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Chapter 11
Mendel’s Theory
Mendel’s Hypothesis
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Before Mendel performed his
experiments people thought that
offspring were just a mixture between
the mother & father’s traits.
For example if you mixed a short plant
& a tall plant the offspring would be of
medium height.
Mendel noticed that each plant has
two “heritable factors”.
Mendel’s Hypothesis Cont.
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1. Individuals have two copies of each
trait (one from each parent)
2. There are alternative versions for
each gene called alleles.
3. When both alleles are present one
can be hidden while the other is
expressed. Dominant & Recessive.
4. Each gamete contributes one allele
during fertilization.
Modern terms
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The trait that is shown more often is
called the dominant trait and is
expressed with a CAPITAL letter.
The trait that is shown less is called
the recessive trait and is expressed
with a lower case letter.
Modern terms cont.
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If the two alleles that an offspring
receives are the same it is said to be
homozygous.
For example AA = homozygous dominant
aa = homozygous recessive
If the two alleles that an offspring
receives are different it is said to be
heterozygous. Aa
Dominance & Recessives
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Most traits have a dominant allele and
a recessive allele.
For example in pea plants a purple
flower is dominant to a white flower.
If an offspring is homozygous
dominant and heterozygous it will be
purple. The only time it will be white
is if the offspring receives both
recessive alleles.
Appearance
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The set of alleles that an organism has
is known as its genotype.
The actual physical appearance of an
organism is its phenotype.
For a purple flower that is
heterozygous, what is the phenotype?
What is the genotype?
Laws of Heredity
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Mendel came up with two laws that
became the basis of heredity.
Law of Segregation – The two alleles
separate when sex cells are formed.
Law of Independent Assortment –
Alleles separate independently from
each other.