Testcross with Two Traits
A testcross of a homozygote (BB) results in offspring of all one phenotype, while a testcross of a heterozygote (Bb) gives a 1 : 1 phenotypic ratio, indicating that one pair of factors is segregating (see section on Monohybrid Crosses). A dihybrid testcross with a dihybrid (i.e., heterozygote; BbLl) gives 1 : 1 : 1 : 1 genotypic and phenotypic ratios, indicating that two pairs of factors are segregating and assorting independently. Testcrosses with individuals that are homozygous for one trait and heterozygous for the second trait give a 1 : 1 phenotypic ratio.
EXAMPLE 2.35 A testcross of a blackcoated, shorthaired individual with an incompletely known genotype (B – L –) is performed with an individual whose genotype is homozygous recessive at all of the loci under consideration (bbll).
Modified Dihybrid Ratios
The classical phenotypic ratio resulting from the mating of dihybrid genotypes is 9 : 3 : 3 : 1. This ratio appears whenever the alleles at both loci display complete dominant and recessive relationships. The classical dihybrid ratio may be modified if one or both loci have codominant alleles or lethal alleles. A summary of these modified phenotypic ratios in adult progeny is shown in Table 22.
Higher Combinations
The methods for solving twofactor (dihybrid) crosses may easily be extended to solve problems involving three (trihybrid) or more pairs of independently assorting autosomal factors. Given any number of heterozygous pairs of factors (n) in the F1, the following general formulas apply:
EXAMPLE 2.36 In the following cross, AaBb × AaBb, n = 2 (both loci are heterozygous). Then, 2^{n} or 4 equals the number of different gametes that are possible from each parent, 3^{n} or 9 equals the number of different genotypes possible, and 4^{n} or 16 equals the possible gametic combinations.
For a conventional trihybrid F2 cross (AaBbCc × AaBbCc), eight phenotypic classes result and the phenotypic ratio is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. The branch diagram below helps illustrate this point, again using the law of products to combine probabilities.
Practice problems for these concepts can be found at:
 1

2
Ask a Question
Have questions about this article or topic? AskRelated Questions
See More QuestionsPopular Articles
 Kindergarten Sight Words List
 First Grade Sight Words List
 10 Fun Activities for Children with Autism
 Signs Your Child Might Have Asperger's Syndrome
 Theories of Learning
 A Teacher's Guide to Differentiating Instruction
 Social Cognitive Theory
 Child Development Theories
 Curriculum Definition
 Why is Play Important? Social and Emotional Development, Physical Development, Creative Development