Genetics Part 2

• Forked Line Method


  When more than two genes are being considered, the Punnett-square method becomes unwieldy. For instance, examining a cross involving four genes would require a 16 x 16 grid containing 256 boxes. For more complex crosses, the forked-line and probability methods are preferred.
• Forked Line Method


  1. Create rows equal to the number of genes being considered

  2. Segregate the alleles in each row on forked lines according to the probabilities for individual monohybrid crosses

  3. Multiply the values along each forked path to obtain the F2 offspring probabilities

  
• For a trihybrid cross, the F2 phenotypic ratio is 27:9:9:9:3:3:3:1

  
• Forked Line Method for trihybrid cross

  1. Probability for color occupies the top row (3 yellow:1 green)

  2. Probability for shape occupies the second row (3 round:1 wrinkled)

  3. Probability for height occupies the third row (3 tall:1 dwarf)

  4. Probability for each possible combination of traits is calculated by multiplying the probability for each individual trait

  
• Probability Method


  A method that gives the proportions of offspring expected to exhibit each phenotype (or genotype) without the added visual assistance of the forked-line method. Both methods make use of the product rule and consider the alleles for each gene separately.
• For a tetrahybrid cross between individuals that are heterozygotes for all four genes, and in which all four genes are sorting independently and in a dominant and recessive pattern, 1/256 of the offspring will be quadruply homozygous recessive.

  
• Probability Method for tetrahybrid cross

  1. Calculate the probability of a homozygous dominant or heterozygous for each gene using the sum rule

  2. Multiply the probabilities for each gene using the product rule to get the overall probability
• The probability of a dominant phenotype at A and B and C and D is 27/64