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| The common fruit fly
(Drosophila melanogaster) is a popular model
organism in genetics research. |
Genetics
Genetics is a discipline of biology. It is the science
of heredity. This includes the study of genes, and the
inheritance of variation and traits of living organisms.
In the laboratory, genetics proceeds by mating carefully
selected organisms, and analysing their offspring. More
informally, genetics is the study of how parents pass
some of their characteristics to their children. It is
an important part of biology, and gives the basic rules
on which evolution acts.
The fact that living things inherit traits from their
parents has been known since prehistoric times, and used
to improve crop plants and animals through selective
breeding. However, the modern science of genetics seeks
to understand the process of inheritance. This began
with the work of Gregor Mendel in the mid-nineteenth
century. Although he did not know the physical basis for
heredity, Mendel observed that organisms inherit traits
via discrete units of inheritance, which are now called
genes.
DNA
Living things are made of millions of tiny
self-contained components called cells. Inside of each
cell are long and complex molecules called DNA. DNA
stores information that tells the cells how to create
that living thing. Parts of this information that tell
how to make one small part or characteristic of the
living thing – red hair, or blue eyes, or a tendency to
be tall – are known as genes.
Every cell in the same living thing has the same DNA,
but only some of it is used in each cell. For instance,
some genes that tell how to make parts of the liver are
switched off in the brain. What genes are used can also
change over time. For instance, a lot of genes are used
by a child early in pregnancy that are not used later.
A living thing has two copies of each gene, one from its
mother, and one from its father. There can be multiple
types of each gene, which give different instructions:
one version might cause a person to have blue eyes,
another might cause them to have brown. These different
versions are known as alleles of the gene.
Since a living thing has two copies of each gene, it can
have two different alleles of it at the same time.
Often, one allele will be dominant, meaning that the
living thing looks and acts as if it had only that one
allele. The unexpressed allele is called recessive. In
other cases, you end up with something in between the
two possibilities. In that case, the two alleles are
called co-dominant.
Most of the characteristics that you can see in a living
thing have multiple genes that influence them. And many
genes have multiple effects on the body, because their
function will not have the same effect in each tissue.
The multiple effects of a single gene is called
pleiotropism. The whole set of genes is called the
genotype, and the total effect of genes on the body is
called the phenotype. These are key terms in genetics. |
|
History of genetics
Pre-Mendelian ideas
We know that man started breeding domestic animals from
early times, probably before the invention of
agriculture. We do not know when heredity was first
appreciated as a scientific problem. The Greeks, and
most obviously Aristotle, studied living things, and
proposed ideas about reproduction and heredity.
Probably the most important idea before Mendel was that
of Charles Darwin, whose idea of pangenesis had two
parts. The first, that persistent hereditary units were
passed on from one generation to another, was quite
right. The second was his idea that they were
replenished by 'gemmules' from the somatic (body)
tissues. This was entirely wrong, and plays no part in
science today. Darwin was right about one thing:
whatever happens in evolution must happen by means of
heredity, and so an accurate science of genetics is
fundamental to the theory of evolution. This 'mating'
between genetics and evolution took many years to
organise. It resulted in the Modern evolutionary
synthesis.
Mendelian genetics
The basic rules of genetics were first discovered by a
monk named Gregor Mendel in around 1865. For thousands
of years, people had already studied how traits are
inherited from parents to their children. However,
Mendel's work was different because he designed his
experiments very carefully.
In his experiments, Mendel studied how traits were
passed on in pea plants. He started his crosses with
plants that bred true, and counted characters that were
either/or in nature (either tall or short). He bred
large numbers of plants, and expressed his results
numerically. He used test crosses to reveal the presence
and proportion of recessive characters.
Mendel explained the results of his experiment using two
scientific laws: |
- Factors, later called genes,
normally occur in pairs in ordinary body cells, yet
separate during the formation of sex cells. These
factors determine the organism's traits, and are
inherited from its parents. When gametes are produced by
meiosis, the two factors separate. A gamete only
receives one or the other. This Mendel called the Law of
segregation.
- Alleles of different genes separate
independently of one another when gametes are formed.
