1. Essential Question: How do organisms reproduce more organisms like themselves?
All organisms are made out of cells
The DNA inside of cells contains instructions to build proteins which determine the type of organism that will develop, how it will be built, and how it will function
DNA is passed down from parents to offspring during reproduction
In sexually reproducing organisms (ex. humans), the DNA comes from 2 parents and is passed down through special cells called gametes

2. NOTES 19 – Genetics: Mendel & Heredity

3. What is genetics?
Genetics – the study of heredity, which means, how traits are passed from parents to offspring

4. Trait – a characteristic of an organism that can be inherited (passed from parents to offspring)
Traits are controlled by genes
EX. Eye color in humans is a trait controlled by genes which are passed from parents to offspring

5. Alleles – Different versions of the same gene
EX. The eye color gene in humans has two alleles, blue and non-blue

6. Who was Gregor Mendel and why was he important?
Gregor Mendel – Mendel was a Czechoslovakian monk who is considered the “father of genetics” for figuring out the basic rules of how traits are inherited by doing experiments with pea plants back in the 1850’s

7. Mendel’s Experiments
Mendel did breeding experiments called test crosses
A test cross is when you choose 2 parents and use them to create offspring to see how a specific trait (or traits) is passed down

8. Mendel’s Famous Experiment
Mendel noticed that pea plants were either tall or short
Mendel noticed that some tall plants when crossed always produced tall offspring and that some short plants when crossed always produced short offspring
Mendel took a tall plant that always produced tall offspring and crossed it with a short plant that always produced short offspring

9. What happened?
All the offspring were tall – what happened to the short trait?
Mendel then took 2 of the tall offspring and crossed them
This time, 75% of the offspring were tall, but 25% of the offspring were short – the short trait reappeared
How can you get short plants from tall plants?

10. What Mendel Figured Out
The Rule of Unit Factors
Sexually reproducing organisms have 2 copies of each gene
One gene comes from the sperm cell and one gene comes from the egg cell
The genes can have different versions called alleles
EX. For eye color, a person can have 2 non-blue alleles, 1 non-blue allele and 1 blue allele, or 2 blue alleles

11. What Mendel Figured Out
The Rule of Dominance
When an organism has 2 different alleles (like 1 blue and 1 non-blue), the trait that shows up is said to be dominant
The allele that does not show up is said to be recessive

12. How can we predict what traits will appear in the offspring of parents?
Thanks to Mendel’s discoveries, we can predict what traits offspring are likely to have as long as we know the traits of the parents

13. Punnett Square
Punnett Square – a chart used to predict the traits of offspring
Capitol letters are used to represent dominant alleles
EX. T for tall allele in pea plants
Lower case letters are used to represent recessive alleles
EX. t for short allele in pea plants

14. Phenotype – The trait the organism shows
EX. tall pea plant or short pea plant
Genotype – The 2 alleles present in the organism for a trait
EX. T T or T t or t t

15. Homozygous – The 2 alleles for a trait are the same
EX. T T or t t
Heterozygous – The 2 alleles for a trait are different
EX. T t

16. Mendel’s Experiment Revisited
Here is another look at Mendel’s experiment using some new terms
Mendel examined the height gene in pea plants
The height gene in pea plants has 2 alleles, tall (T) and short (t)
The tall allele is dominant and the short allele is recessive

17. Mendel’s First Cross
Mendel crossed a homozygous tall plant with a homozygous short plant
T T x t t

18. Mendel crosses a homozygous tall plant (TT) with a homozygous short plant (tt)

19. The tall parent passes on either its first tall allele (T)…

20. …or its second tall allele (T) – either way, it always passes a tall allele

21. The short parent passes either its first short allele (t)…

22. …or its second short allele (t) – either way it always passes a short allele

23. Possible offspring 1 – the tall parent’s first tall allele with the short parent’s first short allele (T t)

24. Possible offspring 2 – the tall parent’s second tall allele with the short parent’s first short allele (T t)

25. Possible offspring 3 – the tall parent’s first tall allele with the short parent’s second short allele (T t)

26. Possible offspring 4 – the tall parent’s second tall allele with the short parent’s second short allele (T t)

27. Genotypic Percentage – 100% of the offspring are heterozygous (T t)

28. Phenotypic Percentage – 100% of the offspring are tall plants

29. Mendel’s First Cross
Each parent only passes 1 of its alleles, so the tall parent always passed on a T and the short parent always passed on a t
All the offspring had the genotype T t, because they received a T from the tall parent and a t from the short parent
Because the tall allele is dominant, all the offspring were tall

30. Mendel’s Second Cross
Mendel crossed two of the offspring from the first cross, which were both heterozygous
T t x T t

31. Mendel crosses a heterozygous tall plant (Tt) with a heterozygous tall plant (Tt)

32. The first tall parent passes on either a tall allele (T)…

33. …or a short allele (t)

34. The second tall parent passes on either a tall allele (T)…

35. …or a short allele (t)

36. Possible offspring 1 – the first tall parent’s tall allele with the second tall parent’s tall allele (T T)

37. Possible offspring 2 – the first tall parent’s short allele with the second tall parent’s tall allele (T t)

38. Possible offspring 3 – the first tall parent’s tall allele with the second tall parent’s short allele (T t)

39. Possible offspring 1 – the first tall parent’s short allele with the second tall parent’s short allele (t t)

40. Genotypic Percentage – 25% of the offspring are homozygous dominant (T T)

41. Genotypic Percentage – 50% of the offspring are heterozygous (T t)

42. Genotypic Percentage – 25% of the offspring are homozygous recessive (t t)

43. Phenotypic Percentage – 75% of the offspring are tall plants

44. Phenotypic Percentage – 25% of the offspring are short plants

45. Mendel’s Second Cross
Each parent only passes 1 of its alleles, but because each parent has 1 of each type of allele, they can pass either a T or a t
Some of the offspring received two T alleles, some received one T and one t, and some received two t alleles
The plants with genotype T T were tall, the plants with genotype T t were tall, and the plants with genotype t t were short