1. Mendelian Genetics
The term ‘Mendelian genetics’ typically relates to the outcomes of simple dominant and recessive gene pairings
Shows specific ratios or patterns of inheritance within a lineage of offspring generations (e.g., F1 and F2 generations)

2. Early ideas of heredity
Constancy of species – heredity occurs within the boundary of the species; not so prior to the Middle ages
(Ex: giraffe and minotaur)

3. View held thru time of Darwin
Direct transmission of traits – child is formed after hereditary material from all parts of parent’s body come together – blending occurs

4. Gregor Mendel (1822 – 1884) Studied garden peas
1st to use mathematics to examine outcomes of crosses
Large # of pea varieties with at least 7 easily distinguished traits
Peas are small, easy to grow, short generation time
Peas can self-fertilize; bisexual

5. Some definitions for tracking traits via Mendelian inheritance
Genotype/Phenotype
Gene/allele
Dominant/Recessive alleles
Homozygous/Heterozygous
P/F1/F2 generations
Genotypic ratio/Phenotypic ratio
Monohybrid cross/Dihybrid cross

6. Mendel conducted studies in 3 stages
Self-crossed flowers to make sure white/purple flowered plants were true-breeding
Crossed true-breeding plants (white X purple)
(X means “crossed with”)
3. Crossed F1 plants to see traits in future generation (F2 generation)

7. Mendel came to understand….
Plant progeny (offspring) did not show blending of traits
For each pair of alternative traits, 1 was not expressed in F1 generation, but re-appeared in F2 generation
Traits segregate among the progeny
Alt, traits are expressed in 3:1 ratio in F2

8. Punnett squares allow analysis using symbols for gametes and genotypes

9. Outcome of crossing true breeding purple-flowered and white-flowered pea plants
F1 progeny: All purple flowered
F2 progeny: 3 purple to 1 white
Self cross each of the F2’s

10. The Mendelian ratio Phenotypic ratio of 3:1 yet,
Genotypic ratio of 1:2:1
When crossing heterozygous individuals of trait controlled by simple dominant/recessive alleles

11. Mendel proposed a simple model of heredity – 5 parts:
Parents transmit “factors’ to offspring
Each individual receives 2 factors which code for the same trait
Not all factors are identical – alternative gene forms are called alleles
Alleles do not influence each other as alleles separate independently into gametes
The presence of an allele does not insure that its trait will be expressed

12. Monohybrid Crosses genotype: total set of alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
phenotype: outward appearance of an individual

13. Monohybrid Crosses
Principle of Segregation – Mendel’s first Law of Heredity
Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.

14. Dihybrid Crosses
Dihybrid cross: examination of 2 separate traits in a single cross
-for example: RR YY x rryy
The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

15. Dihybrid cross between two heterozygous parents
Instead of 4 possible outcomes, there are now 16!!

16. Dihybrid Crosses
Principle of Independent Assortment: Mendel’s 2nd Law.
In a dihybrid cross, the alleles of each gene assort independently.

17. Probability – Predicting Results
Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.
When crossing Pp x Pp, the probability of producing Pp offspring is
probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
¼ + ¼ = ½

18. Probability – Predicting Results
Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.
When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is:
probability of obtaiing rr = ¼
probability of obtaining yy = ¼
probability of rr yy = ¼ x ¼ = 1/16

19. Testcross
Testcross: a cross used to determine the genotype of an individual with dominant phenotype
-cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
-the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent