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Genetic Linkage and Genetic Maps

The Background

Gregor Mendel analyzed the pattern of inheritance of seven pairs of contrasting traits in the domestic pea plant. He did this by cross-breeding dihybrids; that is, plants that were heterozygous for the alleles controlling two different traits.


Producing dihybrids (F1)


Gene Mapping with a Three-Point Cross

The page Genetic Linkage and Linkage Maps shows how carrying out three different dihybrid test crosses in the corn plant reveals:

  • the order of the gene loci
  • the distance between them (in centimorgans, cM)

Here we shall see how a single test cross of a trihybrid corn plant ; that is,

  • one parent is heterozygous for three linked alleles (C,Sh, Bz, on one chromosome; c,sh,bz on the other)
  • and the other parent is homozygous for the recessive version

Continuous Variation: Quantitative Traits

The rules of inheritance discovered by Mendel depended on his wisely choosing traits that varied in a clear-cut, easily distinguishable, qualitative way. But humans are not either tall or short nor are they either heavy or light. Many traits differ in a continuous, quantitative way throughout a population.

This histogram shows the distribution of heights among a group of male secondary-school seniors. As you can see, the plot resembles a bell-shaped