1. Monohybrid Inheritance
Intermediate 2 Biology
Unit 2: Environmental Biology and Genetics

2. 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

3. Glossary Genotype Phenotype Homozygous Heterozygous
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

4. Glossary Dominant Recessive
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.

5. Further definitions P – Parents F1 – first generation
F2 – second generation

6. 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

7. 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

8. Monohybrid Inheritance
This is a genetic cross involving only one gene (characteristic)

9. P phenotype P genotype Gametes F1 genotype F1 phenotype
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

10. Members of the F1 generation are self-pollinated
F1 phenotype F1 genotype Gametes Fertilisation F2 genotype ratio F2 phenotype ratio

11. 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

12. P phenotype P genotype Gametes F1 genotype F1 phenotype
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

13. Members of the F1 generation are self-pollinated
F1 phenotype F1 genotype Gametes Fertilisation F2 genotype ratio F2 phenotype ratio

14. 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 Fertilisation F2 genotype ratio F2 phenotype ratio

15. Polygenic inheritance
This is inheritance when the characteristic is controlled by more than one gene
eye colour in humans
Seed mass in pea plants

16. 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.

17. 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

18. 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.

19. Cross number LL Ll ll
ratio 4 10 25 50
100

20. Human Inheritance
A family tree can be used to trace a particular trait through several generations.

21. Working out genotypes using a family tree
Let T = tongue rolling ability
Let t = inability to roll tongue

22. 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

23. Human Blood Groups There are three alleles for human blood groups IAIB Io
IA and IB are co-dominant
Io is recessive

24. Human Blood Groups genotype Blood Group Phenotype IA IA A IA Io IB IB
IBIo
IA IB AB
Io Io o

25. 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

26. 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

27. evolution Two scientists, two theories
Acquired characteristics – Lamark
Natural Selection - Darwin

28. Natural selection Variation Over-production Struggle for survival
All species of organisms has a large reproductive potential
Struggle for survival
The environment can not support all offspring and this leads to competition

29. 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

30. Charles Darwin Observation 1: Observation 2: Deduction:
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.

31. Observation 3: Deduction: Observation 4:
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.
Populations will slowly change or ‘evolve’ as successful parents pass on characteristics to their offspring.

32. 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

33. Peppered moth “A modern example of evolution”
Pale coloured, speckled peppered moth

34. Peppered moth “A modern example of evolution”
Dark coloured peppered moth

35. Peppered moth “A modern example of evolution”
Peppered moths on tree trunk

36. 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.

37. 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.

38. 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

39. Trial one – on white paper
Number of discs removed
Number of discs replaced
Percentage of discs in each colour
white
newspaper
At start
1st pick
2nd pick
3rd pick
4th pick
5th pick

40. Trial one – on news paper
Number of discs removed
Number of discs replaced
Percentage of discs in each colour
white
newspaper
At start
1st pick
2nd pick
3rd pick
4th pick
5th pick