1. Gregor Mendel's Genetics
Unit 4 - Genetics

2. Gregor Mendel
Austrian monk who founded genetics by experimenting with garden peas
“Father of Genetics”
Grew & bred, counted &observed over 28,000 pea plants
Pea plants can cross-pollinate (2 parents) or self-pollinate (1 parent)
Used true breeders (homozygous dominant – purebred) for one trait and cross-pollinated

3. Principles of Dominance
DOMINANT – Observed Trait of an organism that Masks the Recessive form of the Trait (T, R, S, G, V)
RECESSIVE – Trait of an organism that can be Masked by the Dominant form of a Trait (t, r, s, g, v)

4. Principles of Dominance
Parental Generation – crossing of true breeders
F1 Generation – 1st Offspring Produced
F2 Generation – Offspring of F1 Generation

5. Law of independent Assortment – genes from different traits are
Mendel's Laws
Law of independent Assortment – genes from different traits are
inherited independently of each other
Law of Segregation – Each plant has two different alleles and it
produces two different types of gametes.

6. Punnett Square
Punnett Square – Used to express the possible combinations (probability) for a certain trait that an offspring may inherit from the parents
Monohybrid Cross – cross between
individuals that involves one pair of
contrasting traits
Test Cross – Individual of an Unknown genotype is crossed with a homozygous recessive individual whose phenotype is dominant

7. Important Terms
GENOTYPE: LETTERS – Represent the Alleles of the Trait (TT, Tt, GG, Gg, gg, tt)
PHENOTYPE: PHYSICAL Expression of the Trait (pink, blue, tall, short, curly, straight, etc.)
HOMOZYGOUS: Having TWO IDENTICAL ALLELES for a single trait (TT, tt, GG, gg)
-- Homo Dominant or Recessive
HETEROZYGOUS: Having TWO DIFFERENT ALLELES for a single trait (Tt, Gg, Rr, Ss)

8. Models of Inheritance
Honors pp. 176 &
CP pp

9. Complete Dominance
1. Complete Dominance: One allele is completely dominant over another Heterozygous plants (Tt & TT are indistinguishable) and dominant homozygous plants are indistinguishable.
i. Ex] both pea plants PP and Pp for flower color have purple flowers.

10. Incomplete Dominance
2. Incomplete/Intermediate Dominance an inheritance relationship that occurs when both alleles influence the phenotype. Neither allele of the pair is dominant, but combine and display a new trait.
i. Blending of Traits
ii. Usually results from an inactive or less active gene so the heterozygous phenotype appears intermediate.

11. Incomplete Dominance
1. All offspring in F1 generation show phenotype that is blending both parents.
2. If F1 generation is self-pollinated – the ratio of offspring is in predictable pattern. 1:2:1 – One parent, 2 both parents or blended, 1 other parent.

12. Incomplete Dominance iii. Ex] P1 generation: RR X R’R’
purebred (red) (white)
cross pollinated
F1 generation: RR’ RR’ RR’ RR’
(all pink)
self pollination
F2 generation: RR RR’ RR’ R’R’
1 : : 1
(red) (pink) (white)

13. iv. What if you crossed pink & pink????
Incomplete Dominance
iv. What if you crossed pink & pink???? R R’
RR (red)
RR’ (pink)
R’R’ (white)

14. Codominance
3.) Codominance: Neither of two alleles of the same gene totally masks the other.
i. Both traits appear in F1 generation and contribute to the phenotype of the off spring.
ii. Expression of both traits.
Iii. Two different proteins are produced and both are detected.

15. Codominance Ex] Roan cows and AB blood type
Ex] horse – trait for red hair is codominant with trait for white hair. Horse appears pinkish-brown far away, but if you look closely the coat is of both solid red and solid white hairs. This is a unique color.
More Examples: Erminette Chickens,
Appaloosa Horses, & Roan Horses

16. Codominance
QUESTION??? How are incomplete dominance and codominance different?
- When one allele is incompletely dominant over another, the blended result occurs because neither allele is fully expressed. This is why the F1 generation is a totally different color (pink).
- Two alleles are codominant, both alleles are completely expressed. The result is the combination of the two, NOT blending!

17. Give Examples Showing the Difference between the Three Inheritance Patterns
Black & White Chickens – Black Dominant & White Recessive
Give Genotype & Phenotypes
Complete Dominance:
BB –
bb –
Bb –
Incomplete Dominance:
B’B’ –
BB’ –
Codominance:
WW –
BW –

18. Models of Inheritance (continued)
4.) Multiple Alleles: controlled by three or more alleles of the same gene that code for a single trait.
i. Ex] ABO blood is controlled by three alleles IA, IB, and i
1. Each individual blood type consists of of these alleles
2. Alleles IA and IB are codominant and both are dominant to i allele

19. BLOOD TYPES B blood IBIB or IBi AB blood (codominant) IAIB
PHENOYTPE GENOTYPES
A blood IAIA or IAi
B blood IBIB or IBi
AB blood (codominant) IAIB
O blood (universal donor) ii

20. Models of Inheritance (continued)
5.) Polygenic Traits: trait controlled by multiple genes. Genes may be on the same or different chromosomes.
i. Identified by presence of a wide range of phenotypes
ii. Ex] skin color, hair color, eye color, height
iii. Influenced by environment. Ex] height is controlled by unknown number of genes that play a role in determining the growth of the skeleton. Height is influenced by nutrition and disease.

21. Models of Inheritance (continued)
6.) Sex-Linked Traits: traits controlled by genes located on sex chromosomes. Influenced by the presence of male and female sex hormones.
i. Sex Chromosome - responsible for determining the sex of an organism. Carry genes also for other traits. They are the 23rd pair of chromosomes and are sometimes called X and Y chromosomes.
ii. Males have genotype XY and female have genotype XX. Father determines gender of child.

22. Models of Inheritance (continued)
iii. Ex] color blindness and hemophilia
iv. Genes on X chromosome are X-linked genes and genes found on Y chromosome are Y-linked genes.

23. Models of Inheritance (continued)
v. If a recessive trait, like color blindness, is located on the X chromosome it is not very likely that females will have the phenotype. It is more likely that males have the condition since they only have one X chromosome. Males do not have another X chromosome or a duplicate copy of the gene.

24. Models of Inheritance (continued)
vi. A female that has a recessive gene on one X chromosome is a carrier for that trait. Y linked traits are passed only from a male to male offspring because the genes for these traits are on the Y chromosome
vii. Males are more likely to express a sex linked trait.
Models of Inheritance (continued)

25. Models of Inheritance (continued)
viii. Diagram: (female is heterozygous and a carrier. She contributes to the color blindness of her sons)