Chapter 11Chapter 11

Mendel’s TheoryMendel’s Theory

Mendel’s HypothesisMendel’s Hypothesis

Before Mendel performed his Before Mendel performed his experiments people thought that experiments people thought that offspring were just a mixture offspring were just a mixture between the mother & father’s between the mother & father’s traits.traits.

For example if you mixed a short For example if you mixed a short plant & a tall plant the offspring plant & a tall plant the offspring would be of medium height.would be of medium height.

Mendel noticed that each plant Mendel noticed that each plant has two “heritable factors”.has two “heritable factors”.

Mendel’s Hypothesis Mendel’s Hypothesis Cont.Cont. 1. Individuals have two copies of 1. Individuals have two copies of

each trait (one from each parent)each trait (one from each parent) 2. There are alternative versions 2. There are alternative versions

for each gene called for each gene called allelesalleles. . 3. When both alleles are present 3. When both alleles are present

one can be hidden while the other one can be hidden while the other is expressed. Dominant & is expressed. Dominant & Recessive.Recessive.

4. Each gamete contributes one 4. Each gamete contributes one allele during fertilization.allele during fertilization.

Modern termsModern terms

The The traittrait that is shown more often that is shown more often is called the is called the dominant traitdominant trait and is and is expressed with a CAPITAL letter.expressed with a CAPITAL letter.

The The traittrait that is shown less is that is shown less is called the called the recessive traitrecessive trait and is and is expressed with a lower case expressed with a lower case letter.letter.

Modern terms cont.Modern terms cont.

If the two alleles that an offspring If the two alleles that an offspring receives are the same it is said to receives are the same it is said to be homozygous.be homozygous.

For example AA = homozygous For example AA = homozygous dominantdominant

aa = homozygous recessiveaa = homozygous recessive If the two alleles that an offspring If the two alleles that an offspring

receives are different it is said to be receives are different it is said to be heterozygous. Aaheterozygous. Aa

Dominance & Dominance & RecessivesRecessives Most traits have a dominant allele Most traits have a dominant allele

and a recessive allele.and a recessive allele. For example in pea plants a purple For example in pea plants a purple

flower is dominant to a white flower is dominant to a white flower.flower.

If an offspring is homozygous If an offspring is homozygous dominant and heterozygous it will dominant and heterozygous it will be purple. The only time it will be be purple. The only time it will be white is if the offspring receives white is if the offspring receives both recessive alleles.both recessive alleles.

AppearanceAppearance

The set of alleles that an organism The set of alleles that an organism has is known as its genotype.has is known as its genotype.

The actual physical appearance of The actual physical appearance of an organism is its phenotype.an organism is its phenotype.

For a purple flower that is For a purple flower that is heterozygous, what is the heterozygous, what is the phenotype? What is the genotype?phenotype? What is the genotype?

Laws of HeredityLaws of Heredity

Mendel came up with two laws that Mendel came up with two laws that became the basis of heredity.became the basis of heredity.

Law of SegregationLaw of Segregation – The two alleles – The two alleles separate when sex cells are formed.separate when sex cells are formed.

Law of Independent AssortmentLaw of Independent Assortment – – Alleles separate independently from Alleles separate independently from each other.each other.