Mendelian Genetics Biology 30

Gregor Mendel Used pure-breeding (genetically identical) lines of garden peas Lived 1822-1884 Father of Modern Genetics Examined crosses involving seven different characters

Mendel’s Work

Mendel’s Laws Segregation a) Members of a gene pair segregate into separate gametes b) One-half of the gametes has one member, the other half, the other Independent assortment--during gamete formation, segregation in a gene pair is independent of other gene pairs

Monohybrid Crosses Cross between pure lines differing in a single character of interest What Mendel Observed The F1 were all Yellow However, Green Segregated out in F2 Strong evidence for discrete units of heredity , as "green" unit obviously present in F1, appears in F2 3:1 ratio of Yellow : Green in F2

Mendel also found that Parental, F1, and F2 Yellow peas behaved quite differently When examined each F2 yellow family separately, Mendel found 2/3 of the F2 yellows give 1/2 yellow, 1/2 green 1/3 of F2 yellows gave all yellow progeny

Mendel' s Explanation Genetic information exists as discrete units occurring in pairs YY is the genotype of the pure Yellow line yy the genotype of pure Green line Y dominant to y (y is recessive to Y) YY, Yy (denoted by Y-) = Yellow yy = green

heterozygous (hybrid) Baby Steps BABY STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) Symbol Genotype Name Phenotype RR homozygous (pure) dominant red thoat Rr heterozygous (hybrid) red throat rr homozygous (pure) recessive white throat Parent Pea Plants  ("P" Generation) Offspring  ("F1" Generation) Genotypes: Tt x tt Phenotypes: tall x short Genotypes: 50% (2/4)  Tt 50% (2/4) tt Phenotypes: 50% tall 50% short

Dominant and Recessive Genes - Examples Complete Dominance – the dominant allele masks or covers the recessive allele Homozygous Green pod x Yellow pod Homozygous Green pod x Heterozygous Green pod Heterozygous Purple Flowers x White Flowers Heterozygous Tall Plant x Dwarf Plant Homozygous Inflated pod x Wrinkled Pod Cross between two pods from above (F2=F1xF1)

Test Cross Preformed to determine whether a dominant phenotype is homozygous dominant (pure bred) or heterozygous (hybrid) The cross is always done between a individual with a dominant phenotype and an individual that is homozygous recessive Results if the individual in question is homozygous dominant All offspring will be dominant phenotype Results if the individual in question is heterozygous Offspring will be half dominant and half recessive phenotype A? x aa

Dihybrid Crosses Examples: Crossing pure-breeding lines differing in two characters Examples: Wrinkled Homozygous Yellow Seed x Homozygous Round Green Seed Heterozygous Tall Plant with Homozygous Axial Flowers x Dwarf Plant with terminal Flowers Pure Breed Round Green Seed x Pure Breed Wrinkled Yellow Seed (F1 and F2) – Know this ratio

9:3:3:1 Ratio When crossing two opposing pure breed plants the F1 generation is always heterozygous and the F2 is always 9:3:3:1.

Levels of Dominance When two alleles are equally dominant, they interact to produce new phenotypes Incomplete Dominance – blending of the two traits Co-Dominance– both traits will be shown in the offspring

Examples of Levels of Dominance Example of Incomplete Dominance Four-o’clock plants P1 CRCR (red flowers) x CWCW (white flowers) F1 CRCW (all pink flowers) Example of Codominance Cattle P1 HRHR (red cow) x HWHW (white bull) F1 HRHW (roan calves)

Multiple Alleles When more than two different alleles control a trait The ABO blood group is a nice example, three alleles IA, IB are codominant, IA and IB have dominance over i

Lethal Alleles Manx gene in cats Mm: tail-less mm: normal MM: dead

Multiple Gene Interaction Black Labrador B- E - Brown (or Chocolate) Labrador bb E - Golden Labrador -- ee Individual here most likely is B- ee as black pigment on lips and nose Coat color in Labrador retrievers BB, Bb = Black bb = Brown (Chocolate) Both color genes require E- genotype for pigment deposition.

Using Probability Rule of independent Events The Product Rule Chance has no memory, each event or occurrence must be regarded as an individual event The Product Rule The probability of independent events occurring simultaneously is equal to the product of these event occurring independently