For IGCSE O-Level Biology students Chapter 15

1. CELL DIVISION &
INHERITANCE

2. Cell division
 For an organism to grow, cells must multiply.
 To obtain new cells with the same characteristics all
the information in the old cell must pass to the new
cells.
 Information is stored in the nucleus in a long
molecule called the DNA.
 Before cell division a copy of the DNA is made; this
is called REPLICATION.

3. Chromosome: part of the DNA, containing
genes, seen during cell division.

5. Types of cell division
 Takes place in all body cells
except ovaries & testis
 Produce 2 daughter cells
 Daughter cells have full set
of chromosomes (diploid)
 One division only
 No crossing over
 Parent and daughter cells
are identical
 Takes place only in ovaries
& testis
 Produce 4 daughter cells
 Daughter cells have half set
of chromosomes (haploid)
 2 consecutive divisions
 Crossing over takes place
 Parent and daughter cells
are non-identical.
mitosis meiosis

6. Gene: a part of a chromosome that
codes for a certain character.
 A gene is a segment of DNA carrying a recipe for
making a certain molecule.
 An allele: is a one of different forms of a gene.
 Example there are 2 different genes for petal colour
in a certain flower, one for white one for red.
 Each chromosome holds one allele.
 If both alleles are similar = homozygous
 If alleles are different = heterozygous

7.  Some alleles are stronger than others, they are dominant,
we give them a capital letter e.g. R for red petals or T for
tall plants.
 Weaker alleles are given a small letter r for white petals
or t for short plants.
 Dominant allele: an allele that shows its character in a
homozygous or heterozygous pairs.
 Recessive allele: an allele that shows its character only in
homozygous pairs.
 If a plant has one of these pairs of alleles RR, Rr, rr, what
petal colours will it have?
 RR: Rr: rr:

8. Predicting genotypes
 Mendel when studying
his pea plants observed
that 75% of the plants
were tall while 25%
were short.
 If a plant was short
which pair of alleles
could it have?
 If a plant was tall which
pair of alleles could it
have?

9. Human earlobe
 Humans have either free or fixed earlobes.
 Count number of free and fixed earlobes in class, which
character is dominant?
 Choose a letter to represent the dominant allele=
 Choose a letter to represent the recessive allele=
 Which pair of alleles do you think you have?
 Genotype: the type of alleles that an organism has.
 Phenotype: the observed character that is influenced by
the genes.
Free earlobes Fixed earlobes

10. Genetic diagrams
 Parent Phenotype
 Parent Genotype
 Gametes
 Offspring genotype
 Offspring phenotype
 Ratio

11. Punnet square
r
r
R R
Rr Rr
Rr Rr
Parent genotype
Parent genotype
RR
rr

12. Inheritance of gender

Male
Female

What is the chance a pregnant lady gives birth to
a male child? Which parent is responsible for the
child being male or female?
X Y
X XX XY
X XX XY

13. Your homework!
 Try to find out your genotype for earlobe character.
You will need to observe your parents’ earlobes.
 Draw a Punnet square to show how you inherited
your free or fixed earlobe.
 How many of your brothers or sisters have free or
fixed earlobe? Does it match the ratio you got using
the Punnet square?

14. Codominance
 For some characteristics alleles are neither dominant
or recessive; they are codominant; which means
both alleles show their effect on the organism.
 This results in an organism having a characteristic in
between. For example if alleles for red petal colour
and white petal colour were codominant, a
heterozygous plant will have …………… petals.
