Download Mendel`s Theory

Mendel’s Theory
Section 8-2
Ag Biology
Before Mendel’s Experiments

People thought that
offspring were a blend of
the parents characteristics.

Example
 Cross
a short plant and a
tall plant to produce a
medium plant in height.
Mendel’s Hypotheses

Mendel developed four hypotheses from
his experiments that later became the
Mendelian theory of heredity.
Hypothesis #1

For each inherited trait, an individual has
two copies of the gene- one from each
parent.
Hypothesis #2
There are alternative versions of genes.
 Different versions of a gene are called
alleles.

Hypothesis #3
When two different alleles occur together,
one of them is expressed, while the other
may have no observable effect on the
organism’s appearance.
 Expressed form of the trait is dominant.



Dominant alleles are represented by capital letters.
Not expressed traits when a dominant form
is present are called recessive.

Recessive alleles are represented by lower case letters.
Hypothesis #4



When gametes are formed,
the alleles for each gene in
an individual separate
independently of one
another.
Gametes only carry one
allele for each inherited trait.
When gametes unite during
fertilization, each gamete
contributes one allele.
Terminology

Homozygous- when an individual has the
two alleles of a particular gene that are the
same
RR

rr
Heterozygous- when an individual has
different alleles of a particular gene
Rr
Terminology

Genotype- the set of alleles
an individual has

Phenotype- the physical
appearance of a trait; what
is physically expressed
Laws of Heredity

During meiosis (gamete production),
 Alleles
for a trait separate (Law of segregation)
 The inheritance of one trait does not influence
the inheritance of different trait. (Law of
Independent Assortment)