Mendel’s Principles

Who was Gregor Mendel? He was an Austrian monk born in 1822. He is known as the Father of Genetics for his experiments and observations of the inherited traits of pea plants. He cross-pollinated pea plants which were true-breeding (producing offspring identical to the parents) for several traits.

What are Traits? A trait is a specific characteristic that varies from one individual to the next. Mendel chose 7 pea plant traits to study: seed shape, seed color, flower color, pod shape, pod color, flower position, and plant height. HOWEVER, he studied only ONE TRAIT at a time! This is what helped make his results so conclusive!

Mendel’s Experiment! He wanted to fertilize seeds by joining male & female gametes from 2 different plants with different traits (ex: Tall & short plants). To prevent self-pollination, he cut away the pollen-bearing male parts from one plant. Then, dusted pollen from another plant onto the egg-bearing female part of the first plant. This process is referred to as cross-pollination.

Mendel cross-pollinated true-breeding tall plants with true-breeding short plants. He called these original pair the “parent “ plants or P generation.

Their offspring, or the F1 generation were all tall. The offspring of crosses between parents with different traits are called hybrids.

Mendel then let the F1 generation self-pollinate to create the second generation (F2 generation).

Some of the F2 offspring were tall and some were short. The ratio consistently added up to 3 tall: 1 short.

Mendel’s Results Based on Mendel’s work, we now know that: A gene is a segment of DNA that determines a trait. Ex: eye color Alleles are different forms of a gene. Ex: brown, blue, green, or hazel eyes Mendel’s results led him to three basic principles… Mendel’s Discoveries & Results

The Principle of Dominance: In general, one allele is dominant to another. The recessive allele will be expressed only if the dominant allele is not present. Expressed as the lower case letter of the dominant trait. Ex: Short is a recessive trait: tt The dominant allele is always expressed even if combined with the recessive allele. It masks the recessive one. Expressed with a CAPITAL letter. Ex: Tall is a dominant trait: TT or Tt

The Principle of Dominance continued… The combination of alleles is called the genotype. If both alleles are the same, the individual is homozygous. If the alleles are different, the individual is heterozygous. The physical appearance, or observable trait is the phenotype.

The Principle of Segregation When gametes form, the alleles MUST separate from each other. Each gamete carries only a single allele for each gene.

TT tall no P T n/a n/a yes Tt tall F1 no tt short F2 no 3 4 5 6 8 7 9 10 11 12 13 14 Parents (P) gametes F1 gametes F2 Circle # Genotype Hetero/Homo-zygous Phenotype Generation Gamete? (y / n) 1 3 7 14 TT homozygous tall no P T n/a n/a n/a yes Tt heterozygous tall F1 no tt homozygous short F2 no

Principle of Independent Assortment When examining 2 different traits at the same time (height & seed color), Mendel noticed that all possible combinations were expressed in the F2 generation. Ex: tall & green, tall & yellow, short & green, short & yellow Different traits are inherited independently of each other, so that there is no relation, for example, between a plant’s height and its seed color. Animation