Bio Review Notes #23
GENETICS: TWO-GENE CROSSES
Performance Objectives:
Genes located on different chromosomes separate independently of another (Mendel's law of independent assortment). Mendel showed that this principle explained his observed results.

Mendel's 2-gene (dihybrid) crosses:   Parents chosen from pure lines were homozygous for two traits at once. AABB x aabb gave the same results as AAbb x aaBB. First generation offspring (F1) were heterozygous for all traits (AaBb) and showed all dominant phenotypes. F2 showed a 9:3:3:1 ratio of phenotypes.

Mendel's explanation — the law of independent assortment:   Alleles A and a separate independently of B and b. Four types of F1 gametes (AB, Ab, aB, ab) are thus produced, in equal proportions, and they recombine in all 16 possible ways:
  • 9/16 of F2 show both dominant phenotypes
  • 3/16 of F2 are dominant for the first trait but not the 2nd
  • 3/16 of F2 are dominant for the 2nd trait but not the first
  • 1/16 of F2 show both recessive phenotypes
We now know that independent assortment only works only if the genes are on separate chromosomes.

Other 2-gene crosses:   First, determine the genotypes of the parents. Each parent produces 1, 2, or 4 types of gametes; combine them to determine the F1. Cross F1 x F1 to get the F2. Determine the offspring of each cross by a Punnett square.

Three-gene crosses:   Mendel also studied 3-gene crosses using the methods described above, and was able to explain all the results. Parents were AABBCC x aabbcc (or AABBcc x aabbCC, etc.). F1 were all AaBbCc. F2 phenotype ratios were 27:9:9:9:3:3:3:1. Up to 8 different types of gametes may result in 3-gene crosses.

Index             Syllabus
Prev rev. Aug. 2011 Next