Chapter 11 Mendel’s Theory

Mendel’s Hypothesis Before Mendel performed his experiments people thought that offspring were just a mixture between the mother & father’s traits. 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. 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 noticed that each plant has two “heritable factors”.

Mendel’s Hypothesis Cont. 1. Individuals have two copies of each trait (one from each parent) 1. Individuals have two copies of each trait (one from each parent) 2. There are alternative versions for each gene called alleles. 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. 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. 4. Each gamete contributes one allele during fertilization.

Modern terms The trait that is shown more often is called the dominant trait and is expressed with a CAPITAL letter. 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. The trait that is shown less is called the recessive trait and is expressed with a lower case letter.

Modern terms cont. If the two alleles that an offspring receives are the same it is said to be homozygous. If the two alleles that an offspring receives are the same it is said to be homozygous. For example AA = homozygous dominant For example AA = homozygous dominant aa = homozygous recessive aa = homozygous recessive If the two alleles that an offspring receives are different it is said to be heterozygous. Aa If the two alleles that an offspring receives are different it is said to be heterozygous. Aa

Dominance & Recessives Most traits have a dominant allele and a recessive allele. Most traits have a dominant allele and a recessive allele. For example in pea plants a purple flower is dominant to a white flower. 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. 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 The set of alleles that an organism has is known as its genotype. The set of alleles that an organism has is known as its genotype. The actual physical appearance of an organism is its phenotype. 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? For a purple flower that is heterozygous, what is the phenotype? What is the genotype?

Laws of Heredity Mendel came up with two laws that became the basis of heredity. 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 Segregation – The two alleles separate when sex cells are formed. Law of Independent Assortment – Alleles separate independently from each other. Law of Independent Assortment – Alleles separate independently from each other.