TWO-FACTOR CROSSES INTRODUCTION Crosses that involve two traits, such as pea color and pea shape, are called two-factor crosses. Predicting the outcome of two-factor crosses requires basically the same procedure as one-factor crosses. Keep in mind though that during meiosis, chromosomes assort independently. This means that each of the chromosomes of any pair of homologous pair of chromosomes has an equal opportunity of ending up in a gamete with either chromosome from any other pair of homologous chromosomes. Since genes are located on chromosomes, genes also assort independently as shown below. EXAMPLE What are the genotype and phenotype ratios of the offspring resulting from a cross between two pea plants that are heterozygous for pod color and shape? step 1 Choose colors to represent the alleles. i.e. G for green and g for yellow, N for smooth and n for constricted step 2 Write the genotypes for the parents. i.e. GgNn x GgNn step 3 Determine the gametes that the parents can produce. ie. GN or Gn or gN or gn step 4 Enter the gamete possibilities on the sides of the Punnett square. step 5 Complete the Punnett square. GN GN GGNN Gn GGNn gN GgNN gn GgNn step 6 Gn GGNn GGnn GgNn Ggnn gN GgNN GgNn ggNN ggNn Determine the phenotypes i.e. 9 green smooth 3 green constricted step 7 Determine the genotypes i.e. 1 GGNN : 2 GGNn : 1 GGnn : 2 GgNN : 4 GgNn : 2 Ggnn : 1 ggNN : 2 ggNn : 1 ggnn gn GgNn Ggnn ggNn ggnn 3 yellow smooth 1 yellow constricted PROBLEMS 1. What would be the results of a cross between a pea plant that is both heterozygous green and heterozygous smooth and a pea plant that is yellow and homozygous smooth? green and smooth pea plant genotype: _____________________ yellow and smooth pea plant genotype: _____________________ offspring genotype ratio: ______________________________________________ ______________________________________________ offspring phenotype ratio: ______________________________________________ ______________________________________________ 2. In tomatoes, red fruit color (R) is dominant over yellow fruit color (r) and tall vines (T) are dominant over short vines (t). A variety called Yellow Giant has yellow fruit and tall vines. Another variety, Tiny Boy, bears red fruit on short vines. Could a breeder cross Yellow Giant and Tiny Boy tomatoes and obtain a variety that is short with yellow fruit? Yellow Giant genotype: _____________________________________ Tiny Boy genotype: _____________________________________ offspring genotype ratio: ______________________________________________ ______________________________________________ offspring phenotype ratio: ______________________________________________ ______________________________________________ 3. Suppose that on the newly discovered planet Metatheres there are creatures called prafs. Yellow praf eyes (Y) are dominant over pink (y). Webbed feet (D) are dominant over hoofed feet (d). Suppose a homozygous yellow-eyed, heterozygous web-footed praf mates with a pink-eyed, hoof-footed praf. Could they have a baby praf with yellow eyes and hoofed feet? ________ yellow-eyed, web-footed genotype: pink-eyed, hoof-footed genotype: offspring genotype ratio: _____________________ _____________________ ______________________________________________ ______________________________________________ offspring phenotype ratio: ______________________________________________ ______________________________________________ 4. In Japanese four o’clock flowers the alleles for red flower color (R) and white flower color (W) are codominant. Thus a heterozygous combination (RW) is white with red streaks. What are the results of a cross between a red, heterozygous tall (Tt) and streaked, short plant? red, tall plant genotype: streaked, short plant genotype: offspring genotype ratio: _____________________ _____________________ ______________________________________________ ______________________________________________ offspring phenotype ratio: ______________________________________________ ______________________________________________