The Beginnings

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deoxyribonucleic acid (DNA)

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the chemical composition of the molecules that contain the genes, which are the chemical instructions for cells to manufacture various proteins

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chromosome

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one of the 46 molecules of DNA (in 23 pairs) that virtually each cell of the human body contains and that, together, contain all the genes. Other species have more or fewer chromosomes

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gene

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a small section of a chromosome; the basic unit for the transmission of heredity. A gene consists of a string of chemicals that provide instructions for the cell to manufacture certain proteins

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allele

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a variation that makes a gene different in some way from other genes fro the same characteristics. many genes never vary; others have several possible alleles.

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genome

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the full set of genes that are the instructions to make an individual member of a certain species

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zygote

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the single cell formed from the union of two gametes, a sperm and an ovum

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gamete

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a reproductive cell; that is, a sperm or ovum that can produce a new individual if it combines with a gamete from the other sex to make a zygote

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genotype

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an organism's entire genetic inheritance, or genetic potential

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homozygous

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referring to two genes of one pair that are exactly the same in every letter of their code. most gene pairs are homozygous.

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heterozygous

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referring to two genes of one pair that differ in some way. typically one allele has only a few base pairs that differ from the other member of the pair

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23rd pair

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the chromosome pair that, in humans, determines sex. the other 22 pairs are autosomes; inherited equally by males and females

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xx

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a 23rd chromosome pair that consists of two x shaped chromosomes, one each from the mother and father. XX zygotes become females

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xy

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a 23rd chromosome pair that consists of an X shaped chromosome from the mother and a Y shaped chromosome from the father. XY zygotes become males

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stem cells

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cells from which any other specialized type of cell can form

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monozygotic (MZ) twins

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twins who originate from one zygote that splits apart very early in development. other monozygotic multiple births can occur as well

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dizygotic (DZ) twins

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twins who are formed when two separate ova are fertilized by two separate sperm at roughly the same time

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assisted reporductive technology (ART)

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a general term for the techniques designed to help infertile couples conceive and sustain a pregnancy

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in vitro fertilization (IVF)

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fertilization that takes place outside a woman's body. the procedure involves mixing sperm with ova that have been surgically removed from the woman's ovary. if a zygote is produced, it is inserted into the woman's uterus, where it may implant and develop into a baby

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phenotype

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the observable characteristics of a person, including appearance, personality, intelligence and all other traits

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polygenic

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referring to a trait that is influenced by many genes

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multifactorial

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referring to a trait that is affected by many factors, both genetic and environmental expression; enhancing, halting, shaping or altering the expression of genes, resulting in a phenotype that may differ markedly from the genotype

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epigenitic

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referring to environmental factors that affect genes and genetic expression; enhancing, halting, shaping, or altering the expression of genes, resulting in a phenotype that may differ markedly from the genotype

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human genome project

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an international effort to map the complete human genetic code. this effort was essentially completed in 2001, though analysis is still ongoing

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dominant-recessive pattern

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the interaction of a heterozygous pair of alleles in such a way that the phenotype reveals the influence of one allele (the dominant gene) more than that of the other (the recessive gene)

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carrier

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a person whose genotype includes a gene that is not expressed in the phenotype. such an unexpressed gene occurs in half of the carrier's gametes and this is passed on to half of the carrier's children, who will most likely be carriers, too. generally, only when such a gene is inherited from both parents does the characteristic appear in the phenotype

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x-linked

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referring to a gene carried on the x chromosome. if a male inherits an x-linked recessive trait from his mother, he expresses that trait because the y from his father has no counteracting gene. females are more likely to be carriers of x-linked traits but are less likely to express them

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Four generalities accepted about genotype and phenotype.

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1. Genes affect every aspect of behavior, including social interactions, intellectual abilities, even political values and reactions

2. most environmental influences on children raised in the same home are not shared

3. a person's genes elicit responses that shape development. thus, personality may not be the result of childhood experience but instead may have caused those experiences

4. lifelong, people choose friends and environments that accept and encourage their genetic predispositions. thus genetic effects increase with age

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heritability

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a statistic that indicates what percentage of the variation in a particular trait within a particular population, in a particular context and era, can be traced to genes

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down syndrome

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a condition in which a person has 47 chromosomes instead of the usual 46, with 3 rather than 2 chromosomes at the 21st site.

usually have distinctive characteristics, unusual facial features, heart abnormalities, and language difficuties

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trisomy

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three chromosomes in a given location instead of two

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syndrome

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a cluster of distinct characteristics that tend to occur together

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fragile x-syndrome

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a genetic disorder in which part of the x chromosome seems to be attached to the rest of it by a very thin string of molecules. the cause is a single gene that has more than 200 repetitions of one triplet

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genetic counseling

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consultation and testing by trained experts that enable individuals to earn about their genetic heritage, including harmful conditions that they might pass along to any children they may conceive

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phenylketonuria (PKU)

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a genetic disorder in which a child's body is unable to metabolize an amino acid called phenylalanine. unless the infant immediately begins a special diet, the resulting buildup of phenylalanine in body fluids causes brain damage, progressive mental retardation and other symptoms