Science, Tech, Math › Science Phenotype: How a Gene Is Expressed As a Physical Trait Share Flipboard Email Print Cultura RM / Ray Knight / Getty Images Science Biology Genetics Basics Cell Biology Organisms Anatomy Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate by Regina Bailey Regina Bailey is a science writer and educator who has covered biology for ThoughtCo since 1997. Her writing is featured in Kaplan AP Biology 2016. Updated April 20, 2018 Phenotype is defined as an organism's expressed physical traits. Phenotype is determined by an individual's genotype and expressed genes, random genetic variation, and environmental influences. Examples of an organism's phenotype include traits such as color, height, size, shape, and behavior. Phenotypes of legumes include pod color, pod shape, pod size, seed color, seed shape, and seed size. Relationship Between Genotype and Phenotype An organism's genotype determines its phenotype. All living organisms have DNA, which provides instructions for the production of molecules, cells, tissues, and organs. DNA contains the genetic code that is also responsible for the direction of all cellular functions including mitosis, DNA replication, protein synthesis, and molecule transportation. An organism's phenotype (physical traits and behaviors) are established by their inherited genes. Genes are certain segments of DNA that code for the production of proteins and determine distinct traits. Each gene is located on a chromosome and can exist in more than one form. These different forms are called alleles, which are positioned on specific locations on specific chromosomes. Alleles are transmitted from parents to offspring through sexual reproduction. Diploid organisms inherit two alleles for each gene; one allele from each parent. Interactions between alleles determine an organism's phenotype. If an organism inherits two of the same alleles for a particular trait, it is homozygous for that trait. Homozygous individuals express one phenotype for a given trait. If an organism inherits two different alleles for a particular trait, it is heterozygous for that trait. Heterozygous individuals may express more than one phenotype for a given trait. Traits can be dominant or recessive. In complete dominance inheritance patterns, the phenotype of the dominant trait will completely mask the phenotype of the recessive trait. There are also incidences when the relationships between different alleles don't exhibit complete dominance. In incomplete dominance, the dominant allele does not mask the other allele completely. This results in a phenotype that is a mixture of the phenotypes observed in both alleles. In co-dominace relationships, both alleles are fully expressed. This results in a phenotype in which both traits are independently observed. Genetic Relationship Trait Alleles Genotype Phenotype Complete Dominance Flower Color R - red, r - white Rr Red flower Incomplete Dominance Flower Color R - red, r - white Rr Pink flower Co-dominance Flower Color R - red, r - white Rr Red and white flower Phenotype and Genetic Variation Genetic variation can influence the phenotypes seen in a population. Genetic variation describes the gene changes of organisms in a population. These changes may be the result of DNA mutations. Mutations are changes in the gene sequences on DNA. Any change in the gene sequence can change the phenotype expressed in inherited alleles. Gene flow also contributes to genetic variation. When new organisms migrate into a population, new genes are introduced. The introduction of new alleles into the gene pool makes new gene combinations and different phenotypes possible. Different gene combinations are produced during meiosis. In meiosis, homologous chromosomes randomly segregate into different cells. Gene transfer may occur between homologous chromosomes through the process of crossing over. This recombining of genes can produce new phenotypes in a population. Continue Reading What Makes a Trait Homozygous? Why We Look Like Our Parents Monohybrid Cross: A Breeding Experiment What Does Heterozygous Mean? Understanding Incomplete Dominance in Genetics What Is a Dihybrid Cross in Genetics? Learn the Essentials of Mendel's Law of Independent Assortment How do Alleles Determine Traits in Genetics? The Importance of Genetic Variation Why True-Breeding Plants Are Special What Is Mendel's Law of Segregation? Types of Non-Mendelian Genetics Genes, Mutations, and Genetic Inheritance Pleiotropy: How One Gene Affects Many Traits What Are Traits and How Are They Determined? Can Acquired Traits Be Passed on to Offspring?