Victoria Beckham is going to have another baby.

She thinks it is highly unlikely it will be a girl (she already has three boys)

KS4 starter

© Snapshot Science, 2010 Image: Ron Whisky at wikimedia.org

What do you think?

© Snapshot Science, 2010

KS4 Activity

In the nucleus of most human cells are chromosomes. They are long strings of a substance called DNA.Humans have 23 pairs of chromosomes – 46 altogether.

Where does each member of the pair come from?

…and one from your Dad.

You get one chromosome in each pair from your Mum...

© Snapshot Science, 2010

KS4 Activity

This pair of chromosomes is called the sex chromosomes.It is different in men and women.

Men have XYWomen have XX

© Snapshot Science, 2010

KS4 Activity

The chromosomes form pairs when the sperm and egg (sex cells) fuse during fertilisation.

Sex cells only have 23 chromosomes. Why?

sperm

23

egg

23

46fertilised

egg (zygote)

© Snapshot Science, 2010

KS4 Activity

Sperm…..% are X…..% are Y

X X

XY

Y

XX

X

Egg…..% are X

cell in testis

cell in ovary

Sex cells are produced in the testes and ovaries.When they are formed the chromosome pairs split up. This includes the sex chromosomes.

Predict what will happen in the animation.

1005050

© Snapshot Science, 2010

This Punnett Square can be used to see the probability of the Beckhams having a girl.

Use the results to write what you would say to them.

David

XX

XY

XX

XY

What is the probability of the baby being a girl?