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Mendel’s Theory
Chapter 8 Section 2
Biology
Mrs. James
A Theory of Heredity
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Before Mendel
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People thought that
offspring were a “blend”
of its parents
Ex. Short + tall =
medium height
Mendel & After
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Each sexual organism had
2 separate heritable traits
You get one or the other
Mendel’s Hypotheses
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Mendel had 4 hypotheses
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Based directly on the
results of his experiments
They make up the
Mendelian theory of
heredity
Mendel’s Hypotheses

1st Hypothesis
 For each inherited trait,
an individual has 2 copies
of the gene – one from
each parent.
Mendel’s Hypotheses

2nd Hypothesis
 There are alternative
versions of genes
 Ex. There are purple
& white pea flowers
 Alleles – different
versions of genes
 We receive one allele
from each parent
Mendel’s Hypotheses

3rd Hypothesis
 When two different alleles
occur together, one of them
may be completely
expressed, while the other
may have no observable
effect on the organism's
appearance.
Mendel’s Hypotheses

3rd Hypothesis Cont.
 Dominant – expressed
form of the trait
 Recessive – not expressed
form of the trait
 For every contrasting
form of a trait there is a
dominant and recessive
allele
Mendel’s Hypotheses

4th Hypothesis
 When gametes are formed, the
alleles for each gene in an
individual separate
independently of one another.
Thus, gametes carry only one
allele for each inherited trait.
When gametes unite during
fertilization, each gamete
contributes 1 allele.
Mendel’s Findings in Modern Terms
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Geneticists represent
alleles with letters
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Dominant are capital
letters
Recessive are lowercase
letters that are the same as
the dominant allele’s letter
Ex. Pea plant flowers
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Purple flower = P
White flower = p
Mendel’s Findings in Modern Terms
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Homozygous – 2 alleles
of the same gene that are
the same
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Ex. 2 white flower alleles
are present = pp
Heterozygous – alleles
of the same gene that are
different

Ex. 1 white flower & 1
purple flower allele - Pp
Mendel’s Findings in Modern Terms
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If an individual is
heterozygous:
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The dominant allele is
expressed
The recessive allele is
present, but not
noticeable
Ex. Freckles is dominant

You could have an Ff, and
you will have freckles even
though one allele is not for
freckles
Mendel’s Findings in Modern Terms

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Genotype – the set of
alleles an individual has
Phenotype – the physical
appearance of a trait
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Phenotype is determined
by which alleles are
present
Ex. Pp = purple
pp = white
The Laws of Heredity
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The Law of Segregation
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2 alleles for a trait
segregate when gametes
are formed.
This happens during
meiosis
Each gamete only has one
allele
The Laws of Heredity
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The Law of Independent
Assortment
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Mendel also studied
dihybrid crosses – two
pairs of contrasting traits
Ex. Plant height & flower
color
He wanted to see if the
one trait affected another
He found out that it
didn’t
The Laws of Heredity
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Law of Independent
Assortment
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Alleles of different genes
separated independent of each
other during gamete formation
This applies to genes on
different chromosomes or far
apart on the same
chromosome
Mendel’s laws dominated
biology for almost ½ a century
Any Questions?