GENETICS

Introduction

Genetics

Study of heredity Gregor Mendel is considered the

“father of genetics” Mendel studied the traits of pea

plants A trait is a specific characteristic

such as seed color or height

Pea Plant Traits Studied By Mendel

Mendel’s Experiment

Mendel selected a six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall

These parent plants are known as the P generation and are true breeding, meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves

He cross-pollinated this tall pea plant with pollen from a short pea plant

All of the offspring, known as the F1 generation, grew to be as tall as the taller parent

Mendel’s Experiment (cont.)

Mendel allowed the tall plants in this F1 generation to self-pollinate

After the seeds formed, he planted them and counted more than 1,000 plants in the second generation, known as the F2 generation

Three-fourths of the plants were as tall as the tall plants in the P and F1 generations

One-fourth of the offspring were as short as the short plants in the P generation

Therefore, in the F2 generation, tall and short plants occurred in a ratio of about three tall plants to one short plant

Terms

Genes: parts of a chromosome that determine traits

Alleles: different forms of a gene T = tall t = short

Homozygous: both genes are the same (ex: TT or tt); an individual that is homozygous is referred to as a pure-bred

Terms (cont.)

Heterozygous: the genes are different (ex: Tt); an individual that is heterozygous is referred to as a hybrid

Phenotype: outward expression of a gene (ex: tall)

Genotype: genetic makeup of an individual (ex: TT)

Mendel’s Laws

Mendel’s work with pea plants allowed him to develop his 3 laws of genetics: Law of Segregation Law of Independent Assortment Law of Dominance

Law of Segregation

Paired genes on homologous chromosomes separate during meiosis

The two genes for a trait will separate during gamete formation

Law of Independent Assortment Paired genes segregate randomly and

independently of each other

Law of Dominance When an organism has two different genes for a

trait, one of the genes may be masked or hidden

Law of Dominance (cont.)

Dominant: represented by a capital letter, this gene is always expressed

Recessive: represented by a lowercase letter, this gene is masked by a dominant gene

When recording the results of crosses, it is customary to use the same letter for different alleles of the same gene (ex: T for tall and t for short; G for green pod color and g for yellow pod color)