Fertilisation
M ale
Fem ale
Gam ete m other
cell with 2 sets of
chrom osom es
Gam ete form ation results
in gam etes each with one
set of chrom osom es
1 sperm m eets
an egg
S perm and egg
fuse (fertilisation)
A zygote is form ed.
This cell has 2 sets
of chrom osom es one
from the m ale sperm
and one from the
fem ale egg
Phenotype
Phenotype – the physical appearance of an organism caused by the genetic
information that it has inherited from its parents
Organism
Human
Hamster
Fruit fly
Pea plant
Inherited characteristic
Hair colour
Coat type
Eye colour
Height
Seed colour
Seed shape
2.
Phenotype possibilities
red, brown, blond, black
rough or smooth
red or white
tall or dwarf
green or yellow
round or wrinkled
Genes

Chromosomes are made up of a number of genes e.g.
Genes
Chromosome

Each gene is responsible for controlling an individual inherited characteristic

Genotype – The genes that make up an organism
Forms of a gene

Genes can exist in more than one form within a species
e.g. in hamsters the gene that controls coat colour can be in the form that
results in a white coat or the form that results in a black coat.

Parents of an organism produce gametes that contain one set of chromosomes
and therefore one form of each gene

An organism receives two sets of chromosomes one from the gamete of each
parent

Therefore an organism receives two forms of every gene, one from each parent.

These two forms of each gene can be identical or they can be different
depending on the parents

The different forms of a gene are called alleles
3.
Dominant and recessive forms

The two forms of a certain gene are either dominant or recessive
e.g. in hamsters the gene that controls coat colour - the white coat colour form
is dominant to the black coat colour form.

The dominant form of a gene will always show up in the organisms phenotype

The recessive form of a gene will only show up in the organisms phenotype if
both chromosomes contain the recessive form

Forms of a gene are normally represented using letters

The dominant form is represented using a capital letter

The recessive form is represented using a small letter
e.g. the coat colour gene in hamsters is represented using C (white) or c (black)
CC = hamster with two dominant forms of the coat colour gene. Hamster has a
white coat
Cc = hamster with one dominant form and one recessive form of the coat colour
gene. Hamster has a white coat
cc = hamster with two recessive forms of the coat colour gene. Hamster has a
black coat
Homozygous - Organism which possesses identical forms of a gene
Heterozygous - Organism which possesses different forms of a gene
4.
True breeding organisms
Cross – two organisms being bred together
An organism is said to be true breeding for a certain characteristic (e.g. coat
colour) if both of the forms of a gene that it possesses for that characteristic are
the same (e.g. CC or cc).
If two organisms of the same species are bred together and all of their offspring
are identical for a certain characteristic, then those organisms are said to be true
breeding for that characteristic.
e.g. two hamsters with white coats bred together
Parents (P)
CC
W hite coat
All white coat
CC
CC
CC
W hite coat
x
First
generation (F 1 )
Second
generation
(F 2 )
x
CC
CC
CC
CC
All white coat
The monohybrid cross
This is a particular type of experimental cross that is usually set up as follows:

two organisms of the same species

the phenotypes of the parents are different for one characteristic

the parents are both true breeding for the form of the characteristic that they
possess
5.
e.g. Cross between a hamster that is true breeding for white coat and a hamster
that is true breeding for a black coat
x
Parents (P)
CC
cc
W hite coat
Black coat
First
generation (F 1 )
Cc
All white coat
x
First generation
(F 1 ) crossed
Cc
Cc
W hite coat
Second
generation
(F 2 )
CC
W hite coat
W hite coat
Cc
W hite coat
Cc
W hite coat
cc
Black coat

The phenotype ratio of the F2 generation is 3 white coat: 1 black coat

This means that the white form of the coat colour gene is dominant to the black
form of the coat colour gene
Monohybrid crosses like this will always results in:

An F1 generation that are all the same

A phenotype ratio in the F2 generation of 3 : 1
The full working for this cross is shown on the next page
6.
Cross between a hamster that is true breeding for white coat (CC) and a hamster
that is true breeding for a black coat (Cc)
Parents (P) - phenotype
White
Parents (P) - genotype
CC
Parents (P) gametes
C
X
Black
cc
C
c
First generation (F1)
genotype
Cc
First generation (F1)
phenotype
All white
c
If the F1 hamsters are bred together the F2 generation can be predicted as follows:
F1 phenotype
White
F1 genotype
Cc
F1 gametes
C
X
White
Cc
c
C
These gametes
are put into a
punnet square to
work out all of
the possible
combinations
c
C
c
C
CC
Cc
c
Cc
cc
Second generation (F2)
genotypes
CC
1
Cc Cc
2
cc
1
Second generation (F2)
phenotypes
White
White
Black
Second generation (F2)
phenotype ratio
3 White : 1 Black
7.
The difference between observed and predicted figures
In a monohybrid cross like the one above, the number of the different types of
offspring has been predicted. Often, if the cross is physically carried out the
numbers of offspring of different phenotypes is different from the prediction.
This is because fertilisation is a random process involving an element of chance.
8.