Science, Tech, Math › Science How Do Alleles Determine Traits in Genetics? Share Flipboard Email Print An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. Darryl Leja / NHGRI Science Biology Genetics Basics Cell Biology Organisms Anatomy Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated August 21, 2019 An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. The process by which alleles are transmitted was discovered by scientist and abbot Gregor Mendel (1822–1884) and formulated in what is known as Mendel's law of segregation. Dominant and Recessive Alleles Diploid organisms typically have two alleles for a trait. When allele pairs are the same, they are homozygous. When the alleles of a pair are heterozygous, the phenotype of one trait may be dominant and the other recessive. The dominant allele is expressed and the recessive allele is masked. This is known as complete genetic dominance. In heterozygous relationships where neither allele is dominant but both are completely expressed, the alleles are considered to be co-dominant. Co-dominance is exemplified in AB blood type inheritance. When one allele is not completely dominant over the other, the alleles are said to express incomplete dominance. Incomplete dominance is exhibited in pink flower color inheritance from red and white tulips. Multiple Alleles While most genes exist in two allele forms, some have multiple alleles for a trait. A common example of this in humans is ABO blood type. Human blood type is determined by the presence or absence of certain identifiers, called antigens, on the surface of red blood cells. Individuals with blood type A have A antigens on blood cell surfaces, those with type B have B antigens, and those with type O have no antigens. ABO blood types exist as three alleles, which are represented as (IA, IB, IO). These multiple alleles are passed from parent to offspring such that one allele is inherited from each parent. There are four phenotypes (A, B, AB, or O) and six possible genotypes for human ABO blood groups. Blood Groups Genotype A (IA,IA) or (IA,IO) B (IB,IB) or (IB,IO) AB (IA,IB) O (IO,IO) The alleles IA and IB are dominant to the recessive IO allele. In blood type AB, the IA and IB alleles are co-dominant as both phenotypes are expressed. The O blood type is homozygous recessive containing two IO alleles. Polygenic Traits Polygenic traits are traits that are determined by more than one gene. This type of inheritance pattern involves many possible phenotypes that are determined by interactions among several alleles. Hair color, skin color, eye color, height, and weight are all examples of polygenic traits.The genes contributing to these types of traits have equal influence and the alleles for these genes are found on different chromosomes. A number of different genotypes arise from polygenic traits consisting of various combinations of dominant and recessive alleles. Individuals inheriting only dominant alleles will have an extreme expression of the dominant phenotype; individuals inheriting no dominant alleles will have an extreme expression of the recessive phenotype; individuals inheriting different combinations of dominant and recessive alleles will exhibit varying degrees of the intermediate phenotype. What Are Heterozygous Traits? What Does Heterozygous Mean? Don't Look Like Your Sibs? Mendel's Law of Independent Assortment Explains Why Genes, Traits and Mendel's Law of Segregation Gene vs. Allele: What’s the Difference? Monohybrid Cross: A Breeding Experiment Sex-Linked Traits and Disorders From Genes and the Recessive Disorders What Makes a Trait Homozygous? What Is a Dihybrid Cross in Genetics? What You Should Know about Blood Types Understanding Incomplete Dominance in Genetics What Is Mendel's Law of Segregation? Learn How to Calculate Probabilities in Genetics with Punnett Squares Why We Look Like Our Parents The Difference Between Genotype and Phenotype What Are Traits and How Are They Determined?