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1.6.8Inquiry Process

Chromosome Number: Cross-Breeding — IB Biology HL Revision Notes & Topical Questions

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Part 1 of 5Why Hybrids Are Often Sterile

1.6.8 Chromosome Number: Cross-Breeding

Successful meiosis needs each chromosome to have a partner.

1
Each parent species supplies a haploid set, so the hybrid has one set from each species.
2
If the parents differ in chromosome number the hybrid total may be an odd number.
3
In prophase I of meiosis homologous chromosomes must pair, but the sets from two species may not match.
4
Unpaired chromosomes cannot segregate evenly, so gametes are unbalanced or not formed.
5
The hybrid is therefore sterile, which prevents gene flow between the two species.

Part 2 of 5Examples of Hybrids

Mule
  • Horse (2n = 64) and donkey (2n = 62) give a mule with 63 chromosomes that cannot pair evenly.
Hinny
  • The reverse cross, a male horse with a female donkey, also gives a sterile hybrid.
Liger
  • A male lion crossed with a tigress gives a hybrid that is almost always sterile.
Fertile hybrid
  • Hybrids between closely related species with matching chromosome numbers can sometimes be fertile.
ParentGamete n
Horse32
Donkey31
Mule32 + 31 = 63 (2n)

Part 3 of 5Polyploidy in Plants

Polyploidy in Plants
Polyploid
  • A cell or organism with more than two complete sets of chromosomes, common in plants.
How it forms
  • A sterile hybrid may double its chromosome number, so every chromosome then has a partner.
Result
  • The doubled plant becomes fertile and is reproductively isolated from its parents, so is a new species.
Examples
  • Bread wheat is a hexaploid, formed from the hybridisation of several ancestral grass species.
Why animals differ
  • Most animals have separate sexes and tightly controlled development,
    • so doubling the whole chromosome set usually disrupts development and is lethal.

Part 4 of 5Worked Example

Question

  • A horse has 2n = 64 and a donkey has 2n = 62.
  • Calculate the chromosome number of the hybrid mule and explain why it is usually sterile.

  1. A horse gamete has 32 chromosomes and a donkey gamete has 31.
  2. The mule zygote has 32 + 31 = 63 chromosomes.
  3. The odd number means chromosomes cannot all form homologous pairs.
  4. Without pairing in prophase I of meiosis, gametes with a complete balanced set cannot form.

Answer The mule has 63 chromosomes and is sterile because meiosis cannot proceed normally.

Part 5 of 5Exam Checklist & Tips

1.6.8 – Key Points for the Exam

  • Hybrid Offspring of parents from different species.
  • Polyploid Having more than two sets of chromosomes.
  • Prophase I The stage where homologous chromosomes pair.
  • Reproductive isolation Inability to produce fertile offspring with other groups.

Common Mistakes

  • Stating that odd chromosome numbers cause infertility by themselves, without mentioning pairing at meiosis.
  • Adding diploid numbers instead of haploid numbers when calculating the hybrid.
  • Saying mules reproduce by mitosis only; the problem is meiosis.
  • Believing polyploidy is common in animals; it is rare there.

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