1. Genetics: The Science of Heredity
Chapter 3
Sections 1-2

2. Mendel’s Peas Gregor Mendel: Young priest Gardener
Studied math and Science in Vienna

3. Mendel’s Peas Curious why some plants had different characteristics.
Observed that traits were similar to parents
Traits: physical characteristics of an organism
Heredity: passing of traits from parents to offspring

4. Mendel’s Peas Peas are easy to study
Have many traits that show in 2 forms
Stems: tall or short
Produce large number of offspring in one generation
Self pollinating
Pollen of one plant lands on the pistil of the same plant.

5. Mendel’s Peas Mendel devised a way to “cross pollinate” plants.
Used a paint brush to take pollen from one plant and brush it on the pistil of another.

6. Mendel’s Experiment
Crossed plants with opposite traits
Started with purebreds
Short plants always produced short plants
Allowed peas with one trait to self pollinate for several generations (purebred)
Purebred: plant that always produces off spring with the same form of a trait as parent

7. Mendel’s Experiement First experiment
Crossed tall pea plants with short pea plants
P generation (parents)
Offspring
F1 Generation
(filial)

8. Mendel’s Experiment F1 Generation
All Tall
Crossed these plants- offspring F2 Generation
Mix of tall and short plants

9. Other Traits

10. Dominant and Recessive Alleles
Factors that control traits occur in pairs.
Female contributes one trait and the male the other
Individual alleles control inheritance
Some are dominant and others are recessive
Gene- factors that control traits
Alleles- different forms of a gene
Dominant alleles- traits that always show up when the allele is present
Recessive alleles- traits masked when recessive allele is present

11. Dominant and Recessive Alleles

12. Mendel’s Crosses Purebred plants have two identical alleles
Hybrid plant have two different alleles

13. Using Symbols in Genetics
Standard shorthand method
Capital letter represents dominant alleles
Lower case letter represents recessive alleles

14. Punnett Square Cross between two hybrids
Show possible outcomes of a genetic cross

16. Probability and Genetics
The likelihood that a particular event will occur
Principles of Probability
Coin toss
Heads and tails- 1 in 2 chances of either happening or 1:2 or ½ or 50%
Toss a coin 20 times
You would expect it to be heads or tails 10 times
Did that happen?

17. Mendel and Probability
The first to recognize that the principles of probability can be used to predict the results of genetic crosses.
He used Punnett Squares to show this
F1 was always tall- 100%
F2 was 75% tall and 25% short

18. Phenotype and Genotype
The physical appearance, or visible traits
Genetic makeup, or allele combinations

19. Codominance Alleles are neither dominant or recessive.
Both alleles are expressed in the offspring
See page 92-93