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 = tall

– t = 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 true-breeding 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 white-flowered pea plant.

• You will be provided with two beakers containing 100 lilac beads and 100 white beads – These are to represent heterozygous lilac-

flowered 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

LL Ll ll ratio

4

10

25

50

100

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 Blood Group

Phenotype

IA IA A

IA Io A

IB IB B

IBIo B

IA IB AB

Io Io 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

Pale coloured, speckled peppered moth

Peppered moth “A modern example of evolution”

Dark coloured peppered moth

Peppered moth “A modern example of evolution”

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

Number of discs

replaced

Percentage of discs in

each colour

white newspaper white newspaper white newspaper

At start

1st pick

2nd pick

3rd pick

4th pick

5th pick

Trial one – on news paper

Number of discs

removed

Number of discs

replaced

Percentage of discs in

each colour

white newspaper white newspaper white newspaper

At start

1st pick

2nd pick

3rd pick

4th pick

5th pick