CAMBRIDGE SECONDARY ONE SCIENCE

CHAPTER 3: VARIATION AND INHERITANCE

3.1 KEYS

A key is a method of identifying an unknown organism.

A key is a set of questions or statements about the organism you want to identify,

which you work through in sequence to find the name of the organism.

3.1 KEYS

3.1 KEYS

Does it have legs?

Yes

Does it have more than six legs?

Yes

Crab

No

Does it have four wings?

Yes

Dragonfly

No

Housefly

No

Is its body made up of rings?

Yes

Earthworm

No

Slug

3.1 KEYS

Some keys are arranged differently and are given two statements at each step for you to choose. The choice you made leads to another pair of choices.

Step Descriptions Next step/ Identity1 Has legs 2

Does not have legs 32 Has six legs 4

Has more than six legs Crab

3 Body is made up of rings Earthworm

Body is not made up of rings Slug 4 Has four wings Dragonfly

Has two wings Housefly

3.2 VARIATION

A species is a group of organisms that share common characteristics and can interbreed to produce fertile offspring. No two individuals of the same species are exactly alike. The differences between individuals within a species are called variation.

3.2 VARIATION

All of these snails are of the same species but they show variation

3.2 VARIATION

Causes of variation

Genes Environment

3.3 INHERITANCE

Inheritance is the passing of characteristics from parents to their offspring.

Genes are passed down from parents to offspring. Genes provide a set of

instructions that determine some of the characteristics of an organism. Half of your genes come from your father

and half from your mother. Therefore, children have characteristics from both

parents.

3.3 INHERITANCE

Examples of characteristics determined by genes: (i) Sex (ii) Hair colour (iii) Blood group

3.3 INHERITANCE

Characteristics affected by genes: Sex, hair colour and blood group

3.3 INHERITANCE

In each generation, children inherit genes from both of their parents

3.3 INHERITANCE

The environment also determines the characteristics of an organism. Characteristics determined by genes are affected by genes too. Examples of characteristics determined by the environment: (i) Body weight (ii) Height (iii) Intelligence

3.3 INHERITANCE

Characteristics affected by environment: Body weight, height and intelligence

3.4MORE ABOUT INHERITANCE

Genes are found on chromosomes in the nucleus of a cell.Genes and chromosomes