By
the beginning of the 20th century, it became apparent that a clear
understanding of the processes leading to biological evolution requires knowledge
of how traits are
actually transmitted from one generation to the next. Pursuit of the
answer to this question led to the development of the
scientific field known as
genetics
.
Thanks to Gregor Mendel and other pioneer researchers, we now know that the mechanisms
of genetic inheritance
are ultimately responsible for most biological variation and evolutionary
change. In order to comprehend genetics, however, it is necessary to
also understand the biological basis of life. This begins
with the cell
.
All
plants and
animals are composed of microscopic cells, which are the smallest basic organic units
capable of carrying out the functions that we normally define as life. These
essential cell functions include:
|
1. |
taking in nutrients |
|
2. |
combining the nutrients into
substances for cell growth and repair |
|
3. |
reproducing themselves |
|
4. |
excreting waste matter |
Very simple organisms such as
bacteria and blue-green algae consist of rudimentary prokaryotic
cells. These were
the earliest kinds of cells to appear on Earth. More complex single-celled creatures as well as multicellular plants and animals are composed of
eukaryotic
cells. The
description of cell characteristics that follows is for the eukaryotic type
cells of which humans are mostly composed.
Each of your body's cells is
enclosed by a membrane which maintains its integrity. Within is a
nucleus
with its own surrounding membrane barrier. The
nucleus contains most of the genetic material that determines inheritance. Between the
nuclear and cell membranes are a number of other specialized cell structures, or
organelles. Most of the material outside of the nucleus consists of a water-rich viscous gel referred to as
cytoplasm
. The cell and nuclear membranes are selectively
permeable, which is to say they allow some specific substances to pass through while
containing the cell components within. Oxygen, sugar, and other
nutrients enter through the cell membrane while useful chemicals produced by
the cell as well as carbon dioxide and other waste products of cellular
activity leave the cell.
 |
Generalized animal cell
|
Red blood cells
are an exception to this description in that they
do not have a distinct nucleus and nuclear membrane. In this, they are
more like the primitive prokaryotic cells of bacteria.
NOTE:
There are about 100
trillion cells in each of our bodies. Only 10 trillion of them are
human. The other 90 trillion are viruses, bacteria, and other
microorganisms. We have evolved symbiotic relationships with most of
them. Among other things, they help us digest food and keep us healthy
by reducing inflammation and fighting off harmful microorganisms.
Cells in people and other multicellular organisms reproduce themselves by dividing to form new cells. When
the division process begins, long thread-like structures called
chromosomes
contract and become visible in the nucleus. At
other times, chromosomes are, in effect, not visible because of their stretched out thin
profile. Chromosomes (literally "colored bodies") were given this name,
when they were discovered
in 1882, due to the
fact that they become visible when stained with a synthetic dye
used to enhance the observation of specific cell components.
 |
Contracted human
chromosome
(shown doubled during cell division) |
An individual's chromosomes
can be photographed when they become visible during the cell division process. Then, all of the pictures of individual
chromosomes in a cell may be cut out and organized into a
karyotype
, such as the one shown on the right. This is a
standardized arrangement of the chromosome pictures which allows us to spot major
abnormalities that may be characteristic of particular genetically inherited problems,
such as Down syndrome
. In a karyotype,
the chromosomes are placed into homologous
pairs.
These are pairs that contain genes
for the same traits at the same location (or locus) on
the chromosomes. However, homologous chromosomes may have different alleles
,
or alternate forms of the same gene. Homologous chromosomes are paired early in the cell reproduction process.
The 46
chromosomes in normal humans consist of 22 pairs of autosomes
and one pair of
sex chromosomes. The
autosomes carry the genes that determine most body characteristics. The sex
chromosomes primarily determine sexual traits. If they are X and Y, as in the karyotype shown
here, the individual is almost always a male. In contrast, if both sex chromosomes
are X's, the gender is female.
In
fact, maleness is largely determined by the presence or the absence of a specific gene on
the Y chromosome known as the SRY
(sex-determining region Y) gene. Beginning in the 5th to 7th week after conception
in humans, it triggers other genes that cause the undifferentiated embryonic sex organs to become
testes
.
These, in turn, produce the hormone testosterone
which stimulates the development of other masculine physical traits. If the SRY gene is not functioning or
absent, an XY individual will very likely develop
a female appearing body with breasts and a vagina but will not have a uterus
or ovaries and subsequently will be sterile. Instead of ovaries, there
will be rudimentary internal testes. Late or partial functioning of the SRY gene can result in
individuals being sexually intermediate between male and female.
However, this is not the only process that can cause ambiguous sexual
characteristics.
