You may think you got your mother’s hair color but your father’s curls. The combination of characteristics that makes you an individual is your phenotype, and some of it is influenced by your environment. However, much of your phenotype is determined by the genetic instructions, or genotypes, you inherited from both sides of your family. Although your hair characteristics may seem to physically favor one side over the other, you get the same amount of genetic information from both parents.
In the Beginning
The story of inheritance begins with DNA, molecules that contain the genetic information needed to carry out life processes. Each gene, made up of DNA, has instructions for a specific trait. Some genes carry enough directions to completely control a trait, but others interact to determine characteristics. Genes have two or more variations called alleles. All this information is packaged into chromosomes, the structures that pass genetic directions from parents to offspring.
From Both Sides
During fertilization, the sex cells -- egg and sperm -- combine. These cells, called gametes, create a fertilized egg: a zygote. Each gamete (usually) has 23 chromosomes, so the zygote (usually) ends up with 46. You can think of them as 23 pairs, because both parents pass along alleles for every trait. Therefore, even if you end up with your mother’s straight brown hair, you also have genetic information about hair color and texture from your father.
Not Always the Same
For each trait, parents might contribute the same type of information, but they could pass along different versions. If the alleles are the same, the trait is homozygous, but if they are different, it is heterozygous. Usually a homozygous trait will show up in your phenotype. Heterozygous traits can be much more complicated. Sometimes one allele is dominant, and it is expressed over the recessive gene. This is true with a widow’s peak. Even if you have inherited just one allele for that trait, you will have that V shape above your forehead. You will have a straight hairline only if you receive two “non-widow’s peak” alleles. However, dominance does not mean that one allele completely shuts down the other; both are active.
A Complex System
Many hair traits' operation is more complex than simple dominance. Several genes contribute to the final phenotype. Your hair may be curly, wavy or straight. Texture is determined by multiple genes and alleles, and both differ among world populations. For example, straight hair in Asian countries is caused by alleles from two genes, but the genes are different from the ones for straight-haired Europeans. Also, the alleles for curly hair are not the same in Africa and Europe. Hair color also involves many genes. They control the amount of brown pigment, its production, placement of pigment onto the hair structure and the production of red pigment. Each parent provides an allele for every one of these genes. It is the interaction among them, and not any single gene, that determines the character of your hair.
- UC Berkeley: Understanding Evolution: Genotype versus Phenotype
- UC–San Francisco: UCSF Memory and Aging Center: Genetics
- BBC: Bitesize: Formation of Sex Cells
- University of Washington: Genotypes and Phenotypes
- University of Utah: Inherited Human Traits: A Quick Reference
- Indiana University: Genetics: Reality Rears Its Head
- Thinkstock Images/Stockbyte/Getty Images