Cell division and inheritance allows organisms to grow and pass genetic information between generations. During cell division, DNA is replicated and divided between new cells so they have the same characteristics. There are two types of cell division: mitosis and meiosis. Mitosis produces identical cells while meiosis produces gametes with half the number of chromosomes. Genes determine traits and alleles are different forms of genes. Dominant alleles show up in offspring while recessive alleles only show if an organism is homozygous recessive. Genetic crosses using Punnett squares can predict offspring genotypes and phenotypes from parent genotypes.

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.
