1. Learning Intentions
I understand genetic terminology and can identify vital information for a monohybrid cross.
I can identify if parents are true breeding or homozygous by carrying out a back cross.
I can carry out a monohybrid cross to the F2 generation and state the genotype, and phenotypic ratio produced.
I can state why the observed and predicted results of inheritance are different.
Success Criteria
I can use the layout effectively to show a step-by-step account of inheritance.
I can confidently determine homozygous and homozygous alleles.
I can highlight gametes by circling them.

2. True Breeding,
Dominant
and Recessive

3. The patterns of inheritance have been worked out using breeding experiments.
Breeding experiments involve the mating together or CROSSING of two organisms in order to study their offspring.
The next three slides shows three generations of white and three generations of black mice.

4. P F1 F2 Represents the parents Represents the first generation
Represents the second generation

5. Crossing True-breeding White MiceX P
parents X F1
first generation F2
second generation

6. Crossing True-breeding Black MiceX P
parents X F1
first generation F2
second generation

7. Generation
Example 1
Colours of Mice
(phenotypes)
Example 2 P F1 F2

8. The offspring of the white parents were all white.
When members of this generation were crossed with others from this generation then all of their offspring would also be white.
The offspring of the black parents were all black.
When members of this generation were crossed with others from this generation then all of their offspring would also be black.

9. When the same characteristic is passed repeatedly from generation to generation then we say the organism is.
True Breeding

10. An animal is not always crossed with another animal of the same phenotype.
Look at the next slide.

11. Crossing two different true breeding animalsX P
parents X F1
first generation F2
second generation

12. Black And White All Black Black And White Generation Phenotype(s)
Colour(s) P F1 F2
Black
And
White
All Black
Black
And
White

13. Black is said to be DOMINANT
Perhaps you would expect that when you cross a true-breeding white mouse with a true-breeding black mouse you would get something in between, eg. a GREY mouse.
However, all of the mice in the F1 generation are the same colour – black.
Black is said to be DOMINANT
White is said to be RECESSIVE

14. From this is clear that the black mice in the F1 generation are not true-breeding. Since the black mice in this generation produce both Black and White offspring.

15. The Monohybrid Cross
This involves looking at a single characteristic to see how it is passed from generation to generation.

16. A lot of important work in genetics has been done by studying inheritance in fruit flies.

17. These can be NORMAL or VESTIGIAL.
One characteristic which can be used to illustrate inheritance is wing type in Fruit Flies.
These can be NORMAL or VESTIGIAL.
Normal wings

18. Fruit fly with
normal wings
Fruit fly with
vestigial wings

19. N = normal wings n = vestigial wings
Wing type is controlled by a pair of genes.
Each gene can be one of two types or ALLELES.
N = normal wings
n = vestigial wings
Normal wing is dominant over vestigial wing.
Vestigial wing is said to be recessive.

20. There are three possible genotypes but only two possible phenotypes.
NN = normal wings
Nn = normal wings
nn = vestigial wings

21. If we cross true breeding normal winged flies with true breeding vestigial winged flies it is possible to work out the genotypes and phenotypes of the offspring.

22. NN X nn N n All Nn Genotype all Nn Phenotype all Normal winged
Parent (P)
Always circle the allele found in the gamete
N n
Gametes
All Nn
F1 generation
Genotype all Nn
Phenotype all Normal winged

23. It is more difficult to work out the results of a cross between members of the F1 generation but it can be done.

24. Nn X Nn N or n N or n n N N n NN Nn Nn nn F1 generation Gametes
We can work out the chances of each type of offspring using a punnett square. NN Nn Nn nn

25. NN Nn nn N n Genotypes NN , Nn and nn Phenotypes Normal and Vestigial
F2 Generation
Normal
Normal
Normal
Vestigial
Genotypes NN , Nn and nn
Phenotypes Normal and Vestigial
Phenotype Ratio 3:1

26. Observed ‘v’ Predicted Results
When a monohybrid cross is carried out the actual results that you obtain are not always exactly the results you predicted.
This is because fertilisation is a random process involving the element of chance
Producing large numbers of offspring gives more reliable results.

27. Brown eyes is dominant to blue eyes
Brown eyes is dominant to blue eyes. A brown eyed man (BB) marries a blue eyed woman (bb). What colour of eyes would you expect their children to have? Why?

28. A brown eyed man (Bb) marries a brown eyed woman (Bb) and have 4 children. How many should have brown eyes and how many blue eyes? Would all the brown eyed children have the same genes?

29. Tongue rolling is dominant and non-rolling is recessive
Tongue rolling is dominant and non-rolling is recessive. A tongue rolling woman (Tt) marries a non-rolling man (tt). What fraction of their children should be able to roll their tongues?

30. Two tongue rollers (Tt) get married
Two tongue rollers (Tt) get married. If they have 4 children how many should be able to roll their tongue and how many should not be able to roll their tongue?
Write this as a ratio -
? Rollers : ? Non-rollers

31. 5. In flies normal wings are dominant and short wings are recessive.
A normal winged fly (NN) is mated with a short winged fly.
What genes should their offspring inherit and what should they look like?
If two of these offspring are mated what fraction of their offspring would you expect to have normal wings and what fraction short wings? Write this as a ratio of -
? Normal wings : ? Short wings

32. Family Tree Diagrams

33. T = tongue rolling t = non tongue rolling Tongue Rolling
Tongue rolling is an inherited characteristic which is controlled by a single gene.
The tongue rolling allele is dominant over the non-tongue rolling allele.
T = tongue rolling
t = non tongue rolling

34. The pattern of inheritance can be traced using a family tree diagram, where……
Tongue rolling female
Non tongue rolling female
Tongue rolling male
Non tongue rolling male

35. This is demonstrated in the following example.
It is possible to work out the genotypes of individuals by knowing details a number of generations of a family.
This is demonstrated in the following example.

36. tt Tt tt Tt Tt tt tt Tt tt Family Y Family X ? tt TT tt Tt Tt Family Z
First identify the Homozygous recessive individuals.
Before we work out the genotypes of family Y we can work out family Z
These must be tt
We can now work out the genotype of the male parent
The tongue rollers in the F1 generation must be Tt
We can now work out the genotypes of the children in family X.
We cannot work out the the genotypes of the rest for certain.
The genotype of the last person is impossible to work out.
However, we can make an educated guess of the genotype of one of them. Tt tt Tt Tt tt tt Tt tt
Family Y
Family X ? tt TT tt Tt Tt
Family Z Tt