Sometimes, in the production of sperm cells, an error
can occur that results in a portion of the Y chromosome detaching and becoming
part of an X chromosome. If that portion carries the SRY gene and it is
inherited, the result can be a chromosomal female (XX)
who will develop as a sterile phenotypic male. About 1 in 20,000 normal
appearing males are chromosomal females (XX).
Roughly the same number of phenotypic females are chromosomal males (XY).
Both are sterile.
If you wish to
further explore the question of what distinguishes men from women, select
"Gender Testing of Female Athletes" in the box below. However, be
prepared to remain unclear about the answer.
Gender Testing Of Female
Athletes--how gender can be determined in questionable cases
This link takes you to an
external website. When you are finished, you must click the
"back" button on your browser program to return to this tutorial. |
Sex chromosomes in females and all autosome exist in homologous pairs.
That is to say there are 2 copies of each gene (one on each chromosome).
These copies may or may not be the same allele for a gene (either may be the
dominant or the recessive form). The X and Y chromosomes of males are
mostly not homologous--they only share a few genes. Those genes that are
found on only the X or the Y chromosome are said to be hemizygous
because there is only one copy of each gene. Regardless of whether
that gene is a dominant or a recessive allele, it will be expressed in the
phenotype of males. You will learn more about this phenomenon in the
"Sex Linked Genes" section of this tutorial.
| |
 |
|
Sex Chromosomes
(shown
doubled during cell division) |
The X chromosome is much larger than the Y chromosome
and has many more genes not related to sexual characteristics. One of the X
chromosomes in the cells of human females is partly inactivated early in embryonic
life. This is a self-preservation action to prevent a potentially
harmful double dose of genes.
Recent research points to the "Xist"
gene on the X chromosome as being responsible for a sequence of events that
silences one of the X chromosomes in females. Apparently, it is the X
chromosome from the father that is most likely to be inactivated in
developing brain tissues. This may account in part for the
"feminizing" of female brains and differences in male and female behavior.
The inactivated X
chromosomes become highly compacted structures known as Barr bodies. The
presence of Barr bodies has been used at the Olympics and other international
sports competitions as a test to determine whether an athlete is a man or a
woman. The measurement of testosterone levels has also been used for
this purpose.
Every person has
about
20,000-25,000 genes in each cell. These genes play a crucial role in determining the specific details of what
we are like
biologically. The chromosomes are made up of genes. More accurately, the
chromosomes are mainly DNA molecules and the genes are
sections of them. You will learn more about these molecules later in
this tutorial.
The 23 pairs of human chromosomes do not each have the
same number of genes. Chromosome 1 is the largest, containing about 8%
of all of our genes. Chromosome 21 is the smallest and has only about
1/6 as many genes as chromosome 1.
NOTE: Our 20-25,000 human genes make up
less than 1% of the genes in our bodies. The non-human genes are
mostly in viruses, bacteria, and other microorganisms.
NOTE:
While the gender of children among humans and most other
mammals is determined by males passing on either the X or the Y chromosome,
that is not the case with birds. The sex
chromosomes in male birds are
essentially identical (ZZ). In females they are different (ZW).
Whether females pass on a Z or a W determines the gender of their chicks.
Recent research has shown that the environment and the health of a mother
bird has a major effect on whether she has male or female offspring.
Healthy females have more sons.
NOTE: Many reptile species do not have
the equivalent of X and Y chromosomes to determine gender. Instead,
temperature and other environmental cues cause offspring to be male or
female.
NEWS:
In the January 10, 2002 Nature Science
Update, Henry Gee reported that sex determination in males is not just a
function of the SRY gene on the Y chromosome. The AHC gene on the X
chromosome can act to prevent or reduce the masculinizing effect of the SRY
gene. Apparently, this rare situation can occur in males who inherit a
double dose of the AHC gene. Females normally inherit the AHC gene on
both of their X chromosomes. It is not yet known what the consequences
might be for women who inherit defective AHC genes. However, in studies
with laboratory mice, females seemed to develop relatively normal ovaries
when their AHC gene functions were disrupted.
NEWS: In a March 17, 2005 Nature article by Laura Carrel and Huntington Willard entitled "X-inactivation Profile
Reveals Extensive Variability in X-linked Gene Expression in Females," it
was reported that 15% of the genes on
both X chromosomes escape X-inactivation
in female mammals,
including humans. An additional 10% of X chromosome genes have
variable patterns of inactivation. In other words, many genes from
what were presumed to be inactive X chromosomes are expressed in females and
there are individual differences in which X chromosome genes escape
inactivation.
