1. Section 3
The Cell and Inheritance
By
Mr Hamdy Karim

2. • Key Concepts
• What role do chromosomes play in
inheritance?
• What events occur during meiosis?
• What is the relationship between
chromosomes and genes?

3. Chromosomes and Inheritance
• 1903- Walter Sutton was studying chromosomes in
grasshoppers.
• Found that the body cells of grasshoppers have 24
chromosomes, but the sex cells only had 12
( exactly half)

4. Chromosome pairs
 Sutton observed that when a
sperm cell and an egg cell joined
in fertilization the resulting
fertilized egg again had 24
chromosomes.
 Chromosomes exist in pairs. One
chromosome in each pair came
from the male parent and one
came from the female parent

5. Genes on Chromosomes
• Sutton knew that alleles exist in pairs in an
organism and one alleles comes from the
female parent and the other from the male
parent
• Sutton realized that the paired alleles are
carried on paired chromosomes
• Chromosome theory of inheritance – genes
are carried from parents to offspring on
chromosomes

7. Meiosis
• Meiosis – the process by which the number of
chromosomes is reduced by half to form sex cells –
sperm and egg
• The chromosome pairs separate and are distributed
to two different cells
• Each of these 2 cells has half the number of
chromosomes, but these chromosomes still have 2
chromatids
• In the second part of Meiosis, these chromatids
separate and the cells divide resulting in 4 sex
cells, each with half the number of chromosomes of
the original cell

8. Meiosis• Meiosis
animation

9. Meiosis and Punnett squares
• Punnett squares show what occurs during
meiosis
• When chromosome pairs separate and go
into different sex cells, so do the alleles, too

10. A Lineup of Genes
• Human body cells have 23 pairs
of chromosomes or 46. Each
chromosome has many genes.
(around 35,000 total)
• Each gene controls a trait
• One chromosome pair is from
the female parent and one is
from the male parent