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
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