Chromosome Number

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Understanding:

•  The number of chromosomes is a characteristic feature of members of a particular species

    
Chromosome number is a characteristic feature of members of a particular species

  • Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)
  • In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
    • For instance, a horse (diploid = 64) and a donkey (diploid = 62) may produce an infertile mule (non-diploid = 63)


Chromosome Number and Hybrid Infertility

hybrid infertility


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Application:

•  Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, 

   Oryza sativa and Parascaris equorum

    
Chromosome number does not provide a valid indication of genetic complexity, for instance:

  • Tomatoes (Solanum lycopersicum) have 24 chromosomes and a genome size of 950 million bp, but possess ~32,000 genes
  • Chickens (Gallus gallus) have 78 chromosomes and a genome size of 1.2 billion bp, but possess only ~17,000 genes


Diploid Chromosome Number Comparisons

Diploid number


Comparison of Chromosome Number, Genome Size and Gene Count

                 Graph:                 Comparison                 Diploid Number                  Number of Genes                  Genome Size