In order for members of a species to interbreed, they must have compatible genetic profiles

• The genetic information of each parent must be organised onto the same number of chromosomes, with comparable sizes and gene loci positions
  

• Organisms with different diploid numbers are unlikely to be able to interbreed as the chromosomes cannot form homologous pairs
  

Chromosome number is a characteristic feature of members of a particular species and can be used to deduce evolutionary relationships

• Humans have 46 chromosomes while chimpanzees have 48 chromosomes – indicating that they are different species
  

• However, the two species share a common ancestor and it is hypothesised that chromosome 2 in humans arose from the fusion of chromosomes 12 and 13 in the primate ancestor
  

Comparing chromosomes in humans and modern chimpanzees provides evidence that supports the hypothesis of a past chromosomal fusion event:

• The combined length of chromosomes 12 and 13 in chimpanzees match the length of human chromosome 2
  

• The centromere location of human chromosome 2 matches that of chimpanzee chromosome 12
  

• The banding of the arms of human chromosome 2 matches the long arms of chimpanzee chromosomes 12 and 13
  

• Human chromosome 2 contains telomeric DNA in position where fusion of chromosomes 12 and 13 would have occurred
  

• Human chromosome 2 also contains non-coding DNA in the position that would correspond to the centromere location of a fused chromosome 13
  

Chromosome Comparisons

Karyograms

Karyotyping is the process of pairing and ordering the complete set of chromosomes within a cell to provide a snapshot of an organism’s genetic profile

• The chromosomes can then be photographed to generate a visual representation called a karyogram
  

Karyotyping requires the use of chemical inhibitors to arrest a cell during division while the chromosomes are condensed

• The timing at which the cell is arrested will determine whether chromosomes appear with sister chromatids or not (i.e. pre- vs post-anaphase)
  

The chromosomes are organised in homologous pairs according to size (largest to smallest) and centromere position (metacentric vs acrocentric)

• The chromosomes may be stained in order to generate distinctive banding patterns for further delineation
  

Karyotyping can be used to determine three key things:

• The type of species (chromosome number is a characteristic feature of a species)
  

• The sex of the organisms (in humans, males are XY and females are XX)
  

• The presence of any chromosomal abnormalities (e.g. translocations or aneuploidies)
  

Human Male Karyogram