and eye color are non-Mendelian traits, and their story is a bit more complicated.

Traits are physical characteristics that are passed down from parents to offspring. Examples of traits are the presence of freckles, blood type, hair color, and skin tone. Mendelian traits are traits that are passed down by dominant and recessive alleles of one gene. Alleles are different forms of genes, which are simply parts of DNA that carry information for a certain trait. Non-Mendelian traits are not determined by dominant or recessive alleles, and they can involve more than one gene. We'll examine both types of traits in greater detail, then see examples of how they're expressed in humans.

Mendelian Traits

Alleles are typically expressed by one letter. The capital form of the letter represents the dominant allele, while the lowercase version of the letter represents the recessive allele. Children get one allele for a trait from their father and the other allele for a trait from their mother. These two alleles come together to decide what the actual phenotype of a trait is going to be. A phenotype is the physical representation of a trait, such as brown hair, blue eyes, or freckles. If a child receives either one or two dominant alleles, they will show the dominant phenotype. If a child receives two recessive alleles, they will show the recessive phenotype.

Mendelian traits are named after an Austrian monk named Gregor Mendel who studied how traits are passed through generations of pea plants in the 1800s. In this image, you can see an example of two parent pea plants being crossed by use of a Punnett square, a diagram that helps determine the chance offspring will have a certain phenotype.

Punnett square

The first parent is on top and has two recessive alleles for pea color represented by the two lowercase ys (yy). The second parent on the side has one recessive and one dominant allele for pea plant color represented by the capital and lowercase ys (Yy). Parent one's phenotype will be a green pea color, while parent two's phenotype will be a yellow pea color. When these two parents cross, they each give one allele to the offspring. The possibilities for the alleles of offspring are seen in the four boxes. The offspring will either be Yy, which will make the offspring peas yellow, or yy, which will result in green peas.

Non-Mendelian Traits

Non-Mendelian traits are traits that are not passed down with dominant and recessive alleles from one gene. Polygenic traits are considered non-Mendelian because their alleles are located on more than one gene which allows for more alleles and phenotypes. Examples of polygenic traits are hair color and height.

Other traits, such as blood type, show codominance, where there is no dominant or recessive allele. In this case, both alleles are expressed completely, so an individual who has both alleles will show both phenotypes. This is why individuals with an AB blood type will have both A and B markers on their red blood cells.

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