Mendel’s Principles of Inheritance
By: Florencia Merniez
The Principles of Inheritance was created by Gregor Mendell. He was a scientist who
earned his fame as the founder of science in genetics. These principles, who later became known
as Mendel’s Principles of Heredity, explained the science of genetics. These principles led to the
understanding of traits in living organisms. He experimented with pea plants and formed two
Principles of Inheritance. These principles named the Principle of Segregation and Principle of
Inheritance explain how traits and genes are passed on from parents to offspring.
Mendel started by grouping a few pea plants that were different from each other. Some of
these plants were short, tall skinny, wide, etc.. He crossed tall plants with short plants and he
called the first generation of plants the parental generation. All of the plants offspring came out
tall. Then he crossed the second generation of plants, but this time, only about three fourths of
the offspring were tall, and the remaining one fourth was short. These results explained that
different expressions of a trait were controlled by discrete units, (genes). These genes would
come in pairs, that offspring inherited one unit from each parent.
If the two factors were identical, the individual was called homozygous for the trait. If the
two factors have different information, the individual was called heterozygous. These factors are
called alleles. The genotype of an individual is made up of many alleles it possesses. An
organism’s physical appearance, or phenotype, is determined by it’s alleles as well as by it’s
environment. An individual possesses two alleles for each trait, one allele is given by the female
parent and other by the male parent. These alleles are passed on when an individual matures and
produces gametes. Gametes are the sex cells of an individual, the egg and the sperm.
When the gametes form, the paired alleles separate randomly so that each gamete
receives a copy of one of the two alleles. The presence of an allele does not mean that the trait
will be expressed in the individual that possesses it. In heterozygous individuals, the allele that is
expressed is the dominant. The recessive allele is present but its expression is hidden. In meiosis,
the paternal and maternal chromosomes get separated and the alleles with the traits of a character
are segregated into two different gametes. The two coexisting alleles of an individual for each
trait segregate (separate) during gamete formation so that each gamete gets only one of the two
alleles. Alleles come together at random fertilization of gametes.
Mendel summarized the findings by this experiment into two laws, the Law of
Segregation and the Law of Independent Assortment. The Principle of Segregation explains that
for any particular trait, the pair of alleles of each parent separate and only one allele passes from
each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of
chance. This segregation of alleles occurs during the process of sex cell formation.
The Principle of Independent Assortment explains that different pairs of alleles are
passed to offspring independently of each other. The result is that new combinations of genes
present in neither parent are possible. An example is that a pea plant's inheritance of the ability to
produce purple flowers instead of white ones does not make it more likely that it will also inherit
the ability to produce yellow pea seeds in contrast to green ones. This is because that the fact
that the genes for independently assorted traits are located on different chromosomes.
Independent assortment happens in eukaryotic organisms and produces a gamete with a
mixture of the organism's chromosomes. Of the 46 chromosomes in a human cell, half are
derived from the mother’s egg and have from the father’s sperm cell. This occurs as sexual
reproduction involves the fusion of the egg and the sperm to produce a new organism having the
full complement of chromosomes. During the production of new gametes by an adult, the normal
complement of 46 chromosomes needs to be split to 23 to ensure that the resulting haploid
gamete can join with another gamete to produce a diploid organism. An error in the number of
chromosomes, such as those caused by a diploid gamete joining with a haploid gamete can cause
the organism to be born with problems.
Mendelian traits are also called discrete traits. They are controlled by allleles at genetic
locus. In some cases, a mutation in a single gene can cause a disease that is inherited according
to Mendel's laws. Examples include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and
xerederma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial
disease like arthritis, which is affected by several loci and the environment. Those that receive a
dominant allele from one parent and a recessive allele from the other parent will have the
dominant form of the trait.
For a person to actually have a recessive disorder, they must have two copies of that
recessive allele that causes it. People who have only one copy of a harmful recessive allele are
unaffected, but they can still pass that allele onto offspring. Those are frequently called carriers.
If their mate is also a carrier, its possible for them to have a child who will be homozygous for
that allele, and that child will be affected. The mating between two carriers, the risk of having an
affected child is 25 percent.
WORKS CITED
Orel, Vitezlav. Grgor Mendel: the First Geneticist. Oxford University Press, 1996.
Reid, George. The Laws of Heredity With A Diagrammatic Representation. Methuen &
Co. W.C., London. 1910.
"Mendels Genetics”. Palomar.edu.. 5 January 2002. 11 March 2003.
<http://anthro.palomar.edu/mendel/mendel_1.htm>