Encyclopedia of Biology

instead of being shaped by natural selection in a non-
random way. Especially prevalent in small populations,
where a particular allele can be eliminated by chance
and certain alleles can be favored over time. This can
lead to the loss of genetic variability.

genetic engineering A process that changes the
genetic makeup of cells. During the process a gene is
isolated, modified, and put back into an individual of
the same or different species. The process can be used
to introduce or eliminate specific genes. Used in agri-
culture where plants can be genetically engineered to
resist a pest.

genetic map (DNA map) Achromosome map that
shows the order of and distance between genes. Useful
for finding inherited diseases by following the inheri-
tance of a DNA marker present in affected individuals.
Genetic maps have been used to find the exact chro-
mosomal location of disease genes, including cystic
fibrosis, sickle-cell disease, Tay-Sachs disease, fragile
Xsyndrome, and myotonic dystrophy. Genetic maps
can be cytogenetic, linkage, or physical. A cytogenetic
map produces a visual appearance of a chromosome
when stained and examined under a microscope.
Important are the visually distinct regions, called light
and dark bands, that give each of the chromosomes

a unique appearance. A linkage map is a map of the

relative positions of genetic loci on a chromosome,

determined on the basis of how often the loci are

inherited together. Distance is measured in centimor-

gans (cM). A physical map of a species shows the

specific physical locations of its genes and/or markers

on each chromosome.

genetic recombination The process where offspring

have a different genotype from that of the parent due

to the recombining of genetic materials, usually caused

by crossing over between homologous chromosomes

during meiosis, or random orientation of nonhomolo-

gous chromosomes pairs, gene conversion, or other

means. It occurs during the cell division (meiosis) that

occurs during the formation of spermand egg cells.

This shuffling of genetic material increases the potential

for genetic diversity.

genetics The scientific study of heredity and variation.

genome The complete assemblage of chromosomes

and extrachromosomal genes of a cell, organelle, organ-

ism, or virus; the complete DNA portion of an organism;

the complete set of genes shared by members of any

reproductive body such as a population or species.

142 genetic engineering

holding needle polar bodies

female pronucleus

cell

male pronucleus

DNA

Illustration of gene therapy—an experimentally produced organism in which DNA has been artificially introduced and incorporated into
the organism’s germ line, usually by injecting the foreign DNA into the nucleus of a fertilized embryo.(Courtesy of Darryl Leja, NHGRI,
National Institutes of Health)