Mendelian Inheritance Classical Transmission Genetics

Mendel & The Laws Of Inheritance Gregor Johann Mendel (1822-1884) Austrian monk Conducted studies from 1856-1864, Quantitative analysis

Mendel Used Pea Plants as Experimental Organism Hybridization mating (cross) btwn 2 individuals w/ different characteristics of a trait eg Purple-flowered X white-flowered Hybrids Offspring resulting from a mating Mendel observed no blending For each trait, offspring had characteristic of one or the other of the parents eg either purple or white flowers, not both, not light purple

Mendel Studied Seven Traits That Bred True A variety (phenotype) that always produces the same phenotype is termed true-breeding or pure-breeding Strain; Breed; Agrivar; Variety; homozygosity

Mendel’s Experiments Crossed two variants differing in only one trait a monohybrid cross

X x x

MENDEL’S MONOHYBRID CROSS DATA P Cross F1 generation F2 generation Ratio Tall X dwarf stem All tall 787 tall, 277 dwarf 2.84:1 Round X wrinkled seeds All round 5,474 round, 1,850 wrinkled 2.96:1 Yellow X Green seeds All yellow 6,022 yellow, 2,001 green 3.01:1 Gray X white seed coat All purple 705 gray, 224 white 3.15:1 Axial X terminal flowers All axial 651 axial, 207 terminal 3.14:1 Smooth X constricted pods All smooth 882 smooth, 229 constricted 2.95:1 Green X yellow pods All green 428 green, 152 yellow 2.82:1 F1 showed only one of the parental traits F2 showed ~ 3:1 ratio of both parental traits

Interpreting the Data Data suggested a particulate theory of inheritance Mendel postulated : There are 2 discrete hereditary factors for a trait, one from each parent ALLELES 2. Factors in parent may be identical or different HOMOZYGOUS or HETEROZYGOUS 3. When the two factors of a single trait are different One is dominant - effect can be seen The other is recessive - is masked

T t gametes Dwarf allele masked Tt x Tt

Monohybrid Crosses P (parental): (tall) x (short) - phenotypes T/T x t/t - genotypes F1 (1st filial): T/t (tall) F1 x F1: T/t x T/t (self or sib cross) F2 (2nd filial): 1 : 2 : 1 - genotypic ratio 3 : 1 - phenotypic ratio T/T T/t T/t t/t

Law of Segregation There are two alleles of each gene in an organism. Any given offspring receives only 1 allele of each gene Ergo - the two alleles of a locus are arranged into separate gametes during meiosis and each gamete receives only one or the other allele of any given gene. Each allele of a locus is placed individually into separate gametes during meiosis

Monohybrid test cross test cross cross to test whether dominant phenotype corresponds to homozygous dominant genotype or heterozygous genotype T/T vs T/t

Dominant phenotype F1 x recessive homozygote Monohybrid test cross Dominant phenotype F1 x recessive homozygote T/T x t/t or T/t x t/t or 1 T/t T/t & t/t 1 : 1

Law of Segregation Each allele of a locus will be placed into separate gametes during meiosis. This is true for diploid organisms only

Mendel’s Experiments Dihybrid crosses i.e. Crossing individuals that differ in two traits i.e. Trait 1 = Seed texture (round vs. wrinkled) Trait 2 = Seed color (yellow vs. green) 2 possible inheritance patterns

YR/YR Y/Y; R/R yr/yr y/y; r/r YR/yr Y/y; R/r YR/yr yr/yr YR/YR YRyr F2 generation yr/yr YR/yr YR/YR

DATA FROM ONE OF MENDEL’S DIHYBRID CROSSES P Cross F1 generation F2 generation Round, Yellow peas X wrinkled, green peas All round, yellow 315 round, yellow seeds 101 wrinkled, yellow seeds 108 round, green seeds 32 green, wrinkled seeds

Interpreting the Data F2 phenotype combinations not present in the parentals Round & Green Wrinkled & Yellow These phenotypes = nonparentals Results contradict linked assortment hypothesis support independent assortment hypothesis

Law of Independent Assortment If the genes, assort independently the predicted phenotypic ratio in the F2 generation would be 9:3:3:1 P Cross F1 generation F2 generation Ratio Round, Yellow X wrinkled, green All round, yellow 315 round, yellow 101 wrinkled, yellow 108 round, green 32 green, wrinkled 9.1 2.9 3.1 0.9 Mendel’s data very close to expectations Law of Independent assortment Either allele of one locus may go together with either allele of a second, unlinked locus

Dihybrid Testcross T/t; Y/y x t/t; y/y T/t; Y/y T/t; y/y t/t; Y/y If genes assort independently, the expected phenotypic ratio among the offspring is 1:1:1:1

Forked-line Method (fork diagram) Calculate predicted ratios of offspring by multiplying probabilities of independent events P yellow, round x green, wrinkled Figure: 03-06 Caption: Computation of the combined probabilities of each F2 phenotype for two independently inherited characters. The probability of each plant being yellow or green is independent of the probability of it being round or wrinkled.

Fork Diagram for Trihybrid Cross Figure: 03-09 Caption: Generation of the F2 trihybrid phenotypic ratio using the forked-line method.