Childrens Illustrated Encyclopedia

The science of geneTics has officially existed ever since the word “gene”

was coined in 1909 by the Danish botanist Wilhelm Johannsen (1857-1927).

he invented the term to describe the “particles” of inheritance that pass

characteristics from one generation of plant or animal to the next. The

field of genetics developed over the course of the 20th century, and

produced important discoveries about how genes work. scientists showed

that genes are sections of the long molecules of deoxyribonucleic acid

(DnA) that are connected together to form chromosomes. genes

contain the instructions by which plant and animal cells are built.

genes are passed from both parents to their children

through sexual reproduction. By this process, called

heredity, inheritable characteristics are passed from

one generation to the next.

DnA

Deoxyribonucleic acid

is the full name of DnA.

it is the molecule that

holds the coded

instructions within

genes. its structure is a

double helix, with

chemical bonds that

attach one side of the

helix to the other, rather

like the rungs of a ladder.

each “rung” is made up of a

pair of chemicals selected from

a choice of four chemicals, so the

way in which genetic information is

coded is actually very simple.

hereDiTy

When a plant or animal is created, it inherits

a combination of genetic information from both of

its parents. heredity is the passing of characteristics

from parents to children. it means that a baby

inherits certain features from each of its parents,

but it also ensures that each baby is usually

different from its brothers and sisters.

Albino hamster

has white fur and

red eyes.

muTATion

When new DnA

is being created,

sometimes a mistake

can occur during the

copying process.

These mistakes are called

mutations, and they may appear

as a defect or a new characteristic.

if a mutation turns out to be

useful, it may become common

in future generations.

229

Hamster with

normal coloring

Genetics 229-

genes

each chromosome consists of a

long molecule of DnA. normally,

the molecule is unraveled, but it

coils up to give the chromosome

its characteristic shape (right) when

a cells divides. each gene is a short

section of the chromosome that

contains the instructions needed

to make a particular protein. This

is just one of the large group of

chemicals that control the features,

such as eye color. humans have

around 23,000 genes.

chromosomes

The nucleus of a living cell

contains a number of pairs

of chromosomes. They are

rather like filing cabinets

that store all of the genetic

information of the plant or

animal. chromosomes are

arranged in pairs that carry

identical or different forms

of the same genes.

The sides of the

“ladder” are made up

of phosphate and

sugar molecules.

Each “rung”

is a pair of

chemicals

called bases.

The sex chromosomes determine

whether a cell is male or female.

Males contain an XY pair, while

females contain an XX pair.

X chromosome

Blue eyes

Some of the features

controlled by genes

can be easily seen.

The genes in this girl’s

cells make her eyes

blue, her hair straight,

and her skin fair.

The genes in this boy’s

cells make his eyes

brown, his hair wavy,

and his skin dark.

The DNA molecule looks

something like a twisted

ladder in this model. In

real life it is a chain of tens

of thousands of atoms.

Children resemble their parents

but are not identical to them.

Wavy hair

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