Monohybrid Inheritance
Intermediate 2 Biology
Unit 2: Environmental Biology
and Genetics
Inherited Characteristics
• Inherited characteristics are passed on
from one generation to the next
through sexual reproduction
• Chromosomes are made up of genes
• Alleles are different forms of the same
gene
Glossary
• Genotype
– genes possessed by an organism
• Phenotype
– physical appearance of an organism
• Homozygous
– 2 identical alleles for a particular characteristic
• Heterozygous
– 2 different alleles for a particular characteristic
Glossary
• Dominant
– allele which controls the
development of a
characteristic when it is present
on only one chromosome.
• Recessive
– allele which control the
development of a
characteristic if present on
both chromosomes.
Further definitions
• P – Parents
• F1 – first generation
• F2 – second generation
Genetics and symbols
• The dominant allele is given a capital
letter
• The recessive allele is given a lower
case letter
• In plants, tall is dominant to dwarf
–T
–t
=
=
tall
dwarf
True-breeding
• An individual that is true-breeding has
two identical alleles for the
characteristic
• True breeding plants are homozygous
• In pea plants
– TT is true-breeding tall plant
– tt is a true breeding dwarf plant
Monohybrid Inheritance
• This is a genetic cross involving only
one gene (characteristic)
A cross between a true breeding tall pea
plant with a true-breeding dwarf pea plant.
P phenotype
P genotype
Gametes
F1 genotype
F1 phenotype
Members of the F1 generation
are self-pollinated
F1 phenotype
F1 genotype
Gametes
Fertilisation
F2 genotype ratio
F2 phenotype ratio
Time to practice
• In pea plants, round is dominant to
wrinkled seeds. A cross was carried
out between a pea plant homozygous
for round seeds and a pea plant with
wrinkled seeds.
• Carry out the cross through to the F2
generation
In pea plants, round is dominant to wrinkled seeds. A cross was
carried out between a pea plant homozygous for round seeds
and a pea plant with wrinkled seeds.
P phenotype
P genotype
Gametes
F1 genotype
F1 phenotype
Members of the F1 generation
are self-pollinated
F1 phenotype
F1 genotype
Gametes
Fertilisation
F2 genotype ratio
F2 phenotype ratio
Monohybrid Inheritance
practice questions
• Remember to state
symbols used at start
• Lay out the cross using
headings
P phenotype
P genotype
Gametes
F1 genotype
F1 phenotype
F1 self pollinated
F1 phenotype
F1 genotype
Gametes
Fertilisation
F2 genotype ratio
F2 phenotype ratio
Polygenic inheritance
• This is inheritance when the
characteristic is controlled by more
than one gene
– eye colour in humans
– Seed mass in pea plants
Predicting number of offspring
• A monohybrid cross between two truebreeding parents will always produce
a 3:1 phenotype ratio in the F2
generation.
• Often there is a difference between
observed and expected results due to
fertilisation being a random process
which involves the element of chance.
Using a bead model to illustrate
a monohybrid cross
• This model represents self-pollination of
the F1 generation of pea plants following
a cross between a true-breeding pea
plant with lilac flowers and a whiteflowered pea plant.
• You will be provided with two beakers
containing 100 lilac beads and 100 white
beads
– These are to represent heterozygous lilacflowered pea plants
Using a bead model to illustrate
a monohybrid cross
• Collect three beakers labelled LL, Ll
and ll
• Use the “gamete” beads to form and
classify “zygotes”
• Record your results as a tally mark for
the three genotypes
• Express your results as a F2 phenotypic
ratio.
Cross
number
4
10
25
50
100
LL
Ll
ll
ratio
Human Inheritance
• A family tree
can be used
to trace a
particular trait
through
several
generations.
Working out genotypes using a
family tree
• Let T = tongue rolling ability
• Let t = inability to roll tongue
Co-dominance
• Co-dominant alleles are both
expressed in the phenotype of a
heterozygous organism.
• E.g. Flower colour
– RR = red
– Rr = pink
– Rr = white
• This co-dominant cross would give a
1:2:1 phenotype ratio
Human Blood Groups
• There are three alleles for human
blood groups
– IA
– IB
– Io
– IA and IB are co-dominant
– Io is recessive
Human Blood Groups
genotype
IA IA
IA Io
IB IB
IBIo
IA IB
Io Io
Blood Group
Phenotype
A
A
B
B
AB
o
Environmental impact on
phenotype
• Phenotype is a result of interaction
between
– the genotype
– the effect of the environment on growth and
development.
• Some characters are unaffected by
environmental factors
– Tongue rolling ability
– ABO blood groups
Same genotype, different
phenotypes
• Identical twins in humans
– A twin fed a poorer diet will fail to reach full
potential height
• Clones in plants (asexual reproduction)
– Arrowhead plant (pg193)
• Differences caused by environmental
factors are not passed on to next
generation
evolution
• Two scientists, two theories
– Acquired characteristics – Lamark
– Natural Selection - Darwin
Natural selection
• Variation
• Over-production
– All species of organisms has a large
reproductive potential
• Struggle for survival
– The environment can not support all
offspring and this leads to competition
Survival of the fittest
• Offspring whose
phenotypes are
better adapted to
their environment
have a better
chance of surviving
– Pass on genes to
their offspring
• Offspring whose
phenotype are less
well-suited to
environment die
before reaching
reproductive age
– Fail to pass on genes
Charles Darwin
• Observation 1:
– Living things have the potential to produce
more offspring than are needed to replace
them.
• Observation 2:
– The number of organisms in the wild,
however, usually remains the same.
• Deduction:
– Darwin deduced that some organisms must
die before they reproduce.
• Observation 3:
– Most living things show variation. No two individuals
are usually alike.
• Deduction:
– Darwin concluded that the organisms which survived
were the best adapted or ‘fittest’
• Observation 4:
– Some types of variation are passed on from parents
to their offspring.
• Deduction:
– Populations will slowly change or ‘evolve’ as
successful parents pass on characteristics to their
offspring.
Natural Selection in Action
• Peppered Moth
– Two forms
• Light brown with speckles
• Melanic (dark) form
– Both forms flay at night and rest on tree
trunks during the day
Peppered moth
“A modern example of evolution”
Pale coloured, speckled peppered moth
Peppered moth
“A modern example of evolution”
Dark coloured peppered moth
Peppered moth
“A modern example of evolution”
Peppered moths on tree trunk
• In non-polluted areas, tree trunks are
covered with lichens, the light form is
well camouflaged against the
background.
• In polluted areas, toxic gases kill the
lichens and soot particles darken the
trunks. The melanic (dark) form is
favoured by natural selection.
Natural Selection in Action
Activity
• You will be given two different colours of punched paper
– Spread out 50 discs of each colour onto to the sheet of A3 paper
• Start the stopwatch/ stopclock and time one minute.
– During this time, use the forceps to pick out as many paper discs
as possible
– Place the paper discs in a Petri dish.
• Count how many paper discs of each colour you have
removed.
• If the total number is an odd number, pick out one more grain.
• Add more paper discs the paper to make it up to 100 again,
by adding equal numbers of each colour. Make a note of
how many grains there are of each colour now. The
proportion of the two colours will now be different.
• Time another minute and pick out as
many paper discs as possible.
• Repeat this 4 more times
• Repeat the whole activity on a sheet
of news paper
Trial one – on white paper
Number of discs
removed
white
newspaper
At start
1st pick
2nd pick
3rd pick
4th pick
5th pick
Number of discs
replaced
white
newspaper
Percentage of discs in
each colour
white
newspaper
Trial one – on news paper
Number of discs
removed
white
newspaper
At start
1st pick
2nd pick
3rd pick
4th pick
5th pick
Number of discs
replaced
white
newspaper
Percentage of discs in
each colour
white
newspaper