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Genetics And Heredity


Heredity is the passing on of characteristics from parents
to offspring. All living things pass on traits from one
generation to the next. All children inherit traits from
their parents. A boy may be blond and blue-eyed like his
mother. A girl may have curly hair like her father , and a
nose like her grandmother. Sometimes traits can be traced
to more distant relatives. The traits that one inherits
from parents by way of the sperm and egg are called
biological inheritance. 

All inherited traits may not stay the same throughout a
lifetime. Studies of identical twins, who have the same
biological inheritance, show that environment can cause
changes in physical traits. If one twin had a serious
disease or was accidentally injured, his/her appearance
might change. He/she would then look different from his
twin brother. 

Every person inherits his basic mental ability which
determines his/her capacity to learn. Environment gives
him/her opportunities to develop this capacity. The amount
a person can learn depends both on his/her inherited
ability and on his/her environment -- parents, community,
and school. 

The cell contains bodies that determine which traits are
inherited. These bodies are called chromosomes and genes.
The chromosomes carry the genes. 

Every human body cell contains two sets of 23 chromosomes.
These two sets look very much alike. Each chromosomes in
one set can be matched with a particular chromosome in the
other set. Egg cells and sperm cells have only one set of
23 chromosomes. As a result when an egg and a sperm come
together, the fertilized egg cell will contain the 46
chromosomes of a normal body cell. Half of the chromosomes
come from the mother, and half from the father. 

Whether a fertilized egg cell develops into a girl or a boy
is determined by two special chromosomes called sex
chromosomes. In girls, the two sex chromosomes look exactly
alike. Scientists call them X chromosomes. In boys, the two
chromosomes do not look alike. One of them is an X
chromosome, but the other is shorter and does not match
with its partner. Scientists call this shorter chromosome a
Y chromosome, 

Genes are tiny particles that determine all hereditary
traits. They are carried on the chromosomes. Each
chromosome bears hundreds of genes, arranged in a line
along its length. Every person inherits two genes for a
particular trait -- one from his/her mother and one from
his/her father. But in many traits, the action of one gene
completely overpowers the action of the other. The powerful
gene is called dominant and the other recessive. For
example, suppose a dark-haired father and a red-haired
mother have a dark-haired child. In this case, each of the
child's cells contains a gene for dark hair and a gene for
red hair. But because the child has dark hair, it is
apparent that the gene for dark hair is dominant over the
gene for red hair. 

Recessive genes do not visibly show up when there is a
dominant gene present. A hybrid is a person who has both a
dominant and recessive gene. Their appearance is that of a
person who has no recessive gene, but they carry the
recessive gene, and it is possible their children will also.
There are also times when there is a compromise. If two
genes are both dominant, the resulting offspring will have
a blend of those genes. For example, a black rooster and a
white hen mate, and the resulting offspring are gray. The
same can be said for people.
Genes are made of deoxyribonucleic acid, or DNA for short.
DNA is a threadlike molecule with a diameter of only about
one ten-millionth of an inch. The thread is actually
double, and resembles a tiny twisted rope ladder. A DNA
molecule contains two kinds of purine bases. They are
adenine (called A) and guanine )called G). It also contains
two kinds of pyrimidine bases called cytosine (C) and
thymine (T). A and T fit together and C and G fit together.
No other combinations work. In other words, there are only
four kind of base-pair combinations in a DNA ladder: A-T
and C-G, or T-A and G-C. All genes in every cell contain
these same four bases, A, T, C, and G. The difference
between one gene and another lies in the arrangement of
pairs of bases along the DNA molecule. There are at least
several hundred pairs of bases in each gene. For this
reason, a vast number of arrangements is possible, and
countless kinds of genes can be formed.
Another aspect of genetics is mutation. A gene is an
extremely stable structure. It may remain stable through
thousands of generations, copying itself correctly in
between each cell division. Occasionally, however, a
mistake occurs in the copying process. This mistake may
upset the ordered sequence of base-pairs in a gene and
cause a mutation. 

Some mutations are helpful and may change an animal or
plant so that it is better equipped to live in its
surroundings. For example, once ducks had no webbing on
their feet. Only mutant ducks had webbing at the time. The
mutant ducks were better swimmers, so they survived attacks
from predators while ducks without webbing did not. Thus
only ducks with webbed feet survived, and we have the
modern duck. 

Man's understanding of heredity dates to ancient times. A
6,000 year old engraved stone found in Chaldea, near the
Persian Gulf, traced various traits through five
generations of horses. But ancient men did not know how or
why the principles of heredity worked. Early Greek
philosophers including Aristotle thought blood was the
basic element of heredity. Such phrases as blood line, blue
blood, and blood relative show the influence of Aristotle's

The 1800's brought the first theories about how traits
change from generation to generation. Gregor Mendel, an
Austrian monk, made the greatest single contribution to the
study of heredity and founded the science of genetics. By
careful experiment, observation, and analysis of his
results, Mendel formulated the first laws of heredity. 
Bailey, Marilyn. Evolution: Opposing Viewpoints. Greenhaven
Press, Inc. San Diego California, 1990. 

Bernstein, Vernstein, Martin Schachter, Alan Winkler, and
Stanley Wolfe.
 Concepts and Challenges in Life Science, Second Edition.
Allyn and Bacon Inc. Newton Massachusetts, 1986. 

Coryllel Lipke, Jean. Heredity. Lerner Publications
Company, United States, 1971.


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