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Transcript
Exploring Mendelian Genetics Two trait Crosses 11.3 Dihybrid crosses •All of the crosses discussed so far have involved only a single trait. •Ex. Tt •However, Mendel also conducted crosses that examine the inheritance of two different traits. •He wondered if both traits would always appear together or if they would be expresses independently of each other. Two-Factor Cross • Mendel crossed pure-breeding plants for two traits – Round, Yellow x wrinkled, green – RRYY x rryy • All F1 offspring produced were Round, Yellow – Yellow is dominant to Green – Round is dominant to wrinkled • Genotype of F1 offspring is RrYy • Mendel’s dihybrid crosses with heterozygous plants ( F1 generation) yielded a 9:3:3:1 phenotypic ratio for the F2 generation. • Mendel’s dihybrid crosses led to his second law, the law of independent assortment. A dihybrid cross involves two traits. • The law of independent assortment states that allele pairs separate independently of each other during meiosis. • Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. Dihybrid Cross RrYy x RrYy RY RY Ry rY ry Ry rY ry Dihybrid Cross RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy ry Genotype Ratio RRYY-1/16 RRYy-2/16 RRyy- 1/16 RrYY- 2/16 RrYy- 4/16 Rryy- 2/16 rrYY- 1/16 rrYy- 2/16 rryy- 1/16 Dihybrid Cross RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy wrinkled/Yellow: 3 RrYy Rryy rrYy rryy wrinkled/green: ry Phenotype Ratio Round/Yellow: 9 Round/green: 3 1 Summary of Mendel’s Principles • Principle of Heredity – Inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to offspring. • Principle of Dominance – In cases in which two or more alleles of the gene for a single trait exist, some gene forms may be dominant and others recessive. Summary of Mendel’s Principles • Principle of Segregation – In most sexually reproducing organisms, each adult has two copies of each gene- one from each parent. These genes are segregated from each other when gametes are formed. • Principle of Independent Assortment – The alleles for different genes usually segregate independently from one another.
