Genetics:

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)