Chapter 3Genetics: The science of

heredityMiss Luzma Fabre

Section 1: Mendel´s workGregor Mendel made experiments on plants that revolutionized the study of heredity

Heredity is the passing of physical characteristics from parents to offspring

Each different form of a characteristic is called a trait

Ex: eye color

Mendel observed plant traits such as height and seed color

Genetics is the study of heredity

Mendel´s experiments

Mendel used pea plants

He developed a method by which he cross-pollinated, or “crossed” pea plants: he took pollen from one and brushed it into the second plant

Fertilization is the process in which an egg cell and a sperm cell join to form a new organism

Crossing pea plants

Mendel crossed plants with contrasting traits (ex: tall and short)

He started with purebred plants

A purebred organism is the offspring of many generations that have the same trait

The F1 offspring

Mendel crossed purebred tall plants with purebred short plants (P= parental generation)

The offspring from this cross are called F1 generation

All the F1 plants were tall

The F2 offspring

The F1 generation self-pollinated, resulting in the F2 generation

Some plants were tall and some short in the F2 generation

Tall= ¾ plants

Short= ¼ plants

In all of Mendel´s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants

Dominant and recessive alleles

A gene is the set of information that controls a trait.

Alleles are the different forms of a gene

An orgnaism´s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive.

A dominant allele is one whose trait always shows up in the organism when the allele is present. Ex: Y

A recessive allele is hidden whenever the dominant allele is present. Ex: y

A hybrid is an organism that has 2 different alleles for a trait

Ex: Yy

A dominant allele is represented by a capital letter (Ex. T= tall)

A recessive allele is represented by a lowercase letter (Ex. t= short)

Examples:

Purebred= TT, tt

Hybrid= Tt

Mendel´s contribution

Mendel´s discoveries demonstrated that traits are determined by individual, separate alleles inherited from each parent rather than being a blend from the parents´traits