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Asexual Heredity
Every organism
requires a set of coded instructions for specifying its
traits. For offspring to resemble their parents, there must be
a reliable way to transfer information from one generation to
the next. Heredity is the passage of these
instructions from one generation to another. The DNA
molecule provides the mechanism for transferring these
instructions.
In asexually reproducing organisms,
all the genes come from a single parent. As asexually
produced offspring are produced by the cell division process
of mitosis, all offspring are normally genetically
identical to the parent.
Sexual Heredity
In sexually reproducing organisms, the new individual receives
half of the genetic information from its mother through the
egg and half from its father from his sperm.
Sexually produced offspring resemble, but are not identical
to, either of their parents. Some reasons
for these variations between sexually reproduced offspring and
their parents include crossing over when gametes are
formed in each parent and genetic recombination, which
is the combining of the genetic instructions of both parents
into a new combination in the offspring when fertilization
occurs.
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Genetic
Recombination |
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Note that two of the four offspring in the punnett
square at the right have a completely different genetic
makeup than that of either parent |
The processes of crossing over and genetic recombination
will result in offspring exhibiting variation from the
original parents. The variations shown between different
sexually produced offspring provide the driving force for the
process of natural selection.
Heredity and
Environment
The characteristics of an organism can be described in
terms of combinations of traits. Traits are inherited, but
their expression can be modified by interactions with the
environment. Examples of this include the lack of color
in completely shaded grass, even though it still possesses the
genetic makeup to appear green and the change in fur color of
returning fur in a shaven Himalayan hare at cold
temperatures.
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Effect of Cold on
Himalayan Hare Fur Color |
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The application of an ice pack to a region of shaved
hair results in black hair growing back instead of the
original white color. |
The many body cells in an individual can be very different
from one another, even though they are all descended from a
single cell and thus have identical genetic instructions. This
is because different parts of these instructions are used in
different types of cells, influenced by the cell’s
environment and past history. Poor health
habits can have an adverse effect on the development and
expression of many genes in human cells, resulting in sickness
or even death.
Mutation
A mutation is a change in the genetic material of an
organism.
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Mutations |
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Mutations
which occur in non sex cells of sexually reproducing organisms
will not be passed on to the offspring, although they may
result in disease or death for the organism
involved. One possible consequence of a mutation
in a non sex cell is uncontrolled mitotic cell division or cancer.
Mutations
which occur in sex cells or gametes may be passed to the
offspring. Along with crossing over and genetic
recombination, mutation provides for a source of variation in
sexually reproducing individuals.
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