This he called the Law of Independent Assortment. So
Mendel thought that different traits are inherited
independently of one another. We now know this is only
true if the genes are not on the same chromosome, in
which case they are not linked to each other.
|
Mendel's laws helped explain the results he observed in his
pea plants. Later, geneticists discovered that his laws were
also true for other living things, even humans. Mendel's
findings from his work on the garden pea plants helped to
establish the field of genetics. His contributions were not
limited to the basic rules that he discovered. Mendel's care
towards controlling experiment conditions along with his
attention to his numerical results set a standard for future
experiments. Over the years, scientists have changed and
improved Mendel's ideas. However, the science of genetics
would not be possible today without the early work of Gregor
Mendel.
Between Mendel and modern
genetics
In the years between Mendel's work and 1900 the foundations
of cytology, the study of cells, was developed. The facts
discovered about the nucleus and cell division were
essential for Mendel's work to be properly understood. |
- 1832: Barthélémy Dumortier, the
first to observe cell division in a multicellular
organism.
- 1841, 1852: Robert Remak
(1815–1865), a Jewish Polish–German physiologist, was
the first person to state the foundation of cell
biology: that cells only derive from other cells. This
was later popularized by the German doctor Rudolf
Virchow (1821–1902), who used the famous phrase omnis
cellula e cellula, meaning, all cells from other cells.
- 1865: Gregor Mendel's paper,
Experiments on plant hybridization was published.
- 1876: Meiosis was discovered and
described for the first time in sea urchin eggs, by
German biologist Oscar Hertwig (1849–1922).
- 1878–1888: Walther Flemming and
Eduard Strasburger describe chromosome behaviour during
mitosis.
- 1883: Meiosis was described at the
level of chromosomes, by Belgian zoologist Edouard van
Beneden (1846–1910), in Ascaris (roundworm) eggs.
- 1883: German zoologist Wilhelm Roux
(1850–1924) realised the significance of the linear
structure of chromosomes. Their splitting into two equal
longitudinal halves assured each daughter cell got the
same chromosome complement. Therefore, chromosomes were
the bearers of heredity.
- 1889: Dutch botanist Hugo de Vries
suggests that "inheritance of specific traits in
organisms comes in particles", naming such particles
(pan)genes.
- 1890: The significance of meiosis
for reproduction and inheritance was described only in
1890 by German biologist August Weismann (1834–1914),
who noted that two cell divisions were necessary to
transform one diploid cell into four haploid cells if
the number of chromosomes had to be maintained.
- 1902–1904: Theodor Boveri
(1862–1915), a German biologist, in a series of papers,
drew attention to the correspondence between the
behaviour of chromosomes and the results obtained by
Mendel. He said that chromosomes were "independent
entities which retain their independence even in the
resting nucleus... What comes out of the nucleus is what
goes into it".
- 1903: Walter Sutton suggested that
chromosomes, which segregate in a Mendelian fashion, are
hereditary units. Edmund B. Wilson (1856–1939), Sutton's
teacher, and the author of one of the most famous
text-books in biology, called this the Sutton–Boveri
hypothesis.
|
At this point, discoveries in cytology merged with the
rediscovered ideas of Mendel to make a fusion called
cytogenetics, (cyto = cell; genetics = heredity) which has
continued to the present day.
Rediscovery of Mendel's work
During the 1890s several biologists began doing experiments
on breeding. and soon Mendel's results were duplicated, even
before his papers were read. Carl Correns and Hugo de Vries
were the main rediscovers of Mendel's writings and laws.
Both acknowledged Mendel's priority, although it is probable
that de Vries did not understand his own results until after
reading Mendel. Though Erich von Tschermak was originally
also credited with rediscovery, this is no longer accepted
because he did not understand Mendel's laws. Though de Vries
later lost interest in Mendelism, other biologists built
genetics into a science.
Mendel's results were replicated, and genetic linkage soon
worked out. William Bateson perhaps did the most in the
early days to publicise Mendel's theory. The word genetics,
and other terminology, originated with Bateson.
