Exploring Mendelian Genetics

Law of Independent Assortment Does the segregation of one pair of alleles affect the segregation of another pair of alleles? –Mendel performed an experiment to follow two different genes from generation to generation in a Two-Factor Cross = Dihybrid Cross

Dihybrid Cross: F 1 Generation Mendel crossed true-breeding plants that produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy). All F 1 offspring from this cross had round yellow peas (RrYy) This cross produced hybrid plants for the next cross…

Dihybrid Cross Cross a Round, Yellow peas plant (RRYY) with plants that produce Wrinkled, Green peas (rryy). RRYY x rryy R = round seed r = wrinkled seed Y = yellow seed y = green seed RY ry RrYy All offspring produced have Round, Yellow peas. Round, Yellow

Dihybrid Cross: F 2 Generation –Mendel crossed two F 1 plants (RrYy) heterozygous for both seed shape and color –Using a Punnett square, we can see that alleles for seed shape segregate independently of those for seed color = Law of Independent Assortment – the inheritance of one character has no effect on the inheritance of another.

Dihybrid Cross Cross two plants that are heterozygous for round, yellow seeds (RrYy). RrYy x RrYy R = round seed r = wrinkled seed Y = yellow seed y = green seed RYRyrYry RY Ry rY ry RRYY RRYyRrYYRrYy RRYyRRyyRrYy Rryy RrYY RrYyrrYY rrYy RrYy RryyrrYyrryy Phenotype Ratio - Round, Yellow Round, Green Round, Yellow Round, Green Wrinkled, Yellow Wrinkled, Green 9 Round, Yellow 3 Round, Green 3 Wrinkled, Yellow 1 Wrinkled, Green 9 : 3 : 3 : 1 Ratio

Dihybrid Cross Genotype Ratio – 1 RRYY :2 RRYy : 2 RrYY : ….Ahhh! Punnett square shows a 9:3:3:1 phenotypic ratio – 9 yellow round : 3 yellow wrinkled : 3 green round: 1 green wrinkled

Blind dog Phenotypes (a) Possible phenotypes of Labrador retrievers B_N_ Blind dog Genotypes (b) A Labrador dihybrid cross Black coat, normal vision B_nn Black coat, blind (PRA) bbnn Chocolate coat, blind (PRA) bbN_ Chocolate coat, normal vision Mating of double heterozygotes (black coat, normal vision) BbNn Phenotypic ratio of offspring 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) Blind

Summary of Mendel’s Principles –Inheritance of biological information is determined by genes and there are different versions (alleles) –Each adult organism has two copies of each gene – one from each parent (could then be heterozygous or homozygous) –Some forms of a gene may be dominant or recessive –Alleles for different genes usually segregate independently of one another (law of segregation/law of independent assortment)