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

CROSSING OVER

• Complete linkage between genes on the same chromosome is rare.
• The location of a gene on a chromosome is called a locus (plural loci). The loci of the genes on a chromosome are arranged in a linear sequence.
• Crossing over is the exchange of genetic material between homologous segments of non sister chromatids of homologous chromosomes that occurs during meiosis and contributes to genetic variability.
• The important features of the concept of crossing over are as follows,
  • The two alleles of a gene in a heterozygote occupy corresponding positions in the homologous chromosomes.
  • Crossing over involves the breakage of each of the two homologous chromosomes and the exchange of parts.
  • Crossing over occurs in the post replication tetrad stage of prophase-1 of meiosis during which 4 chromatids are present for each pair of homologous chromosomes.
  • Chromosomes with recombinant combinations of linked genes are formed by the occurrence of crossing over in the region between the two loci.
  • The probability that crossing over will occur between two loci increases with increasing distance between the two loci on the chromosome.
• Crossing over is an exception to Linkage.
• Crossing over and independent assortments are two important mechanisms for the generation of new combinations of genes.
• The first cytological demonstration of genetic crossing over has been given by Stern (working with Drosophila) and H. B. Creighton and B. McClintock (working with maize) in 1931.