Chapter 3: Forming a New Life

• The first year is characterized by the development of motor skills, including reaching, grasping, crawling, standing, walking, and running.
• fertilization


  the union of sperm and ovum to produce a zygote; also called conception
• zygote


  one-celled organism resulting from fertilization
• dizygotic twins


  fraternal twins; twins conceived by the union of two different ova (or a single ovum that has split) with two different sperm cells; they are no more genetically alike than other siblings
• monozygotic twins


  identical twins; twins resulting from the division of a single zygote after fertilization; they are genetically similar
• DNA


  chemical that carried inherited instructions for the development of all cellular forms of life
• genetic code


  sequence of bases within the DNA molecule; governs the formation of proteins that determine the structure and functions of living cells
• chromosomes


  coils of DNA that consist of genes
• genes


  small segments of DNA located in definite positions on particular chromosomes; functional units of heredity
• human genome


  complete sequence of genes in the human body
• autosomes


  in humans, the 22 pairs of chromosomes not related to sexual expression
• sex chromosomes


  pair of chromosomes that determines sex: XX in the normal human female, XY in the normal human male
• alleles


  two or more alternative forms of a gene that occupy the same position on paired chromosomes and affect the same trait
• homozygous


  possessing two identical alleles for a trait
• heterozygous


  possessing differing alleles for a trait
• dominant inheritance


  pattern of inheritance in which, when a child receives different alleles, only the dominant one is expressed
• recessive inheritance


  pattern of inheritance in which a child receives identical recessive alleles, resulting in expression of a nondominant trait
• polygenic influence


  pattern of inheritance in which multiple genes at different sites on a chromosome affect a complex trait
• mutations


  permanent alterations in genes or chromosomes that may produce harmful characteristics
• phenotype


  observable characteristics of a person
• genotype


  genetic makeup of a person, containing both expressed and unexpressed characteristics
• multifactorial transmission


  combination of genetic and environmental factors to produce certain complex traits
• epigenesis


  mechanism that turns genes on or off and determines functions of body cells
• incomplete dominance


  pattern of inheritance in which a child receives two different alleles, resulting in partial expression of a trait
• sex-linked inheritance


  pattern of inheritance in which certain characteristics carried on the X-chromosome inherited from the mother are transmitted differently to her male and female offspring
• down syndrome


  chromosomal disorder characterized by moderate-to-severe mental retardation and by such physical signs as downward-sloping skin fold at the inner corners of the eyes. Also called trisomy-21
• genetic counseling


  clinical service that advises prospective parents of their probable risk of having children with hereditary defects
• behavioral genetics


  the study of relative hereditary and environmental influences on behavior
• heritability


  statistical estimate of contribution of heredity to individual differences in a specific trait within a given population
• concordant


  term describing tendency of twins to share the same trait or disorder
• reaction range


  potential variability, depending on environmental conditions, in the expression of a hereditary trait
• canalization


  limitation on variance of expression of certain inherited characteristics
• genotype-environment interaction


  the portion of of phenotypic variation that results from the reactions of genetically different individuals to similar environmental conditions
• genotype-environment correlation


  tendency of certain genetic and environmental influences to reinforce each other, may be passive, reactive (evocative), or active. Also called genotype-environment convariance
• niche-picking


  tendency of a person, especially after early childhood, to seek out environments compatible with his or her genotype
• nonshared environmental effects


  the unique environment in which each child grows up, consisting of distinctive influences or influences that affect one child differently that another
• obesity


  extreme overweight in relation to age, sex, height, and body type as defined by having a body mass index of above the 95th percentile
• temperment


  characteristic disposition, or style of approaching and reacting to situations
• schizophrenia


  mental disorder marked by loss of contact with reality; symptoms include hallucinations and delusions
• gestation


  period of development between conception and birth