25.21: Glossary: D

data (singular, datum)

observations collected in a scientific investigation in order to test a hypothesis

dead zone

area in the ocean or other body of water where low oxygen levels following excessive growth of algae have killed virtually all aquatic organisms

death rate

number of deaths per year per 1,000 people in a population

deciduous teeth

first set of 20 teeth that emerge in infancy and early childhood and are lost and replaced by permanent teeth during middle childhood; also called baby teeth

decomposer

organism that breaks down the remains of dead organisms or other organic wastes

demographic transition

major population shift in which the death rate falls and the population growth rate increases, followed later by the birth rate falling and the population growth rate decreasing

demography

scientific study of human populations

dendrite

extension of the cell body of a neuron that receives nerve impulses from other neurons

dependent variable

variable in a scientific experiment that is affected by another variable, known as the independent variable; also called outcome variable

depressant

type of psychoactive drug that calms the brain, reduces anxious feelings, and induces sleepiness

dermis

inner layer of skin that is made of tough connective tissue and contains blood vessels, nerve endings, hair follicles, and glands

desalination

process in which salt is removed from ocean water to produce fresh water for human use

descriptive statistics

statistics such as the mean and standard deviation that describe and summarize data

developmental adjustment

nongenetic phenotypic change that occurs during development in infancy or childhood and that may persist into adulthood and become irreversible

diabetes mellitus

disease caused by problems with the pancreatic hormone insulin, which lead to high blood glucose levels and symptoms such as excessive thirst and urination; includes type 1 and type 2 diabetes

diabetic nephropathy

progressive kidney disease caused by damage to capillaries in the glomeruli of the kidneys due to poor blood sugar control in people with diabetes

diaphragm

large, dome-shaped muscle below the lungs that allows breathing to occur when it alternately contracts and relaxes

diastole

part of a heartbeat in which the atria contract and pump blood into the ventricles, while the ventricles relax and fill with blood from the atria

differentiation

process by which unspecialized cells become specialized into one of many different types of cells, such as neurons or epithelial cells

diffusion

type of passive transport that does not require the help of transport proteins

digestion

process of breaking down food into nutrients that can be absorbed by blood or lymph

digestive system

organ system that breaks down food, absorbs nutrients, and eliminates any remaining wastes

diploid

having two of each type of chromosome, or twice the number of chromosomes as a haploid gamete

directional selection

type of natural selection for a polygenic trait in which one of two extreme phenotypes is selected for, resulting in a shift in the phenotypic distribution toward that extreme

disaccharide

sugar consisting of two monosaccharides

disease

condition associated with impairment of normal body functioning, generally causing signs or symptoms of illness

disease burden

impact of a disease on a population as measured by financial cost, mortality, morbidity, or other indicators

disease outbreak

event in which two or more people contract an infectious disease from a common source of infection

disruptive selection

type of natural selection for a polygenic trait in which phenotypes in the middle of the phenotypic distribution are selected against, resulting in two overlapping phenotypes, one at each end of the distribution

diverticulitis

disease in which one or more pouches (diverticula) in the large intestine become infected and inflamed

diverticulosis

condition in which pouches called diverticula form in the wall of the large intestine

DNA (deoxyribonucleic acid)

double-stranded DNA that makes up genes and chromosomes

DNA replication

natural process of copying DNA prior to cell division

domain

taxon in the revised Linnaean system that is larger and more inclusive than the kingdom

dominant

referring to an allele that masks the presence of another allele for the same gene when the two alleles occur together in a heterozygote, or referring to a trait that is controlled by such an allele

dormancy

state in which a plant slows down cellular activity and may shed its leaves

dorsal cavity

major human body cavity that includes the head and the posterior (back) of the trunk and holds the brain and spinal cord

double-blind experiment

scientific experiment in which neither the subjects nor the researchers know in which group subjects have been placed to help avoid bias in the outcome

double helix

double spiral shape of the DNA molecule

doubling time

time it would take a population to double in size at its current rate of growth

drowning

respiratory impairment from being in or under a liquid, whether or not death results

drug abuse

use of a drug without the advice of a medical professional and for reasons not originally intended

drug addiction

compulsive use of a substance such as a psychoactive drug despite negative consequences that such use may entail

drug dependence

condition in which stopping use of a substance such as a psychoactive drug produces physical and/or psychological symptoms

duodenum

first and shortest of three parts of the small intestine where most chemical digestion occurs