|
Heredity
- Mendel's work
By
Jeanne Smith, Contributor
HEREDITARY
IS determined by discrete conserved
factors. It is very easy to identify
subtle features in children which
link them to their parents. Very often
we hear statements such as "you
have your father's eyes" or "you
have your mother's nose". Siblings
usually also look alike.
You
probably know families of tall people.
The inheritance of traits in a family
is usually quite obvious, however
the rules governing heredity are not
as obvious. Gregor Mendel (1822-84)
an Austrian experimental biologist,
who also became a monk, carried out
the first extensive studies of inheritance
in the 1800s. Mendel studied the inheritance
of characteristics of pea plants.
He studied only a few characteristics
which could be described plainly and
showed either one definite form or
another.
Mendel
carried out independent studies of
the inheritance of characteristics
such as flower colour, seed shape
and seed colour. Mendel's idea that
each characteristic is determined
by a specific factor called a gene,
provided the basis for further genetic
studies. Mendel was aware of the genetic
role of chromosomes, however his experimental
results were confirmed in the first
decade of the 1900s, after the rediscovery
of his laws, and fundamental to modern
genetic thought. Mendel selected pure-breeding
(homozygous) strains of peas for his
work. These plants had been inbred,
that is bred only with themselves
for several generations and showed
only one form of the trait being studied.
In
each of his experiments, Mendel crossed
two strains of peas (hybridisation)
to produce hybrids. In one of his
experiments, Mendel crossed pure-breeding
purple flowered plants with pure-breeding
white flowered plants. The resulting
pea plants all produced purple flowers.
This first set of offspring he called
the first filial (or F1) generation
(the word filial comes from the word
Filius which means brother). Mendel
then crossed the plants from the F1
generation to produce a second filial
(or F2) generation. Mendel noted that
the white flower absent from the F1
generation was present in the F2 generation.
Mendel
also studied the ratios in which the
characteristics occurred. He found
that all the features studied showed
the same pattern.
MENDEL
CONCLUDED THAT:
*
the characteristics are determined
by factors he called genes.
*
each plant carried two genes for each
characteristic.
*
one gene he described as dominant,
in this case purple flower colour
is dominant to white. That is, in
the heterozygous condition only the
allele for purple colour will affect
the phenotype.
*
The other gene was described as recessive.
The effects of this gene are hidden
in the presence of the dominant gene.
A
capital letter is used to represent
a dominant allele and a lower case
letter is used to represent a recessive
allele, for example:
P
represents purple flowers (dominant)
p
represents white flowers (recessive)
*
The genotype PP is the homozygous
dominant condition and produces purple
flowers.
*
The genotype Pp is the heterozygous
condition and also produces purple
flowers.
*
The genotype pp is the homozygous
recessive condition and produces white
flowers.
*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
|
