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Continuity
of a species
By
Jeanne Smith, Contributor
THE
CHROMOSOME is the structure which
contains hereditary information for
growth and reproduction of an organism.
They are found in the nucleus of the
cell. Each species has a specific
number of chromosomes, and its own
set of genes. The set of all genes
shared by the members of a population
is known as its gene pool. The gene
is the unit of inheritance, the particular
set of genes carried by an organism
is its genotype. This determines its
phenotype which is the physical appearance
(or other manifestations) of the organism.
The functions of the body and behaviour
of the organism are also genetically
determined.
The
genes of the species are passed on
from one generation to the next by
reproductive cells. The reproductive
cells of a sexually reproducing organism
are called gametes. Gametes contain
half the total number of chromosomes
(a single set) possessed by the members
of the species. This is called a haploid
number of chromosomes.
Gametes
fuse in sexual reproduction to restore
the complete (double set) of chromosomes.
This is called the diploid number
of chromosomes. Humans have 46 chromosomes
in each body cell, and many thousands
of genes. This means that each chromosome
carries several genes. Each cell in
an individual's body has the same
combination of chromosomes and genes.
However unless you are an individual
twin, no two individuals are exactly
alike genetically. Most of our cells
contain homologous chromosomes; these
are two chromosomes one of which came
from each parent. They are identical
in component parts and structure.
Humans therefore have 23 pairs of
chromosomes. Each chromosomes in the
pair carries genes for the same characteristic
in the same locus. The locus is the
location of a gene on the genetic
map of the chromosome. Having homologous
pairs of chromosomes means that each
cell will contain two of a particular
kind of gene. A particular form of
a gene which produces a distinct characteristic
or phenotype is called an allele.
Each
chromosomes in the pair therefore
has one allele. The combination of
alleles on homologous chromosomes
may be homozygous or heterozygous.
In the homozygous condition, there
are identical alleles at the same
gene locus in the diploid condition.
In the heterozygous condition, there
are two different alleles at the gene
locus in the diploid condition. The
genotype, or genetic combination determines
the phenotype of the organism.
*Jeanne
Smith teaches Biology at the Queen's
school. Send your questions and comments
to the CXC Study Guide, the Gleaner
Company Ltd., 7 North Street, Kingston;
or email us at jcampbell@gleanerjm.com
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