Since
Mendel's time, our knowledge of the mechanisms of genetic inheritance has grown
immensely. For instance, it is now understood that inheriting one allele can, at
times, increase the chance of inheriting another or can affect how and when a trait is
expressed in an individual's phenotype. Likewise, there are degrees of
dominance and recessiveness with some traits. The simple rules of Mendelian inheritance
do not apply in these and other exceptions. They are said to have
non-Mendelian inheritance patterns.
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Polygenic
traits:
stature, body shape,
hair and skin color |
Polygenic
Traits
Some traits are determined by the combined effect of more than one pair of
genes. These are referred to as polygenic
,
or continuous, traits.
An example of this is human stature. The combined size of all of the body parts from
head to foot determines the height of an individual. There is an
additive effect. The sizes of all of these body
parts are, in turn, determined by numerous genes. Human skin, hair, and eye color
are also polygenic traits because they are influenced by more than one allele
at different loci. The result is the perception of continuous gradation
in the expression of these traits.
NOTE: whether an
individual achieves his or her genetically programmed height is
significantly affected by thyroid gland hormones and human growth hormones (HGH) produced in the
pituitary gland . A deficiency in the amount of these hormones during
childhood and puberty can result in stunted growth. Too much of them
can cause excessive growth resulting in exceptional height.
Differences in diet and other environmental factors during the crucial
growth years can also be important in determining stature and other
complex traits. Usually, about 10% of an individual's height is due to
the environment.
Intermediate
Expression
Apparent
blending can occur in the phenotype when there is incomplete dominance resulting in
an intermediate expression of a trait in heterozygous individuals. For
instance, in primroses, snapdragons, and four-o'clocks, red or white flowers are
homozygous while pink ones are heterozygous. The pink flowers result because the
single "red" allele is unable to code for the production of enough red pigment
to make the petals dark red.
Another example of an
intermediate expression may be the pitch of human male voices. The lowest and
highest pitches apparently are found in men who are homozygous for this trait (AA and aa), while the intermediate
range baritones are heterozygous (Aa). The
child-killer disease known as Tay-Sachs
is also characterized by incomplete dominance. Heterozygous individuals
are genetically programmed to produce only 40-60% of the normal amount of an
enzyme that prevents the disease.
Fortunately for Mendel, the
pea plant traits that he studied were controlled by genes that do not exhibit an
intermediate expression in the phenotype. Otherwise, he probably would not have
discovered the basic rules of genetic inheritance.
Codominance
For some traits, two alleles
can be codominant. That is to say, both are expressed in heterozygous
individuals. An example of this is people who have an AB
blood type for the ABO blood system. When they are
tested, these individuals actually have the characteristics of both type A and type B blood. Their
phenotype is not intermediate between the two.
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Type AB blood testing as both
A and B
|
Multiple-allele
Series
The ABO blood type system is
also an example of a trait that is controlled by more than just a single pair of
alleles. In other words, it is due to a multiple-allele series. In this
case, there are three alleles (A, B,
and O), but each individual only inherits two of them (one
from each parent).
Some traits are controlled
by far more alleles. For instance, the human HLA system,
which is responsible for identifying and rejecting foreign tissue in our bodies, can have
at least 30,000,000 different genotypes. It is the HLA system which causes the
rejection of organ transplants. The more we learn about human genetics the more it becomes
clear that multiple-allele series are very common. In fact, it now appears that they
are more common than simple two allele ones.
Modifying
and Regulator Genes
There are two
classes of genes that can have an effect on how other genes function. They are
called modifying genes and regulator genes.
Modifying genes alter how
certain other genes are expressed in the phenotype. For instance, there is a
dominant cataract
gene which will produce varying degrees of vision impairment depending on the presence of
a specific allele for a companion modifying gene. However, cataracts
also can be promoted by diabetes and common environmental factors such as excessive ultraviolet radiation,
and alcoholism. Nearly half of all people in North America over 65
years of age eventually develop them.
Regulator genes can either initiate or
block the expression of other genes. They control the production of a variety of
chemicals in plants and animals. For instance, the time of production of specific proteins
that
will be new structural parts of our bodies can be controlled by such regulator genes.
Shortly after conception, regulator genes work as master switches
orchestrating the timely development of our body parts. They are also responsible for changes that occur in our bodies as we grow older.
In other words, they control the maturation and aging processes. Regulator genes
that are involved in subdividing an embryo into what will become the major
body parts of an individual are also referred to as homeotic
, homeobox
,
or Hox
genes. They are responsible for setting generalized cells on the path
to become a head, torso, arms, legs, etc.
