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Often called fraternal twins, occurring when two eggs each get fertilized by two different sperm, resulting in two zygotes in the uterus at the same time.
Referring to a gene that influences the expression of a trait only when paired with an identical gene.
Times during which certain environmental influences can have an impact on the development of the infant.
The knowledge that an object exists even when its not in sight.
The scientific study of the changes that occur in people as they age from conception until death.
Research design in which participants are first studied by means of a cross-sectional design but are also followed and assessed for a period of no more than six years.
Research group in which one participant or group of participants are studied over a long period of time.
Any factor that can cause a birth defect.
The period from two to eight weeks after fertilization, during which the major organs develop.
First two weeks after fertilization, during which the zygote moves down to the uterus and begins to implant in the lining.
Piaget's first stage of cognitive development in which the infant uses its senses and motor abilities to interact with objects in the environment.
A mental concept formed through experiences with objects and events.
Research design in which several different age-group of participants are studied at one particular point in time.
Identical twins formed when one zygote splits into two separate masses of cells, each of which develops into a separate embryo.
The time from about eight weeks after conception until the birth of the child.
Referring to a gene that actively controls the expression of a trait.