Specific Learning Objectives:
• Describe meiosis
• Describe the characteristics of human gametes
• Explain the inheritance of gender

An adult human is made up of about 50 billion cells.
That’s 50 000 000 000 000 cells!
Everyone started out as just one single cell.
How does one cell become 50 thousand million?

New cells are produced by old cells dividing.
Why are new cells needed?
 Growth and repair of body cells.
 Production of specialized sex cells.
New cells are needed for two main reasons, so there are two different types of cell division …

Mitosis
• A cell reproduces itself for growth or repair
• Makes identical copies of itself
Meiosis
• Sex cells ( Gametes ) production
• Where a cell forms with half the chromosomes from parent cell

Mother Each body cell in the baby contains 23 pairs of chromosomes
MEIOSIS
Another name for sperm and egg cells is
Gametes
These sex cells join to make one cell during
Zygotes
MITOSIS
Embryo
Father
One part of the chromosomes in each pair comes from the father, the other from the mother
This cell grows into a baby by multiplying & dividing many times

• Our chromosomes come in pairs (humans have 23 pairs to make 46 chromosomes).
• Chromosomes carry the genes that determine our characteristics.
• The sperm and egg are sex cells ( gametes ) are formed by the process of meiosis. Each gamete contain half the chromosome pairs.
• With the fusion of a sperm and egg, the chromosomes carried in each are combined together.

In human body cells there are a total of 46 chromosomes
The sex of a child is determined by specific chromosomes called X and
Y chromosomes.
1 2 3 4 5
6 7 8 9 10
Males have the X and Y chromosomes
Females have X and X chromosomes
11 12 13 14 15
16 17 18 19 20
21 22 23
XY

Chromosomes and sex cells
What happens to the sex chromosomes when sex cells are formed?
X meiosis
X X X female body cell
(XX)
All ova have one X chromosome.
X X Y Y meiosis male body cell
(XY)
Sperm have an X or Y chromosome.
Why is there an equal chance of a baby being a boy or a girl?

Possible combinations can be worked out using a Punnett square: X
X
X Y