Chromosome Number: Cross-Breeding (HL) (HL IB Biology)

Chromosome number & cross-breeding

• With rare exceptions, each individual in a species always has the same number of chromosomes
  • Human cells contain 46 chromosomes
  • Chimpanzee cells contain 48 chromosomes
  • Dog cells contain 78 chromosomes
• The diploid (2n) chromosome number is halved when meiosis takes place, producing haploid (n) gametes
• During fertilisation the nuclei of (n) gametes fuse together to form the nucleus of a (2n) zygote
• Both gametes must contain the same number of chromosomes in order for the zygote to be viable.
  • If a zygote has too many or too few chromosomes it may not survive
• Because species have different chromosome numbers, they will produce haploid gametes which also have different chromosome numbers
  • Human gametes contain 23 chromosomes
  • Chimpanzee gametes contain 24 chromosomes
  • Dog gametes contain 39 chromosomes
• Even if two different species are able to mate with each other, e.g. in the case of closely related species, they will be unable to produce fertile offspring because
  • The gametes will be unable to fuse
  • The gamete fusion does not lead to a viable zygote due to the uneven number of chromosomes
  • The gamete fusion does lead to a viable zygote, but the new diploid cells contain an uneven number of chromosomes and so will be unable to carry out meiosis themselves; this leads to an infertile interspecific hybrid
    • A well-known example of this is the mating of a horse and donkey to produce a mule:
      • Horses have 64 chromosomes, so produce gametes with 32 chromosomes
      • Donkeys have 62 chromosomes, so their gametes contain 31 chromosomes
      • The fusion of a horse and a donkey gamete gives rise to a mule zygote containing 63 chromosomes
      • These chromosomes cannot pair up during meiosis, so mules cannot produce gametes of their own

Infertile hybrid mule diagram

Crossing a horse with a donkey produces a mule; mules have an odd number of chromosomes so cannot carry out meiosis