Mendel’s workCOULTER

Gregor Mendel• A priest in the mid 19th century tended a garden in the a European

monastery.

• Heredity is the passing of physical characteristics from parents to offspring.

• Mendel wondered why different pea plants had different characteristics.• Some were tall, others were short, some plants produced green seeds, others

had yellow seeds,

• Traits: each different form of a characteristic• Mendel observed that the pea plants’ traits were often similar to those of their

parents. Sometimes the plants had different trait from those of the parents

• Genetics: the study of heredity

Mendel’s experiment• The flower petals

surround the pistil and the stamen.

• New organism begins to form when egg and sperm join in a process called fertilization.

• Before fertilization can occur pollen (stamen) must reach the pistil; this process is called pollination.

Pollination • Pea plants are self pollinating

plants.

• Pollen from the flower lands on the pistil of the same flower.

• Mendel developed a method to cross-pollinate pea plants.

• He removed pollen from the flower of one plant.

• He brushed the pollen onto a flower on a second plant.

Crossing pea plants• Mendel decided to cross plants with contrasting traits.

• He started his experiment with purebred plants

• Purebred organisms are the offspring of many generations that have the same trait.

Crossing pea plants: the F1 offspring• Mendel crossed purebred

tall plants with purebred short plants.

• Scientists call these parent plants the parental generation, or P generation.

• The offspring from this cross are the first filial generation, or the F1 generation.

• Filial is Latin for daughter or son.

Crossing pea plants: F2 offspring• When the plants of the F1

generation were fully grown, Mendel allowed self-pollination

• Mendel was surprised to see the F2 generation was a mix of tall and short.

• The shortness trait reappeared even though none of the F1 parent plants were short.

• About 3/4th were tall and 1/4th were short.

Crossing pea plants: experimenting with other traits• In all of Mendel’s crosses, only one form of a trait

appeared in the F1 generation. However in the F2 generation, the “lost” form of the trait always reappeared in about 1/4th of the plants.

Dominant and recessive alleles• Mendel reasoned:

• Individual factors, genetic information, must control the inheritance of traits in peas.

• Factors that control each trait exist in pairs (female contributes one, male contributes one).

• One factor in a pair can mask, or hide the other factor.

Dominant and recessive alleles: genes/alleles• Gene: the factor that controls the trait.

• Alleles: the different forms of a gene.

• An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive.• Each pea plant inherits two alleles from its parents—one from egg

and one from sperm.

• Dominant allele: is one whose trait always shows up in the organism when the allele is present.

• Recessive allele: is hidden whenever the dominant allele is present.

Dominant and recessive alleles: Mendel’s crosses• In Mendel’s height cross, the

purebred tall plant in the P generation had two alleles for tall stems. The purebred short plant had two alleles for short stems.

• The F1 plants each inherited an allele for tall stems and short stems.

• The F1 plants are called hybrid

• Hybrid: organism has two different alleles for a trait.

• Take a look at the F2 generation…

Symbols for alleles• Dominant allele is represented by a capital letter.• Example: tall stem is represented by T

• Recessive allele is represented by a lower case letter.• Example: short stem is represented by t

• When a plant inherits two dominant alleles (purebred) it is represented as TT

• When a plant inherits two recessive alleles (purebred) it is represented as tt

• When a plant inherits one dominant and one recessive (hybrid) it is represented as Tt

Significance of Mendel’s work• Mendel’s discovery of genes and alleles eventually changed

scientists’ ideas about heredity.

• Before Mendel, most believed trait were a blend of parents. So, if a tall and short were crossed, offspring would be medium.

• Mendel discovered traits are determined by individual separate alleles inherited from each parent.

• Unfortunately the importance of Mendel’s work was not recognized until after his death.

• In the 1900, three different scientists discovered Mendel’s work.

• Mendel is often called the father or genetics.