1. Genetics and Inheritance
Alleles and Appearance

2. Alleles and Appearance
Haploid gametes
Diploid zygote

3. Alleles and Appearance
Remember:
A gene is part of a chromosome.
Each inherited characteristic is controlled by 2 copies of its gene.
The 2 copies of the gene that determine a particular characteristic may be the same or different.
Different forms of the same gene are called alleles.

4. Alleles and Appearance
Remember that each gene occupies a specific site on a chromosome.
Consider the gene for height in pea plants.
Allele for ‘tall’
Allele for ‘dwarf’
What will the appearance of the pea plant be?

5. Alleles and Appearance Note
Alleles are described as dominant or recessive.
A dominant allele always shows up in an organisms appearance (masks the recessive allele).
A recessive allele only shows up if there are two of them.

6. Alleles and Appearance
Alleles are symbolised using letters
Dominant alleles are given capital letters, while recessive alleles are given the same letter but in lower case.
For example, in pea plants the dominant allele for tallness has the symbol ‘T’ and the recessive allele for dwarfism has the symbol ‘t’.

7. Alleles and Appearance
Dwarf = Recessive (t)
Tall = Dominant (T)
t t
T t
T T
Tall
Dwarf
Tall

8. Alleles and Appearance Notes
Collect and complete the cut-out “Symbols for Alleles”, by choosing suitable symbols (letters) for the alleles.

9. Alleles and Appearance
The two descriptions of an organisms characteristics that you need to know are:
Genotype
and
Phenotype

10. Alleles and Appearance Notes
The genotype of an organism is the alleles it carries for a particular characteristic. Genotypes are usually written as letter symbols, e.g. Tt.

11. Alleles and Appearance Notes
The phenotype of an organism is the outward appearance of one of it’s characteristics and is usually described in words, e.g. tall.
Phenotype is dependent upon genotype.

12. Alleles and Appearance Notes
More vocabulary!
A homozygous organism has two identical alleles of a gene, e.g. TT
A heterozygous organism has two different alleles of a gene, e.g. Tt

13. Alleles and Appearance Notes
Collect and complete the cut-out “Genotypes and Phenotypes in Mice”
then
Collect and complete the cut-out “Predicting Phenotypes”. In fruit flies, the grey body allele (G) is dominant to the black body allele (g).

14. Alleles and Appearance
Think:
Can different genotypes give the same phenotype?
Can the same genotype give different phenotypes?

15. Genetics and Inheritance
The monohybrid cross

16. The Monohybrid Cross Notes
A monohybrid cross is a breeding experiment which follows the inheritance of just one characteristic, for example coat colour in mice.

17. The Monohybrid Cross Notes
Organisms which are true breeding pass on the same characteristics to their offspring over many generations.
This means that the offspring of true breeding black mice are always black and the offspring of true breeding white mice are always white.

18. The Monohybrid Cross Think!
Will the genotype of a true breeding organism be homozygous or heterozygous?

19. The Monohybrid Cross Let’s look at the example of mice.
In mice, black coat (B) is dominant to white coat (b).

20. The Monohybrid Cross P (parents) X F1 (first filial generation)
F2 (second filial generation)
Filial =
son / daughter

21. The Monohybrid Cross P X F1 X F2 BB BB Gametes all ‘B’ BB BB

22. The Monohybrid Cross
What would the genotype of true breeding white mice be?
What would be the genotype and phenotype of the F1 and F2 generation?

23. The Monohybrid Cross bb Parents x b All offspring (F1) bb bb x b
Gametes all b
All offspring (F1) bb bb x
Gametes all b
Second generation
(F2) bb

24. The Monohybrid Cross X P X F1 F2

25. The Monohybrid Cross
Another type of cross is when each parent is a true breeding individual, but each has a different phenotype.
What would happen if you crossed a true breeding black mouse with a true breeding white mouse?

26. The Monohybrid Cross P F1 F2 X X b b B B B b B b bb BB Bb Bb BB bb Bb

27. The Monohybrid Cross B b BB Bb bb
This can be shown in a punnet square:
Gametes B b BB Bb bb
Gametes

28. The Monohybrid Cross Notes
Practice crosses:
Pea plants can be tall (T) or dwarf (t).
If a true breeding tall plant was crossed with a true breeding tall plant what would the phenotype and genotyope of the F1 and F2 generations be?

29. The Monohybrid Cross Notes
Pea plants can be tall (T) or dwarf (t).
If a true breeding tall plant was crossed with a true breeding dwarf plant what would the phenotype and genotyope of the F1 and F2 generations be?

