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148 Human Body

FIND OUT MORE. Cells 73 • Genetic Engineering 210–211 • Genetics 209

HOW DO GENES AFFECT CHARACTERISTICS?

Genes determine most of your physical characteristics,

such as eye colour. Eye colour is controlled by only

a handful of genes, but other characteristics, such

as height, involve lots of genes. Genes can also

affect mental characteristics such as personality and

intelligence, but these are heavily influenced by your

experiences as well. The study of genes and how they

affect you is called genetics.

HOW ARE GENES INHERITED?

People inherit half their genes from their mother

and half from their father. The genes are passed

on in chromosomes carried by sperm and ova. The

chromosomes interact with each other in different

ways, so children from the same parents may share

characteristics, but are also very different. Apart from

identical twins, everyone has a unique set of genes.

HOW DOES AN EMBRYO FORM?

Sperm and ova come together as a result of

sexual intercourse (sex). During sex, a man places

his penis inside a woman’s body. Sperm leave the

penis and swim into the woman’s reproductive

organs. If the ovaries have released an ovum, a

sperm cell may fuse with it and form an embryo.

This is called fertilization.

WHERE DOES THE EMBRYO DEVELOP?

The embryo develops inside an organ called the uterus

(womb). It sinks into the soft lining of the uterus and

absorbs food from the mother through an organ called

the placenta, which develops from the embryo.

The uterus has a very stretchy wall so that it

can expand as the baby grows.

WHERE DO SEX CELLS FORM?

Male cells are called sperm. They are produced by the

million in the testes – a pair of ball-shaped glands

that hang outside the body in the scrotum. A female

sex cell is called an ovum (plural ova). A woman’s

ova form before she is born, in two organs

called ovaries, in her abdomen.

New life is created by reproduction. The mother and

father produce sex cells, which join to form an embryo.

These sex cells also determine the child’s. HEREDITY.

The process of inheriting characteristics from parents is called

heredity. Many of our characteristics are passed on through genes

• instructions made up of DNA molecules in our chromosomes.

4 OVUM AND SPERM

When sperm cells find an ovum, they try to break

through its coat. The first to get through fertilizes the

egg and the rest die. Nuclei from the sperm and the ovum

then fuse to make a single nucleus.

1 CHROMOSOMES OF A WOMAN

Chromosomes are microscopic threads in our cell nuclei. Each one

contains a long DNA molecule. We have 46 chromosomes in 23 pairs.

One of each pair comes from our mother and the other from our father.

Reproduction

1 HUMAN FOETUS

At five months old, the human foetus weighs under 500 g (1 lb)

but has fully developed lips, eyes, fingers, and toes. The mother can

feel movements of the foetus inside the uterus (womb).

HEREDITY

The umbilical cord

carries food and

oxygen to the baby

from the placenta –

the organ inside the

womb that links the

baby to the mother’s

blood supply

The sex chromosomes

are the only chromosomes

that differ between men

and women

Sperm

attach to the ovum

and try to enter it

Ovum – ova are

the largest cells

in the human body

reproduction