Mendel's Monohybrid Cross

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Introduction

Gregor Mendel (1822-1884) was an Austrian monk who discovered the basic rules of inheritance. From 1858 to 1866, he bred garden peas in his monastery garden and analyzed the offspring of these matings. The garden pea was good

Crossing Over and Genetic Recombination in Meiosis

Crossing over occurs between equivalent portions of two nonsister chromatids.

Link to an illustrated discussion of crossing over examined at the level of a cell undergoing meiosis.

Each chromatid contains a single molecule of DNA. So the problem of crossing over is really a problem of swapping portions of adjacent DNA molecules.

It must be done with great precision so that neither chromatid gains or loses any genes. In fact

The Evidence of Creighton and McClintock

In 1932, the geneticists Harriet Creighton and Barbara McClintock provided an elegant demonstration that the recombination of genes linked on a chromosome requires the physical exchange of segments of the chromosome with its homologous partner.

During their studies of linkage in corn, they developed a strain of corn that had one chromosome (number 9 of 10 pairs) with two unusual features:

  • a knob at one end of the chromosome and
  • an extra piece at