MEIOSIS

HEREDITY (MENDEL’S THEORY)

NATURAL AND ARTIFICIAL VARIATION AND SELECTION

KEY TERMS

Cells: the basic unit of structure in the of structure in living things that can carry on all life functions.

Chromosomes: structures composed from DNA that contain cells (genetic material)

Diploids: Cells that have the full number of chromosomes in homologous pairs

Gametes: the sperm and egg cells or sex cells that combine to form a zygote during fertilization.

Haploids: cells that have the normal number of chromosomes.

Spores: a reproductive structure with a protective covering formed from non living matter; E.g. mud created toads and frogs.

CELL REPRODUCTION

Cell reproduction is the process by which cells produce more cells. Cell

reproduction is very pertinent for growing, repairing dead cells and

producing new organisms.

TYPES OF CELL REPRODUCTION

Mitosis Meiosis

CHARACTERISTICS OF MEIOSIS

Cells in the testes and ovaries divide by

meiosis.

Meiosis produces cells that are not genetically

identical.

Meiosis have only half of the number of chromosomes as the

original cells. In meiosis the cells divide twice.

STAGES OF MEIOSIS

Four chromosomes in the nucleus

Each chromosome duplicates itself

Cells divide into two

Cells divide again to form four sex cells with only

two chromosomes

IMPORTANCE OF MEIOSIS

Meiosis is a source of genetic variations because it provides new combinations of chromosomes for the resulting gametes.

COMPARISON BETWEEN MEIOSIS AND MITOSIS

FEATURE MITOSIS MEIOSIS

Number of cell division

1 2

Number of cells formed

2 4

Number of chromosomes in cells formed

Same as original cell (diploid)

Half the number of original cells (haploid sex cells)

Types of cells formed

Body cells Sex cells

Genetic variation in cells formed

None Variation

HEREDITY

Heredity is the passing of traits to offspring from its parents or ancestor. This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell organism.

GREGOR JOHANN MENDEL

Mendel worked out the main rules of inheritance. Many others had studied the inheritance of trait earlier, but Mendel blended his knowledge of science and mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arrive at the laws of inheritance.

MENDEL’S EXPERIMENTS

Mendel used Pea plant for his experiment, which are usually self pollinating even when they are alone. He selected pea plant so that he could control and change their self-pollination to cross-pollination. He called them ‘P’ generation.

MENDEL’S FIRST EXPERIMENT

At first Mendel cross-pollinated short and tall pea plants of pure breed.

When the plants of F1 generation were fully grown he allowed them to self pollinate.

SECOND EXPERIMENTDIHYBRID

Natural Selection

Natural selection is Darwin’s most famous theory; 

Individuals with characteristics which increase their

probability of survival will have more opportunities to

reproduce and their offspring will also benefit from the

heritable, advantageous character.

Natural Selection

Artificial Selection

Artificial selection is the intentional reproduction of

individuals in a population that have desirable traits. 

A process in the breeding of animals and in the cultivation 

of plants by

which the breeder chooses to perpetuate only those forms 

having

certain desirable inheritable characteristics.

Artificial Selection

SIMILARITIES AND DIFFERENCES

Similarities

Both natural and artificial

selection reduces changes

in living organisms.

Differences

The selective force of natural selection is the environment while the agent of change in artificial selection arises from human.

REFERENCES

Coble C.R, Fichter G.S et al. (1980) Life Science. New Jersey. Prentice-Hall Inc. pp. 358-361.

Hallman R. (2001) Reviewing The Living

Environment: Biology. New York. Amsco School Publications Inc. pp 100-102.