Unit 6 Section 1: What is Heredity

The German friar Gregor Mendel spent part of his life studying plants. As he bred different pea plants in his monastery, he made discoveries that would forever change the course of scientific discovery.

• Describe the results of Mendel's experiments

• Identify what alleles are

• Identify the role alleles play in controlling the inheritance of traits

What Did Mendel Observe?

• Heredity: the passing of physical characteristics from parents to offspring

• Genetics: The scientific study of heredity

• Father Mendel wanted to know why different pea plants had different characteristics

Traits

• Trait: The scientific name for a specific characteristic in an individual

  • Father Mendel noticed that overall plants had traits similar to their parents

Mendel's Experiements

• Mendel used thousands of plants to study traits

  • He would hand-pollinate to cross-breed pea plants with specific traits

    • Breed a tall plant with a short plant

• Purebred: the offspring of many generations that have the same form of a trait

The F₁ Offspring

• Mendel crossed Purebred Tall with Purebred short plants

  • Parental or P Generaion

• All offspring from that breeding was the First Filial (F₁) Generation

  • Fila is Latin for daughter or son

• All were tall, with no short plants

The F₂ Offspring

• Mendel then took the F₁ Generation and allowed them to self-pollinate, expecting all the plants to be tall.

  • Instead, 75% were tall and 25% were short

• He repeated the experiment with traits other than tallness and always had the same result

  • one trait disappeared for the F₁ generation but came back in the F₂ generation

Mendel's Conclusions

• Father Mendel figured that some form of information controls what traits each offspring showed

  • The traits exist in pairs (tall or short, green or yellow, etc.)

  • One piece of the pair comes from each parent

  • One form of the trait is stronger and can hide the other

Genes & Alleles

• Gene: a factor that controls a trait

• Alleles: The different forms of a gene

  • The gene for tallness in plants has two forms: tall and short

Types of Alleles

• An organism's traits are controlled by the alleles it inherits from its parents.

• Dominant Allele: a trait that always shows up in the organism when the allele is present

  • Shown by a Capital letter for the trait

    • Example: Tall plant alleles are shown by a "T"

• Recessive Allele: a trait that is hidden when a dominant allele is present

  • Shown by a lowercase letter for the trait

    • Example: Short plant alleles are shown by a "t"

Alleles in Mendel's Crosses

• In the Parent generation, all the parents were purebred, so they all had the same alleles for their traits

  • Either TT or tt

• The F₁ generation was a mix

  • Hybrid: an organism with two different alleles for a trait

    • Only the Dominant trait is shown

    • all the F₁ plants had Tt for their alleles

  • They can pass on either a tall or a short allele to their offspring

Significance of these discoveries

• Before these experiments, people thought all offspring just showed a blend of their parent's traits

• Mendel's Pea plants showed that traits are actually determined by specific values

  • His work was not known for a long time and was rediscovered in the 1900s

  • Mendel is known as "The Father of Genetics"