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 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.

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 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.

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.

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.