In each of these problems you examine a single phenotypic trait that is determined by 2 genes Each of the genes has a Gene Interaction and Epistasis dominant allele and a recessive allele In each problem you start with a set of three true breeding strains For example three plants with either red purple or white flowers In these problems two main goals are to determine the genotype of each of the 3 true breeding strains You will determine the genotype of each of the true breeding strains by a crossing pairs of the 3 given strains then b determine the genotype and phenotype of the 4th true breeding strain performing an intercross on the first generation F1 offspring in each cross for example F1 F2 ratio all ratio 3 1 Cross 1 red x purple red F1 Intercross red purple Review Traits Controlled by One Gene In the simplest case genetic traits are controlled by a single gene with two alleles and simple dominance The traits Mendel studied in peas are like this For example pea color was controlled by a dominant yellow allele and a recessive green allele In this situation there are four possible genotypes There are only two functional genotype categories 4 Total Genotypes Genotype Phenotype GG yellow Gg yellow gG yellow gg green since there are just two phenotypes 2 Functional Genotypes Genotype Phenotype G yellow gg green The three genotypes with at least one dominant allele are yellow all have yellow peas We use the abbreviation G this functional genotype category This notation represents all the genotypes with at least one dominant G allele the indicates it doesn t matter if the second allele is dominant or recessive True breeding genotypes Two of these genotypes are true breeding homozygous GG yellow and gg green If we cross these true breeding strains all the first generation F1 offspring will inherit a dominant allele from the 1st parent and a recessive allele from the other Yellow GG x Green gg F1 100 Yellow Gg If we perform an intercross on these first generation offspring of the offspring will fall in the G category and will fall in the gg category so we sill see yellow and green offspring in the ratio 3 to 1 Yellow Gg x Yellow Gg Phenotype Ratio Genotype Yellow 3 G Green 1 gg Traits Controlled By Two Genes As Mendel showed when there are two genes with two alleles of each there are 16 possible genotypes If each gene controls a different phenotype there are four different phenotype categories For instance yellow peas and smooth skin yellow peas and wrinkled skin green peas and smooth skin green peas and wrinkled skin and the 16 genotypes fall into 4 functional genotype classes At least one dominant of both genes A least one dominant allele of A but not B A least one dominant allele of B but not A Genotypes No dominant alleles of either gene aabb AABB AaBB aABB AABb AaBb aABb AAbB AabB aAbB 9 A B aaBB aaBb aabB 3 A bb Ratio Summary Phenotype yellow smooth yellow wrinkled green smooth green wrinkled In this situation there are four doubly homozygous true breeding genotypes True breeding AABB aaBB AAbb aabb 1 aabb When two genes control a single phenotypic trait we have the same four functional genotype classes and can have as many as four phenotypes Two Genes and Two Traits Genotype Phenotype A B yellow smooth aaB yellow wrinkled A bb green smooth aabb green wrinkled Two Genes and One Trait Genotype Phenotype A B red flowers aaB orange flowers A bb yellow flowers aabb white flowers Epistasis But different true breeding genotypes can have the same phenotype and the four true breeding strains may only have 3 different phenotypes or even 2 different phenotypes as described below AAbb Aabb aAbb 3 aaB Crossing Strains That are True Breeding for Two Genes One Gene Segregating If we cross these two true breeding genotypes Each offspring will inherit a dominant A allele from both parents a dominant B allele from the first parent and a recessive b allele from the second parent In this cross we say one gene segregated B When we intercross these F1 offspring We will obtain four F2 offspring genotypes All strains will have 2 dominant A alleles Three strains will have at least one at least one dominant B allele will have the same phenotype The fourth strain with two recessive bb alleles will generally have a different phenotype Since the four genotypes have equal frequencies there will be a 3 1 ratio in the F2 offspring frequencies with the two phenotypes Two Genes Segregating If we cross these two true breeding genotypes Each offspring will inherit a dominant A allele recessive b allele from the first parent and a recessive a allele dominant B allele from the second parent In this cross we say two genes segregated both A and B When we intercross these F1 offspring This is like Mendel s di hybrid cross and we will obtain 16 genotypes as Mendel did 9 will have at least one dominant allele of both genes 3 will have at least one dominant A allele and recessive bb alleles 3 will have recessive aa alleles and at least one dominant B allele 1 will have recessive aa alleles and recessive bb alleles If there is no epistasis we will observe four phenotypes in the familiar 9 3 3 1 ratios AABB x AAbb AABb AABb x AABb AABB AABb AAbB AAbb AAbb x aaBB AaBb AaBb x AaBb AABB AaBB aABB AABb AaBb AabB aABB aABb aAbB AAbb Aabb aAbb aaBB aaBb aabB aabb Pathways that result in Genetic Epistasis Two genes can act together in many ways to create a phenotype We use flower pigmentation as a phenotype examples to discuss these alternative pathways A Two Genes have the Same Basic Effect A1 Either gene alone yields the full phenotypic effect A dominant allele of either gene yields full yellow pigment A2 Either gene alone yields a partial phenotypic effect together they yield an enhanced effect A dominant allele of either gene yields lavender pigment and a dominant allele of both genes yields darker purple pigment aabb aabb AAbb Aabb aAbb aaBB aaBb aabB AAbb Aabb aAbb aaBB aaBb aabB 1 3 3 aaB A bb 9 A B 9 A B 9 genotypes yield purple flowers 6 yield lavender 1 yields white aabb 15 genotypes yield yellow flowers 1 genotype yields white B Two Genes work in succession on a single path B2 A dominant allele of 1 gene creates a phenotype A dominant B1 A dominant allele of 1 gene creates an intermediate product allele of the other acts on this product to create a phenotype A dominant allele of the other creates the new phenotype A dominant allele of A creates an intermediate product and a A dominant allele of A creates a product