Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Genotype and phenotype

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In the process of attenuation, a gene that codes for an

enzyme required for the synthesis of an amino acid is not

active until the level of that amino acid lowers to some thresh-

old level. At this level, the molecules called ribosomesphysi-

cally stall as they move down the beginning of the RNA

template that encodes protein. The stalling prevents the for-

mation of a signal that otherwise stops the onward movement

of the ribosomes. After the pause, because the stop signal has

not formed, the ribosomes resume their movement and the

protein is produced. When the level of the critical amino acid

is higher, the ribosomes do not stall, encounter a stop signal,

and the synthesis of the protein does not occur.

These processes operate simultaneously for many genes

in a bacterium. For some of these genes, the controlling fac-

tors are independent of one another. But for other proteins, a

common factor, such as a sensory protein that can sense

changes in the environment and provide a signal to the various

regulatory processes, also operates. This genetic regulation

pattern is referred to as global regulation. An example of

global regulation is a phenomenon called diauxic growth,

which is exemplified by the lactose operon(also called the lac

operon). Diauxic growth allows a bacterium to preferentially

utilize one nutrient (such as glucose) when two nutrients (such

as glucose and lactose) are present. When the preferred source

is exhausted, metabolismcan switch so as to utilize the second

source (lactose). This nutrient preference involves genetic reg-

ulation of protein production.

Other genetic regulatory mechanisms operate in

response to fluctuations in temperature, pH, oxygen level, the

attraction or repulsion of a bacterium from a compound

(chemotaxis), and the production of a spore.

See also Cell cycle (prokaryotic), genetic regulation of;

Microbial genetics

GENETICALLY ENGINEERED VACCINES•

seeVACCINE

GGenotype and phenotypeENOTYPE AND PHENOTYPE

The term genotype describes the actual set (complement) of

genes carried by an organism. In contrast, phenotyperefers to

the observable expression of characters and traits coded for by

those genes. Although phenotypes are based upon the content of

the underlying genes comprising the genotype, the expression

of those genes in observable traits (phenotypic expression) is

also, to varying degrees, influenced by environmental factors.

The term genotype was first used by Danish geneticist

Wilhelm Johannsen (1857–1927) to describe the entire genetic

or hereditary constitution of an organism, In contrast,

Johannsen described displayed characters or traits (e.g.,

anatomical traits, biochemical traits, physiological traits, etc.)

as an organism’s phenotype.

Genotype and phenotype represent very real differences

between genetic composition and expressed form. The geno-

type is a group of genetic markers that describes the particular

forms or variations of genes (alleles) carried by an individual.

Accordingly, an individual’s genotype includes all the alleles

carried by that individual. An individual’s genotype, because

it includes all of the various alleles carried, determines the

range of traits possible (e.g., a individual’s potential to be

afflicted with a particular disease). In contrast to the possibil-

ities contained within the genotype, the phenotype reflects the

manifest expression of those possibilities (potentialities).

Phenotypic traits include obvious observable traits as height,

weight, eye color, hair color, etc. The presence or absence of a

disease, or symptoms related to a particular disease state, is

also a phenotypic trait.

A clear example of the relationship between genotype

and phenotype exists in cases where there are dominant and

recessive alleles for a particular trait. Using an simplified

monogenetic (one gene, one trait) example, a capital “T”

might be used to represent a dominant allele at a particular

locus coding for tallness in a particular plant, and the lower-

case “t” used to represent the recessive allele coding for

shorter plants. Using this notation, a diploid plant will possess

one of three genotypes: TT, Tt, or tt (the variation tT is identi-

cal to Tt). Although there are three different genotypes,

because of the laws governing dominance, the plants will be

either tall or short (two phenotypes). Those plants with a TT

or Tt genotype are observed to be tall (phenotypically tall).

Only those plants that carry the tt genotype will be observed

to be short (phenotypically short).

In humans, there is genotypic sex determination. The

genotypic variation in sex chromosomes, XX or XY deci-

sively determines whether an individual is female (XX) or

male (XY) and this genotypic differentiation results in consid-

erable phenotypic differentiation.

Although the relationships between genetic and envi-

ronmental influences vary (i.e., the degree to which genes

specify phenotype differs from trait to trait), in general, the

more complex the biological process or trait, the greater the

influence of environmental factors. The genotype almost com-

pletely directs certain biological processes. Genotype, for

example, strongly determines when a particular tooth devel-

ops. How long an individual retains a particular tooth, is to a

much greater extent, determined by environmental factors

such diet, dental hygiene, etc.

Because it is easier to determine observable phenotypic

traits that it is to make an accurate determination of the rele-

vant genotype associated with those traits, scientists and

physicians place increasing emphasis on relating (correlating)

phenotype with certain genetic markers or genotypes.

There are, of course, variable ranges in the nature of the

genotype-environment association. In many cases, genotype-

environment interactions do not result in easily predictable

phenotypes. In rare cases, the situation can be complicated by

a process termed phenocopy where environmental factors pro-

duce a particular phenotype that resembles a set of traits coded

for by a known genotype not actually carried by the individ-

ual. Genotypic frequencies reflect the percentage of various

genotypes found within a given group (population) and phe-

notypic frequencies reflect the percentage of observed expres-

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