Mendel's experimental results have later been the object of
some debate. Fisher analyzed the results of the F2 (second
filial) ratio and found them to be implausibly close to the
exact ratio of 3 to 1. It is sometimes suggested that Mendel
may have censored his results, and that his seven traits
each occur on a separate chromosome pair, an extremely
unlikely occurrence if they were chosen at random. In fact,
the genes Mendel studied occurred in only four linkage
groups, and only one gene pair (out of 21 possible) is close
enough to show deviation from independent assortment; this
is not a pair that Mendel studied. |
|
Tools of genetics
Mutations
During the process of DNA replication, errors sometimes
occur. These errors, called mutations, can have an
effect on the phenotype of an organism. In turn, that
usually has an effect on the organism's fitness, its
ability to live and reproduce successfully.
Error rates are usually very low—1 error in every 10–100
million bases—due to the "proofreading" ability of DNA
polymerases. Error rates are a thousandfold higher in
many viruses. Because they rely on DNA and RNA
polymerases which lack proofreading ability, they get
higher mutation rates.
Processes that increase the rate of changes in DNA are
called mutagenic. Mutagenic chemicals increase errors in
DNA replication, often by interfering with the structure
of base-pairing, while UV radiation induces mutations by
causing damage to the DNA structure. Chemical damage to
DNA occurs naturally as well, and cells use DNA repair
mechanisms to repair mismatches and breaks in
DNA—nevertheless, the repair sometimes fails to return
the DNA to its original sequence.
In organisms which use chromosomal crossovers to
exchange DNA and recombine genes, errors in alignment
during meiosis can also cause mutations. Errors in
crossover are especially likely when similar sequences
cause partner chromosomes to adopt a mistaken alignment;
this makes some regions in genomes more prone to
mutating in this way. These errors create large
structural changes in DNA sequence—duplications,
inversions or deletions of entire regions, or the
accidental exchanging of whole parts between different
chromosomes (called translocation).
Punnett squares
Developed by Reginald Punnett, Punnett squares are used
by biologists to determine the probability of offspring
to having a particular genotype.
If B represents the allele for having black hair and b
represents the allele for having white hair, the
offspring of two Bb parents would have a 25% probability
of having two white hair alleles (bb), 50% of having one
of each (Bb), and 25% of having only black hair alleles
(BB).
Pedigree chart
Geneticists (biologists who study genetics) use pedigree
charts to record traits of people in a family. Using
these charts, geneticists can study how a trait is
inherited from person to person.
Geneticists can also use pedigree charts to predict how
traits will be passed to future children in a family.
For instance, genetic counselors are professionals who
work with families who might be affected by genetic
diseases. As part of their job, they create pedigree
charts for the family, which can be used to study how
the disease might be inherited.
Twin studies
Since human beings are not bred experimentally, human
genetics must be studied by other means. One recent way
is by studying the human genome. Another way, older by
many years, is to study twins. Identical twins are
natural clones. They carry the same genes, they may be
used to investigate how much heredity contributes to
individual people. Studies with twins have been quite
interesting. If we make a list of characteristic traits,
we find that they vary in how much they owe to heredity.
For example: |
- Eye colour: entirely inherited
- Weight, height: partly inherited,
partly environmental
- Which language a person speaks:
entirely environmental.
|
The way the studies are done is like this. Take a group of
identical twins and a group of fraternal twins. Measure them
for various traits. Do a statistical analysis (such as
analysis of variance). This tells you to what extent the
trait is inherited. Those traits which are partly inherited
will be significantly more similar in identical twins.
Studies like this may be carried further, by comparing
identical twins brought up together with identical twins
brought up in different circumstances. That gives a handle
on how much circumstances can alter the outcomes of
genetically identical people.
The person who first did twin studies was Francis Galton,
Darwin's half-cousin, who was a founder of statistics. His
method was to trace twins through their life-history, making
many kinds of measurement. Unfortunately, though he knew
about mono and dizygotic twins, he did not appreciate the
real genetic difference. Twin studies of the modern kind did
not appear until the 1920s. |
|
Genetics of prokaryotes and
viruses
The genetics of bacteria, archaea and viruses is a major
field or research. Bacterial mostly divide by asexual
cell division, but do have a kind of sex by horizontal
gene transfer. Bacterial conjugation, transduction and
transformation are their methods. In addition, the
complete DNA sequence of many bacteria, archaea and
viruses is now known.
