Chromosome Number: Cross-Breeding (DP IB Biology)

Chromosome Number: Cross-Breeding

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