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Biology LibreTexts

25.18: Glossary: R

  • Page ID
  • race

    discrete category within the human species that groups together individuals based on a few readily observable traits such as skin color and hair texture


    association of racial traits such as skin color with unrelated traits such as intelligence, often leading to prejudice and discrimination against people based only on how they look

    radiation therapy

    treatment of cancer using ionizing radiation such as X rays to kill cancerous tissues


    radioactive gas from underground rocks that may cause lung cancer


    starting substance in a chemical reaction

    reading frame

    grouping of nitrogen bases in DNA into three-base codons


    protein on a cell membrane that binds with a hormone, neurotransmitter, or other chemical


    referring to an allele that is masked by the presence of another allele for the same gene when they occur together in a heterozygote; or referring to a trait controlled by such an allele

    recombinant DNA

    DNA that results when DNA from two organisms is combined


    short part of the large intestine between the colon and anus where feces is stored until it is eliminated through the anus

    red blood cell

    type of cell in blood that contains hemoglobin and carries oxygen


    rapid motor response to a sensory stimulus in which nerve impulses travel in an arc that includes the spinal cord but not the brain; in infants, may refer to one of several instinctive behaviors, such as crying and sucking, that help an infant survive

    regulatory element

    region of DNA where a regulatory protein binds

    regulatory protein

    protein that regulates gene expression

    relative dating

    method of dating fossils by their location in rock layers that determines which fossils are older or younger but not their age in years

    renal pelvis

    funnel-like end of a ureter where it enters the kidney and where urine collects before it is transported through the ureter

    renal tubule

    tubular structure of a nephron in a kidney through which filtered substances pass and where some filtered substances are reabsorbed by the blood and additional substances are secreted from the blood

    renewable resource

    natural resource that can be replenished by natural processes as quickly as humans use it

    replacement fertility rate

    fertility rate at which women average only enough children by the end of their reproductive years to replace themselves and their partner in the population


    reproducing a scientific investigation and getting the same result


    process by which living things give rise to offspring

    reproductive system

    organ system responsible for the production and fertilization of gametes and, in females, the carrying of a fetus


    part of a biogeochemical cycle that holds an element or water for a long period of time


    exchange of gases between the body and the outside air

    respiratory center

    one of several areas in the medulla and pons of the brain stem that help control unconscious breathing

    respiratory system

    organ system that brings oxygen into the body and releases carbon dioxide and other waste gases into the atmosphere

    respiratory tract

    continuous system of passages through which air flows into and out of the body

    resting potential

    difference in electrical charge across the plasma membrane of a neuron that is not actively transmitting a nerve impulse

    reticular layer

    lower layer of the dermis that gives the dermis strength and elasticity and contains many dermal structures such as glands and hair follicles


    layer of photoreceptor cells at the back of the eye, including rods that are particularly sensitive to dim light and cones that are sensitive to light of different colors

    Rhesus (Rh) blood group system

    system of blood cell antigens controlled by two genes with many alleles on chromosome 1, producing five common antigens of which the main antigen is the D (Rh+) antigen


    bony “cage” enclosing the thoracic cavity and consisting of the ribs, thoracic vertebrae, and sternum

    ribosomal RNA (rRNA)

    type of RNA that helps form ribosomes and assemble proteins


    organelle inside all cells where proteins are made


    disorder in children caused by vitamin D deficiency and characterized by soft, weak bones and skeletal deformities


    skin infection by the fungus Trichophyton that causes a characteristic ring-shaped rash

    RNA (ribonucleic acid)

    single-stranded nucleic acid that helps make proteins

    RNA world hypothesis

    hypothesis that RNA was the first biochemical molecule to evolve and that early life was based on RNA, rather than DNA or proteins


    precipitation that falls on land and flows over the surface of the ground