Where Do the Physical Traits We Inherit Come From?

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Mom and Dad

  • Physical traits are inherited through genes. Your mother and father each contributed to the way you look by passing on their genetic material to you. Most of the traits that are one can see and identify in another person, e.g. hair color, eye color, height, weight, skin color, fingerprints and shape of face and facial features are called "polygenic" or continuous traits. Unlike the Mendelian traits that include diseases such as Marfan's Syndrome, Dwarfism and Elephant Man Syndrome, it is not an all or nothing situation. Think of your eye color, hair color, etc. as an average of the traits that your parents posses. Whichever group of traits, either mom's or dad's, that was dominant over the other, is the one that is expressed in the way that you look. For example, if your mother has blue eyes and your father has brown eyes, in most cases you will have brown eyes like dad. The reason for this is that brown is dominant over blue. How dark or light your brown eyes are is related to the other genes' contributions to your eye color.

Grandma and Grandpa

  • Our genes are actually comprised of much more than what we see. Thus the way you look is called your phenotype and your actual genetic makeup is your genotype. For example, a brown-eyed (phenotype) person may have a genotype of Bb for one brown trait and one blue trait. Since brown is usually dominant, this person, who also carries a blue eyed trait, is brown-eyed. This distinction can also be thought of as dominant and recessive or expressed or unexpressed. In this manner, brown-eyed parents may produce blue-eyed children. How can this be? The brown-eyed parents probably have genotypes of Bb. Somewhere back on the family tree, a blue-eyed person contributed his genes to the lineage. This recessive blue-eyed trait may linger undetected for generations, until it meets enough of another blue-eyed trait to be expressed as the phenotype of an individual. Diseases and deficiencies such as Tay Sachs, Albinism, Color Deficiency, Cystic Fibrosis and PKU are recessive genes that also work in this manner. These physical problems are unexpressed in people called "carriers." When one carrier has children with another carrier, the trait is expressed.

The Environment

  • In many cases, what we inherit from our families is a set of potentials. Mendelian traits are unlikely to be affected by outside influences, but many polygenic traits are. For example, a baby may be born with the genetic potential to grow to a height of 6'3" -- but if his family is poor and he does not get the proper nutrition, he will not grow to his full height. In the same manner, exposure to sunlight, altitude, temperature, and environmental pollutants can affect the degree to which the genetic potential is reached. It is important to remember that environmental factors play a role in the translation from genotype to phenotype.

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