1. Genetics:
Mendel & The Gene Idea

2. Mendelian genetics Character (heritable feature, i.e., fur color)
Trait (variant for a character, i.e., brown)
True-breed (all offspring of same variety)
Hybridization
(crossing of 2 different true-breds)
P generation (parents)
F1 generation (first filial generation)
F2- the next generation, etc….

3. Leading to the Law of Segregation
Alternative versions of genes (alleles) account for variations in inherited characteristics
For each character, an organism inherits 2 alleles, one from each parent
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
The alleles for each character segregate (separate) during gamete production (meiosis).

4. Genetic vocabulary…….
Homozygous: pair of identical alleles for a character.
Example: PP or pp
They are “true-breeding”
Heterozygous: two different alleles for a gene
Example- Pp
Phenotype: an organism’s traits
It describes what you “see”
Genotype: an organism’s genetic makeup (PP, Pp, or pp)
Testcross: breeding of a recessive homozygote X dominant phenotype (but unknown genotype)

5. The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters?
Monohybrid cross vs. dihybrid cross
The two pairs of alleles segregate independently of each other.
Mendel’s Law of Independent Assortment

6. Now you can do more predicting…
Dihybrid Cross- A mating between 2 parents that are both heterozygous for the same traits.
Ex. RrYy x RrYy
Always should give a 9:3:3:1 phenotypic ratio
In fact if we know the genotypes, we can use laws of probability:
Rule Of Multiplication- probability that independent events will occur simultaneously is a product of their individual probabilities
Rule Of Addition- probability of an event occurring in 2 or more independent ways is the sum of their individual probabilities.

7. Of course, Genetics is not purely Mendelian...
Incomplete dominance: results in a phenotype which is intermediate between the dominant and recessive trait.
Ex: pink snapdragons
Codominance: two alleles are equally expressed and affect the phenotype in separate, distinguishable ways.
Ex: Variegated flowers

8. but sometimes, even “Complete Dominance” really isn’t:
It’s like a continuum: example AA, Aa, aa
Complete
Dominance
Co-
dominance
Incomplete Dominance
“A” phenotype
and “a” phenotype
expressed equally
AA and
Aa have
same
phenotype
Aa shows
intermediate
phenotype

9. Example: Tay-Sacs Disease
Tay-sacs is a disease in which brain cells lack a lipid metabolizing enzyme.
It all depends on the level at which the phenotype is studied:
Organismal level- “complete dominance”
Heterozygote is symptom-free because half the number of functional enzymes is enough to do the job.
Biochemical Level- “Incomplete Dominance”
Lab Enzyme reaction rates are intermediate for carriers.
Molecular Level- “Codominance”
Both functional and dysfunctional versions of the enzyme are produced.

10. More non-mendalian….
Pleiotropy: ability of a single gene to have multiple phenotypic effects.
Ex: sickle-cell anemia & malaria resistance
Epistasis: a gene at one locus affects the phenotypic expression of a gene at a second locus.
Dihybrid cross deviates from 9:3:3:1
Ex: mice coat color
Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character
Ex: human skin pigment and height
Multiple alleles: more than 2 possible alleles for a gene.
Ex: human blood types
A, B, AB, O
(A&B codominant, both dominant to O)

11. Human disorders The family pedigree
Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell
Dominant disorders: •Huntington’s
Testing: •amniocentesis •chorionic villus sampling (CVS)