30. The Monohybrid Cross Notes
R = red flowers r = yellow flowers
A plant homozygous for red flowers is crossed with a plant that has yellow flowers.
Work through the cross to the F2 generation.
What is the phenotype and genotype of the F1?
What is the phenotype and genotype ratios of the F2?

31. The Monohybrid Cross Notes
R = red flowers r = yellow flowers
A plant heterozygous for red flowers is crossed with a plant that has yellow flowers.
Work through the cross to the F1 generation.
What is the phenotype and genotype ratios of the F1?

32. The Monohybrid Cross Notes
There is often a difference between the observed and predicted numbers of different types of offspring.
This is because fertilisation is a random process involving the element of chance.

33. The Monohybrid Cross How can we tell if an organism is true breeding?
If a mouse has a black coat, it’s genotype could be
BB – true breeding, or
Bb – not true breeding.

34. The Monohybrid Cross Notes
A test cross is used to identify the genotype of an individual showing a dominant characteristic by crossing it with a homozygous recessive individual.

35. The Monohybrid Cross
BB or Bb?

36. The Monohybrid Cross B b Bb All offspring: black bb
Ratio = Black : White
1 : 1 X BB

37. The Monohybrid Cross Notes
Now read Tp ‘Human Inheritance’ and complete the Activity on p

38. Genetics and Inheritance
Co-dominance

39. Co-dominance Notes
Co-dominance is a pattern of inheritance where both alleles are expressed equally.
Offspring from a cross between two true-breeding parents have a phenotype in between each parent.

40. Co-dominance – Think! In horses
B = black coat
W = white coat
What would be the phenotype of a horse with the following genotypes:
BB?
WW?
BW?

41. Co-dominance and Blood Group
Blood group is determined by three alleles:
A, B and O
A and B are co-dominant to one another
Both are completely dominant to O
There are four blood group phenotypes:
A, B, AB and O.
Work out on the board

42. Co-dominance Notes
A man with blood group A whose father was blood group O marries a woman with blood group AB.
Draw a punnet square to show all the possible genotypes that their children could have.
Which phenotype could not occur among their offspring?

43. Genetics and Inheritance
Polygenic Inheritance

44. Polygenic Inheritance
Variations between individuals fall into two categories:
discontinuous or continuous
Discontinuous characteristics fall into distinct categories – examples?
Continuous characteristics show a range of differences from one extreme to the other – examples?

45. Controlled by alleles of a single gene Single-gene inheritance
Discontinuous:
Controlled by alleles of a single gene
Single-gene inheritance
Blood Groups
O B A AB
Tongue rolling
Roller
Non-roller

46. Controlled by alleles of two or more genes Polygenic inheritance
Continuous:
Controlled by alleles of two or more genes
Polygenic inheritance
Skin Colour
Height

47. presented as a bar graph. Ranges of data can be
Discontinuous data
Continuous data
Pea seed shoot lengths (mm)
No. of seeds
1-5
6-10
11-15
16-20
21-25
No. of pupils
Tongue rolling ability
Roller
Non-roller
Distinct data can be
presented as a bar graph.
Ranges of data can be
presented as a histogram or
line graph.

48. Polygenic Inheritance Notes
A characteristic that is controlled by more than one gene and is expressed as a range of phenotypes is said to show polygenic inheritance.
Include an example!

49. Genetics and Inheritance
Organisms in genetic experiments

50. Organisms in genetic experiments
Qualities of an organism which make it suitable for genetic experiments are:
Short life cycle, grow quickly
Easy to breed, produce many offspring
Easy to observe contrasting characteristics

51. Drosophila
Drosophila (fruit flies) are suitable for carrying out genetic experiments as they
are very small
are easy to breed
produce large numbers of offspring
have a short life cycle
have many different phenotypes, e.g. body colour
are easy to distinguish, e.g. males have rounded abdomens, females have pointed.
Complete the RISE cutout

52. Using Drosophila Notes
The steps involved in setting up a cross to study the colour of the abdomen in Drosophila flies
Steps
Notes
1. Parents are chosen, one with a grey body and the other with an ebony body.
Parents must be true breeding to ensure dominant alleles are not hiding recessive alleles.

53. Using Drosophila Notes
Steps
Notes
2. Breed the parents.
This produces the F1 generation.

54. Using Drosophila Notes
Steps
Notes
3. The F1 phenotype is observed.
This identifies the dominant allele.

55. Using Drosophila Notes
Steps
Notes
4. The F1 generation is self-crossed.
This produces the F2 generation.

56. Using Drosophila Notes
Steps
Notes
5. The F2 generation is examined and the number of each phenotype counted.
This allows the phenotype ratio to be calculated.

57. Using Drosophila Notes
Steps
Notes
6. The experiment is repeated.
This improves the reliability of the results.