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Mendel and Heredity
Chapter Eight
The passing of characters
(traits) from parents to offspring
is called heredity.
Genetics is the study of
heredity or how genes are
passed from parents to their
offspring. Much of our
understanding of this field is
owed to the studies of Gregor
Johann Mendel, an Austrian
monk who studied garden peas.
In 1842 Gregor Mendel entered a monastery in
Brunn, Austria. One of his tasks was to care
for the garden. His inquiring mind acquired
and stored data about garden peas that he
would use later in his studies.
In 1851 he entered the University of Vienna
where he learned statistics (study of
probabilities) in his math courses. These
math courses would prove valuable to him
when he began to organize his data in his
Mendel repeated the experiments of a British farmer
who had crossed a variety of garden peas that had
purple blooms and white blooms. All of the offspring of
the cross had purple flowers, however, in the next
generation, two white blooms resurfaced.
Mendel would use his math to study the traits.
The garden pea was an excellent
selection for Mendel’s studies:
• Several characters of garden peas exist in
two clear forms with no intermediates such
as purple or white flowers.
• The male and female parts of the flower
are found in the same bloom and can
easily be cross-pollenated.
• The garden pea grows on a small plant,
matures quickly, and produces many
offspring.
Mendel’s Garden Pea Characters
Mendel intended to cross
various parent varieties and
study the occurrence of one
trait. This is a monohybrid
cross.
Mendel’s experimental plan was to:
• All parent plants to develop true breeding. That
is, these parents’ offspring always would show
the same traits. The groups are sometimes
defined as pure. These were called the P
generation.
• Mendel would then cross two P generations to
develop the F1 generation or first filial.
• He would then cross two F1 generations to
develop an F2 generation.
Mendel expressed the
occurrence of each trait as a
ratio. He found that traits
always occurred in the F2
generation in a ratio of 3:1.
Mendel’s Theory
Section Two
Before Mendel, it was thought
that offspring inherited a blend
of their parent’s traits. Mendel
did not support this hypothesis.
He believed that offspring
inherited two factors, one from
each parent, that would control
their traits.
Mendel developed four hypothesis from
his experiments.
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For each inherited character, an individual has two
copies of a gene– one from each parent.(Pair
Principle)
There are alternative forms of a gene (alleles) that
control different traits.
When the two alleles occur together, one may be
completely expressed (dominant) and one may
have no observable effect (recessive).(Law of
Dominance)
Gametes carry only one allele for each inherited
character and, during fertilization, each gamete
contributes one allele.
Mendel’s hypothesis have developed into two laws of heredity:
• Law of Segregation– The two alleles for a
trait separate when gametes are formed.
• Law of Independent Assortment– Alleles
of different genes separate independently
when gametes are formed.
The two alleles for a gene that an individual
carries can be classified in two ways:
 Homozygous– The two genes that
determine a character are the same.
 Heterozygous– The two genes that
determine a character are different.
Two terms are used to describe a
character:
 Genotype is the exact genes that
determine a trait
 Phenotype describes the appearance
of a trait.
Studying Heredity
Section Three
A Punnet Square is a
diagram that predicts all the
outcomes of a cross by
considering all the possible
combinations of genes. The
probability or liklihood that
a character will be expressed is
stated as a fraction of the
whole.
To study a trait in a particular
family, geneticists prepare a
pedigree which is a history of
how the trait has been
expressed over several
generations.
To study a trait, a geneticist must
determine if it is:

Autosomal—That is, the trait is carried on
autosomes and appears in both sexes equally.

Sex-Linked—Most sex-linked traits are recessive
and are carried on the X chromosome. The only
way a female would express the trait would be if
she inherited it on both of her chromosomes. A
male would always express the trait if it was
present on his X chromosome.
Complex Patterns of
Heredity
Section Four
When several genes influence a
trait, it is considered polygenic
inheritance. Examples include
height, weight, eye color, hair
color and skin color.
Other polygenic inheritance
patterns include:

Incomplete dominance as in Japanese Four
O’Clocks.

Multiple alleles as in blood type.

Codominance as in a roan.

Environmentally influenced traits as in the coat
color of arctic foxes.
Sometimes mutated genes cause a disease or
condition. As these genes are passed to new
generations, they are considered genetic disorders.
• Sickle Cell Anemia — Recessive gene that causes a
defect in the red blood cells.
• Cystic Fibrosis —Recessive gene that keeps certain
enzymes from being produced that digest certain
foods and mucus.
• Hemophilia – Sex-linked mutated gene that affects
blood clotting.
• Huntington’s Disease – Dominant gene that caused
slow failure of the person’s nervous system.
Genetic disorders cannot be
cured. Treatment is possible as
with a baby with PKU. A person
with a genetic disorder may
want to go to genetic counseling
with a doctor before becoming a
parent.
Gene therapy is a pioneer field
of replacing defective genes
with healthy ones. For instance,
a form of cold virus has been
used to transfer healthy genes
into lung cells.