 Gene
Control--video clip from
Teachers' Domain
View in:
QuickTime
or
Windows
Media Player
(length = 2 mins 58
secs) |
Can
We Slow Aging--video clip from Nova ScienceNow
about a regulator gene that controls the aging process
(length =
11 mins 30
secs) |
Incomplete
Penetrance
Some genes are incompletely
penetrant. That is to say, their effect does not normally occur unless certain
environmental factors are present. For example, you may inherit the genes
that are responsible for
type 2 diabetes
but never get the disease unless you become greatly overweight, persistently
stressed psychologically, or do not get enough sleep on a regular basis. Similarly, the genes that cause the chronic
autoimmune disease, multiple sclerosis
, may be triggered by
the Epstein-Barr
virus
and possibly other specific environmental stresses. New research
suggests that abundant exposure to the sun in childhood can provide some
protection from developing MS. Subsequently, people who grow up in
tropical and subtropical regions of the world have significantly lower rates
of MS as adults.
Sex
Related Genetic effects
There are three categories
of genes that may have different effects depending on an individual's gender. These are
referred to as:
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1. |
sex-limited genes |
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2. |
sex-controlled genes |
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3. |
genome imprinting |
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Human gender
differences in facial hair |
Sex-limited genes
are ones that are inherited by both men and women but are normally only expressed in the
phenotype of one of them. The heavy male beard is an example. While women have
facial hair it is most often very fine and comparatively sparse.
In contrast, sex-controlled
genes are expressed in both sexes but differently. An example of this is gout
, a
disease that causes painfully inflamed joints. If the gene is present, men are
nearly eight times more likely than women to have severe symptoms.
Some genes are known to have
a different effect depending on the gender of the parent from whom they are inherited.
This phenomenon is referred to as genome imprinting
or genetic imprinting. Apparently, diabetes
, psoriasis
, and
some rare genetically inherited diseases, such as a form of mental retardation known as
Angelman syndrome
, can follow this inheritance pattern.
Recent research by Catherine Dulac of Harvard University points to genetic
imprinting as being an important factor in causing male and female brains to
develop somewhat differently. She suggests that this is due to the
fact that some of the genes inherited from the opposite sex parent are
likely to be turned off following conception.
Pleiotropy
A single gene may be
responsible for a variety of traits. This is called pleiotropy
. The complex of
symptoms that are collectively referred to as sickle-cell
trait
,
or sickle-cell anemia, is an example. A single gene results in irregularly shaped red blood cells
that painfully block blood vessels, cause poor overall physical development, as well as related
heart, lung, kidney, and eye problems. Another pleiotropic trait is albinism
.
The gene for this trait not only results in a deficiency of skin, hair, and eye
pigmentation but also causes defects in vision.
Stuttering
Alleles
Lastly, it is now known that some genetically
inherited diseases have more severe symptoms each succeeding generation due to segments of
the defective genes being doubled in their transmission to children (as illustrated below).
These are referred to as stuttering alleles or unstable alleles.
Examples
of this phenomenon are Huntington's
disease, fragile-X syndrome,
and the myotonic form of muscular
dystrophy
.
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Unstable allele doubling each generation
|
Mendel
believed that all units of inheritance are passed on to offspring unchanged.
Unstable alleles are an important exception to this rule.
Environmental
Influences
The phenotype of
an individual is not only the result of inheriting a particular set of
parental genes. The specific environmental characteristics of the uterus in which
a fertilized egg is implanted and the health of the mother can have major impacts on the phenotype of the future
child. For instance, oxygen deprivation or inappropriate hormone levels
can cause lifelong, devastating effects. Likewise, accidents, poor
nutrition, and other environmental
influences throughout life can alter an individual's phenotype for many traits.
Geneticists
study identical or monozygotic
twins to determine which traits are inherited and which ones were acquired
following conception. Since monozygotic twins come from the same zygote, they
are essentially identical in their genetic makeup. If there are any differences in their
phenotypes, the environment is virtually always responsible. Such differences show
up in basic capabilities such as handedness, which had been assumed to be
entirely genetically determined. In rare instances, one monozygotic twin will be clearly right-handed while the other will be
left-handed. This suggests that there
may be both genetic and environmental influences in the development of this
trait.
Summary
Researchers have
identified more than 5,000
genetically inherited human diseases and abnormalities. As we learn more about the inheritance patterns
for these traits, it is becoming clear
that at least some of the twelve exceptions to the simple Mendelian rules of
inheritance described here are, in fact, relatively common. It would not
be surprising if other "exceptions" were discovered in the future.
However, it is important to keep in mind that there are at least 18,000
human traits controlled by genes that follow the basic Mendelian rules of
inheritance.
NEWS: Susan Lolle et al. reported
in the March 2005 issue of Nature that they have discovered a plant
species that can overwrite the genetic makeup inherited from parents.
These cress plants seem to be able to revert back to the DNA sequences of
their grandparents including genetic information that was lost in the
intervening generation. The researchers suggest that since the DNA
sequences were not present in the parents that there may be a
"template-directed process that makes use of an ancestral RNA-sequence
cache." The implication is that this is a form of inheritance that
does not follow the basic tenets of classical genetics. ("Genome-wide
Non-Mendelian Inheritance of
Extra-genomic Information in Arabidopsis",
Nature, Vol. 434, No 7032, March 24, 2005)
