1. Gregor and his peas

2. Early theories suggested that offspring were a blend of their parents factors (called the “Blend Theory”). This could not explain the appearance of recessive traits from one generation to the next.

3. A monk by the name of Gregor Mendel suspected that heredity depended on contributions from both parents and that specific characteristics from each parent were passed on to their offspring.

4. Mendel studied pea plants. He would breed these and then observed the offspring produced.

5. Mendel noticed that pea plants had a number of special characteristics He did experiments on how these characterisitics were passed on.

6. Mendel crossed a round seeded plant with a wrinkled seed plant. All the offspring had round seeds. He decided that round seed was dominant over wrinkled seed

7. Mendel then got the round seeded offspring and self crossed them. He found ¾ of the offspring had round seeds and ¼ had wrinkled seeds. How did he explain this?

8. Mendel decided that there were two types of genes for seed shape. They are called alleles. Each plant has two alleles for their seed shape. One allele comes from each parent.

9. From his experiments Mendel decided that round seed was dominant over wrinkled. Round seed = S Wrinkled seed = s

10. Plants had different combinations of these alleles SS Ss ss

11. If the plant has one dominant allele it shows that characterisitics. To show the recessive allele they have to have two recessive alleles.

12. SS Two dominat alleles = round Ss One dominant allele = round ss Two recessive alleles = wrinkled

13. Mendal started with true breeding pea plants. True breeding means that when self pollinated the plant only produces one type of young. He started off with a true breeding round seed and crossed it with a true breeding wrinkled seed.

14. Genotype. The actual alleles of the plant. Phenotype What the plant looks like.

15. Genotype: SS Phenotype: round Genotype: Ss Phenotype: round Genotype: ss Phenotype: wrinkled

17. All the offspring had round seeds. Mendel then wondered what would happen if he crossed the offspring with each other. Round seed is dominant over wrinkled seed.

18. Mendels results showed how it is possible for two “normal” parents to have recessive offspring.

20. Mendel was able to explain a number of traits in pea plants in this way.

21. Heterozygous Contains two different alleles. Homozygous Contains two alleles the same Dominant SS Recessive ss

22. You have a round seed plant. How do you tell if it is heterozygous (Ss) or homozygous (SS)? You do a Test cross. This is where you cross it with a homozygous recessive.

23. The punnet square SS s s Ss Homozygous – put in the genotype Put in the homozygous recessive genotype

24. From the punnet square we see that all the offspring are heterozygous and have round seeds. SS s s Ss

25. The punnet square Ss s s Ssss Ssss Heterozygous – put in the genotype Put in the homozygous recessive genotype

26. Colour Ratios From the punnet square we see that half the offspring are round and half are wrinkled. Ss s s Ssss Ss ss

27. By looking at the offspring from a test cross we can work out if the parent was homozygous or recessive. If any offspring show the recessive trait the parent was heterozygous. If all offspring do not show the recessive trait the parent was homozygous.