Although many bacteria were given generic and specific
names, like Staphylococcus aureus, the whole idea of a
species is rather meaningless for an organism which does
not have sexes and crossing-over of chromosomes.
Instead, these organisms have strains, and that is how
they are identified in the laboratory.
Genes and development
Gene expression
Gene expression is the process by which the heritable
information in a gene, the sequence of DNA base pairs,
is made into a functional gene product, such as protein
or RNA. The basic idea is that DNA is transcribed into
RNA, which is then translated into proteins. Proteins
make many of the structures and all the enzymes in a
cell or organism.
Several steps in the gene expression process may be
modulated (tuned). This includes both the transcription
and translation stages, and the final folded state of a
protein. Gene regulation switches genes on and off, and
so controls cell differentiation, and morphogenesis.
Gene regulation may also serve as a basis for
evolutionary change: control of the timing, location,
and amount of gene expression can have a profound effect
on the development of the organism. The expression of a
gene may vary a lot in different tissues. This is called
pleiotropism, a widespread phenomenon in genetics.
Alternative splicing is a modern discovery of great
importance. It is a process where from a single gene a
large number of variant proteins can be assembled. One
particular Drosophila gene (DSCAM) can be alternatively
spliced into 38,000 different mRNA.
Epigenetics & control of
development
Epigenetics is the study of changes in gene activity
which are not caused by changes in the DNA sequence. It
is the study of gene expression, the way genes bring
about their phenotypic effects.
These changes in gene activity may stay for the
remainder of the cell's life and may also last for many
generations of cells, through cell divisions. However,
there is no change in the underlying DNA sequence of the
organism. Instead, non-hereditary factors cause the
organism's genes to behave (express themselves)
differently.
Hox genes are a complex of genes whose proteins bind to
the regulatory regions of target genes. The target genes
then activate or repress cell processes to direct the
final development of the organism.
Extranuclear inheritance
There are some kinds of heredity which happen outside
the cell nucleus. Normal inheritance is from both
parents via the chromosomes in the nucleus of a
fertilised egg cell. There are some kinds of inheritance
other than this.
Organelle heredity
Mitochondria and chloroplasts carry some DNA of their
own. Their make-up is decided by genes in the
chromosomes and genes in the organelle. Carl Correns
discovered an example in 1908. The four o'clock plant,
Mirabilis jalapa, has leaves which may be white, green
or variegated. Correns discovered the pollen had no
influence on this inheritance. The colour is decided by
genes in the chloroplasts.
Infectious heredity
This is caused by a symbiotic or parasitic relationship
with a microorganism.
Maternal effect
In this case nuclear genes in the female gamete are
transcribed. The products accumulate in the egg
cytoplasm, and have an effect on the early development
of the fertilised egg. The coiling of a snail, Limnaea
peregra, is determined like this. Right-handed shells
are genotypes Dd or dd, while left-handed shells are dd.
The most important example of maternal effect is in
Drosophila melanogaster. The protein product
maternal-effect genes activate other genes, which in
turn activate still more genes. This work won the Nobel
Prize in Physiology or Medicine for 1995.
Aspects of modern genetics
Much modern research uses a mixture of genetics, cell
biology and molecular biology. Topics which have been
the subject of Nobel Prizes in either chemistry or
physiology include: |
- Alternative splicing, where one gene
codes for a variety of relared protein products.
- Genomics, the sequence and analysis
the function and structure of genomes.
- Genetic engineering, the changing of
an organism's genome using biotechnology.
- Mobile genetic elements, types of
DNA which can change position in the genome.
- Horizontal gene transfer, where an
organism gets genetic material from another organism
without being the offspring of that organism.
|
Genetics of human behaviour
Many well-known disorders of human behaviour have a genetic
component. This means that their inheritance partly causes
the behaviour, or makes it more likely the problem would
occur. Examples include: |
- Autism
- ADHD (attention deficit disorder)
- Risk taking
|
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Also, normal behaviour is also heavily influenced by
heredity: |
- Learning and cognitive ability
- Personality
|
 Kiddle: Genetics
Wikipedia: Genetics |
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