+VARIATION IN CHROMOSOME NUMBER PART 2+

NATURAL AND EXPERIMENTAL WAYS TO PRODUCE VARIATION IN CHROMOSOME NUMBER

• Variations in chromosome numbers are due to nondisjunction of the chromosomes.
• Meiotic nondisjunction produces haploid cells with extra or fewer chromosomes.
• Mitotic nondisjunction produces patches of tissues that have an altered chromosome numbers.
• Interspecies crosses can create allopolyploids.
  

MITOTIC NONDISJUNCTION

• Occurs after fertilization has occurred.

1. The result is an organism which contains a group of cells genetically different from one another.
2. This is called mosaicsm.

• The size and location of mosaic region depend on when and where the event occurred.
• If event occurs in first mitotic division, result is a bilateral gynandromorphy.
  

CHANGES IN EUPLOIDY

• An autopolyploid = increase in the numbers of chromosome sets in a single species.
• Alloploidy is the result of an interspecies cross.
• Allodiploids have one set of chromosomes from each parent.
• Allopolyploid contains a combination of interspecies chromosome sets.
• An allotetraploid has 2 complete sets of chromosomes from 2 species.
  

FERTILITY AND EUPLOIDY

• Allodiploid are usually sterile, unless the species is closely related.
• Evolutionary related chromosomes from 2 different species are called homologous.
• Allotetraploids are usually fertile because they possess 2 complete sets of the chromosomes (including sex chromosomes) from each species.
  

EXPERIMENTAL TREATMENTS AND POLYPLOIDY

• Polyploid and alloploid plants are important in agriculture.

1. Several mechanisms have been developed to generate these chromosome combinations.
2. e.g. drug colchicines binds to tubulin and interferes with normal chromosome segregation, thus promoting nondisjunction.
   

CELL FUSION AND HYBRID PLANTS

• In cell fusion, individual calls are mixed together and made to fuse, thus allowing the crossing of 2 species that would not naturally occur.

1. This requires the removal of cell wall, creating a protoplast.
2. Once fused, cell contains 2 nuclei, called a heterokaryon.
3. This cell will then undergo nuclear fusion to produce a hybrid cell with a single nucleus.
   

MONOPLOIDS AND AGRICULTURE

• A monoploid allows researchers to generate a strain of a crop that is homozygous for all of its genes.
• These true-breeding strains may then be crossed to produce plants that are heterozygous for many genes.
• These plants are usually more vigorous called hybrid vigour or heterosis.
• The use of anther culture can produce diploid strains of crops that are homozygous for all genes.
• Animal species may be produced by parthenogenesis, which involves inducing them to begin development without fertilization.

1. The haploid zygote usually only lives for a short period before dying, but this provides useful information for developmental biologists.