The DNA in cells contains the genetic information for each individual, including the physical features that we associate with how someone looks, such as hair color, eye color, freckles and dimples. Children inherit physical traits from their parents when parents pass copies of their genes to their children.
Mechanics of Heredity
Humans have 46 chromosomes within the nucleus of each cell. Each parent contributes 23 chromosomes to their child, and together the chromosomes form 23 homologous pairs in the offspring. Chromosomes contain all the genetic information for each person in their DNA, which is carried by genes.
Individuals possess two copies of each gene because they inherit one of each gene from their mother and one from their father. There is more than one version of each gene, which are called alleles. Some genes have two alleles that code for a particular trait, while other genes have many alleles.
What Are Physical Traits?
Physical traits are observable characteristics that children inherit from their parents. Some physical traits, such as freckles, are expressed completely due to dominant or recessive inheritance of a single gene. Other traits are expressed in varying degrees because they are influenced by multiple genes, such as left- or right-handedness.
A partial list of inherited traits includes:
- Curly hair
- Free or attached ear lobes
- Hair color
- Eye color
Dominant and Recessive
One factor that determines which allele becomes inherited by a child depends on whether the allele is dominant or recessive. A dominant physical trait only needs a dominant allele inherited from one parent to be expressed. A recessive physical trait requires a recessive allele inherited from both parents.
Most traits do not follow a simple pattern of dominant or recessive inheritance. For some genes, the allele that is expressed (whether it is dominant or recessive) completely masks the expression of the other allele. Sometimes the expressed trait is a blend of both alleles, or else the expression falls somewhere between the two alleles.
In some cases, recessive traits are less common than dominant traits because they require both parents to carry the recessive allele. However, in some populations the recessive expression of certain traits is more common than the dominant allele, despite that only one parent needs to pass on the allele.
Variations in Genes: Eye and Hair Color
Some physical features are determined by a single gene, while other traits manifest due to the expression of two or more genes. These genes are expressed in various degrees, resulting in a specific version of a physical trait. Some characteristics passed on from parents to offspring are due to a combination of expressed genes and do not necessarily follow the pattern of dominant or recessive inheritance.
More than 20 genes play a role in controlling the proteins involved in expressing hair color. The proteins activate the production of two pigments responsible for hair color—pheomelanin for red hair and eumelanin for blond, black or brown hair. While one gene can determine black or brown hair, the expression of multiple genes determines shades that range from lightest blond to black or a combination of shades, like reddish brown.
Variations among genes involved with melanin determine eye color. Genes that activate the production of a large amount melanin result in brown eyes and a small amount of melanin produces blue eye color, with shades of green and hazel falling between the two ends of the spectrum. Eye color is mainly determined by two genes, with several other genes playing a smaller part in how eye color is expressed.
- University of Utah Genetic Science Learning Center: Observable Human Characteristics
- University of Utah Genetic Science Learning Center: What Are Dominant and Recessive?
- University of Hawai'i Manoa: Biology 171- Characteristics and Traits
- National Institutes of Health Genetics Home Reference: What is a Gene?
- National Institutes of Health Genetics Home Reference: Is Eye Color Determined by Genetics?
- National Institutes of Health Genetics Home Reference: Is Hair Color Determined by Genetics?
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