1. Patterns of Inheritance
Chapter 9

2. Gregor Mendel Austrian monk “Father of Modern Genetics”
Famous for his work with peas

3. Mendel’s Peas

4. Cross-Pollinating

5. General Background Vocabulary
Self-pollination: pollen from flower fertilizes the same plant
True-breeds: pure gene lines – offspring match parent due to self-fertilization)
Cross-pollination: pollen will fertilize a different plant
Hybrid: Offspring that result from a cross between organisms with different traits; express a combination of traits from
Character: heritable feature that varies among individuals (ex. seed color)
Trait: possible variations for a particular character (ex. yellow seeds vs. green seeds)

6. Mendel’s Crosses
Mendel noticed that one trait was always expressed over the other in the F1 offspring
However, trait that disappeared always reappeared in about 25% of the F2 offspring.

7. Mendel’s Initial Conclusion
Biological inheritance is determined by chemical factors passed from one generation to the next
Geneticists now refer to these heritable factors as genes
Genes can come in more than one form, each form is an allele ex. B or b (The “B” gene w/ 2 alleles)
Additional relevant terminology:
Homozygous: two identical alleles (AA or aa)
Heterozygous: two different alleles (Aa)
Phenotype: Physical appearance
Genotype: Genetic make-up
Homozygous dominant (AA)
Homozygous recessive (aa)
Heterozygous (Aa)

8. The Principles Dominance & Segregation
Dominance: Certain alleles will be expressed over others, the expressed alleles are dominant to the unexpressed recessive alleles
Segregation: Each parent carries two alleles for each gene. During meiosis, the pairs are separated to that only one allele is sent to the offspring in the gamete from each parent.

9. Genetics & Probability
Probability is the likelihood of an event happening
Consider flipping a coin
Likelihood of flipping heads = 50%
(1 of 2 possibilities)
Likelihood of flipping heads twice?
50% x 50% = 25%
To predict outcomes of genetic crosses we use punnett squares

10. The Test Cross

11. Exploring Mendelian Genetics
Does segregation of one set of alleles influence the segregation of another pair of alleles?
Mendel’s Two Factor (dihybrid) Crosses
Followed two traits at a time.
Same method as his original single-factor crosses
Cross-pollinated to produce the F1 and allowed them to self-pollinate
Mendel found that alleles for different traits did not influence each others segregation. This is referred to as the principle of independent assortment.

12. Pedigrees
Pedigrees are like genetic “family trees”. They are used to show the inheritance of traits within families and to predict genotypes and/phenotypes of certain individuals.
The following key explains the symbols and layout of a typical pedigree:

13. Autosomal Dominant Pedigree

14. Autosomal Recessive Pedigree