1. Inheritance
Lesson 3

2. Lesson 3-Learning Intention
True breeding
F1 offspring
F2 offspring

3. What we know so far..
An organism’s PHENOTYPE is the characteristic which is in the PHYSICAL APPEARANCE
Each characteristic is controlled by TWO GENES. Each copy of the gene is called an ALLELE.
The two genes which control the characteristic is called the GENOTYPE

4. What could the genotype of a true breeding organism be?
An organism is said to be TRUE BREEDING if, when crossed with another organism of the same strain, it always produces offspring of exactly the same kind
What could the genotype of a true breeding organism be?

5. True Breeding Organisms
Contain 2 genes which are the same for that characteristic- HOMOZYGOUS.
Homozygous - 2 identical alleles for any particular characteristic (2 identical forms of the same gene).

6. Example
The invisible woman has super strength- her genotype is HOMOZYGOUS DOMINANT (SS)
Superman is also HOMOZYGOUS DOMINANT for super strength (SS)
Both parents can only pass on the gene for super strength to their offspring (as that is all they have)
All offspring will possess the same genotype as their parents for super strength (SS)

7. Heterozygous organisms
Organisms that have two DIFFERENT alleles are heterozygous (one dominant and one recessive)
The dominant gene HIDES the recessive gene and so the dominant gene will appear in the PHENOTYPE (appearance)
The organism can still pass on the recessive allele to offspring

8. Homozygous and heterozygous: copy
If an organism possesses 2 identical forms of the same gene, the organism is said to be HOMOZYGOUS and is true breeding. If an organism possesses 2 different forms of a gene, it is said to be HETEROZYGOUS.

9. Monohybrid crosses Only one characteristic (MONO) is considered
Alleles represented by letters- capital for the dominant gene and small letter for recessive

10. what is the parent’s GENOTYPE?
Example
For example: - in peas, round seed is dominant to wrinkled seed. This can be represented in symbols:
Round = dominant allele = R
Wrinkled = recessive allele = r
2 true breeding parents- one has wrinkled seeds and one has dominant seeds
what is the parent’s GENOTYPE?

11. First generation offspring
Parents round seeds x wrinkled seeds (true breeding)
Genotype
Alleles
What would be the possible genotype of any offspring?

12. First generation offspring
Parents round seeds x wrinkled seeds (true breeding)
Genotype RR rr
Alleles
What would be the possible genotype of any offspring?
All F1 offspring would be round seeds Rr (heterozygous) R r R r

13. Second generation offspring
The seeds from the first generation offspring were self crossed
F1 self-cross round seeds x round seeds
Genotype
Gametes

14. Second generation offspring
The seeds from the first generation offspring were self crossed
F1 self-cross round seeds x round seeds
Genotype Rr Rr
Gametes
What would the possible genotypes be for these offspring? R r R r

15. Second generation offspring
The seeds from the first generation offspring were self crossed
F1 self-cross round seeds x round seeds
Genotype Rr Rr
Gametes
What would the possible genotypes be for these offspring?
RR : Rr : Rr : rr
Phenotype ratio : 3 round seeds : 1 wrinkled seed R r R r

16. Sex determination
The sex chromosomes determine the gender of an individual
Two types of sex chromosome- X and Y
Males have an X and a Y (XY)
Females have two Xs (XX)

17. Sex Cells Female sex cells (eggs) can only pass on an X chromosome
Male sex cells (sperm) can contain either an X or a Y
Fertilisation is a RANDOM process
The egg may be fertilised by an X or a Y sperm

18. Sex determination
If egg is fertilised by an X sperm- genotype is XX and zygote develops into a girl
If egg fertilised by a Y sperm- genotype XY and zygote develops into a boy

19. Sex Determination: copy
The sex chromosomes (which are inherited from the parents) determine the gender of an individual. There are 2 types of sex chromosome; X and Y. Males have an X and a Y chromosome (XY genotype) Females have two X chromosomes (XX genotype) Females can only pass on an X chromosome in their eggs. There is a 50/50 chance of the egg being fertilised by an X or Y sperm.

20. Past paper questions

21. Past paper questions

22. Past paper questions

23. Past paper questions

24. Past paper questions