AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

Stern’s classical experiment

STERN'S CLASSICAL EXPIRIMENT

• C. Stern in 1931 by direct cytological evidence, demonstrated that crossing over involves the interchange of parts of homologous chromosomes.
• The female Drosophila carries XX chromosome and the male Drosophila carries one X chromosome and one Y chromosome.
• He made the two X chromosomes of the female are different from each other X chromosomes by treating such flies with X-rays.
• One X chromosome had a part of a Y chromosome attached to one end.
• The other X chromosome has been broken into two unequal segments.
• Thus, both aberrant X chromosomes were cytologically detectable.
• The broken X chromosome contains a recessive gene (c) for carnation eye colour and a dominant gene (B) for bar eye shape (one fragment having both of the genes).
• While its homologue X chromosome contains the dominant gene (C) for red colour and the recessive gene (b) for round eye shape.
• Female flies heterozygous for these two morphologically distinguishable X-chromosomes were produced by crossing. These heterozygous females with trans-configuration were crossed to males with carnation eye colour (c) and round eye shape (b).

 To view "Stern's Classical Experiment Animation"

• Fertilization produced following four kinds of female offspring,
  • Carnation Bar
  • Red Round
  • Carnation Round &
  • Red Bar.
  • The crossing over was indicated phenotypically showed microscopic evidence of exchanges between homologous chromosomes.
  • The physical or cytological basis of crossing over was thus established