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Summary Genetics
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GENETICSGENERAL
- Genetics is the study of heredity and variation in living organisms.
- It tries to explain both the similarities and differences between parents and their
offspring.
- Hereditary characteristics are transferred by gametes. During fertilization the
male gamete from the father and the female gamete from the mother fuse to
form a zygote which develops into a new organism.
- Each organism inherits certain general characteristics from its parents which are
unique to a particular species. There are characteristics that ultimately determine
that you are a human and not a dog. Each organism also inherits special
characteristics from both parents that make it unique.
- Each cell contains many genes which carry the information for making many
proteins. But not all of these genes are used by any one cell. Just a few genes will
be “switched on” in any one cell at any one time.
- Every chemical reaction inside a living organism is catalyzed by enzymes.
Enzymes are proteins. So, by providing information for making enzymes, genes
affect all te chemical reactions in an organism’s body.
- The complete set of genes of a particular organism is known as its genome. [The
human genome consists of about 25 000 different genes].
- It is a copy of all essential DNA coding required to synthesize all the proteins
needed by an organism.
- Each somatic cell possesses a complete set of genetic instructions [genes].
Red blood cells are an exception as they do not have nuclei and thus no
chromosomes or genes.
- Although these genes are all found in somatic cells, only certain genes are
activated / switched on in specific cells. This, only the proteins that provide the
structure and function of a particular cell, are synthesized.
Genes are switched on by complicated mechanisms involving master / control genes.
- Each organism has its own unique gene combination that is determined by the
sequence of the nitrogenous bases on the DNA molecule.
- Scientists determine the evolutionary relationships between organisms by
studying the DNA sequences of different organisms.
99,9% of all human DNA is identical; the difference of 0,1% makes each human
unique.
- The first major discovery in understanding the structure and functioning of the
gene was made by an Austrian Monk, Gregor Mendel.
GREGOR MENDEL [1822 – 1884]
- He studied and taught Natural Sciences and Mathematics.
- Carried out experiments on pea plants in his free time to study the inheritance of
characteristics from one generation to another.
- Through his experimental findings, he formulated the basic laws of genetics.
- At the time, Mendel’s work was not recognized, and he only became known as
“The Father of Genetics” years after his death when the importance of his
investigations was realized. The value of his work was only recognized 20 years
after his death.
MENDEL’S HEREDITARY EXPERIMENTS
, - Mendel incidentally chose pea plants to do his hereditary experiments. Although
he didn’t realize it at the time, pea plants were a good choice because they show
clear, observable, contrasting characteristics / traits.
- He studied seven of these contrasting characteristics, namely: tall and short
plants; smooth and wrinkled seeds; elbow and green seeds; inflated and
constricted pods; green and yellow pods; purple and white flowers; and axial and
terminal flowers.
- A further advantage was that pea plants are naturally self-pollinating and do not
cross-pollinate.
- Mendel cut off the anthers [containing pollen] and then performed cross-
pollination himself, using pollen of his own choice.
- He “painted” ripe pollen onto the stigma w. a small brush and could thus control
the cross.
- Initially, Mendel did his experiments w. parent plants that were true breeding for a
specific characteristic.
A plant is true breeding for a specific characteristic when self-pollination takes place
and all the offspring produced have the same characteristic as the parent, e.g. if a tall
plant self-pollinates and only produces tall plants, it is a true breeding plant for the
“tall” characteristic.
- He studied how characteristics, such as the colour of flowers, were passed on to
the next generation.
- He chose to investigate the pea flower as it has a closed flower and is not
pollinated by insects, but by self-pollination.
- Mendel removed the stamens from purebred white flowers to prevent them from
self-pollinating and then he took pollen from pure-breeding red flowers and
dusted the pollen onto the stigma of the white flowers.
- He then covered the white flowers w. small paper bags to ensure that no further
cross pollination took place.
- Mendel noticed that when he crossed the true-breeding red flowers w. white
flowers, he always got an F1generation of red flowers.
- When he allowed these flowers to breed freely amongst themselves to get the F2
generation he always got a ratio of 75% red flowers and 25% white flowers.
- He also took true-breeding tall pea plants and crossed them true- breeding dwarf
plants.
Conclusions from Mendel’s experiments
The following conclusions were drawn from Mendel’s experiments:
- A hereditary characteristic [e.g. plant length in peas] is determined by two
“factors” [genes].
These “factors” were only named “genes” after Mendel’s death.
- One gene of the gene pair comes from the individuals’ mother and the other gene
from the father. The gene pair is carried on two chromosomes of a homologous
chromosome pair.
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