Genetics: Mendel & The Gene Idea

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….

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).

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)

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

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.

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

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

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.

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)

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