1.6.8Inquiry Process
Chromosome Number: Cross-Breeding — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Why Hybrids Are Often Sterile
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
- Mule Horse (2n = 64) and donkey (2n = 62) give a mule with 63 chromosomes that cannot pair evenly.
Hinny
- Hinny The reverse cross, a male horse with a female donkey, also gives a sterile hybrid.
Liger
- Liger A male lion crossed with a tigress gives a hybrid that is almost always sterile.
Fertile hybrid
- Fertile hybrid Hybrids between closely related species with matching chromosome numbers can sometimes be fertile.
| Parent | Gamete n |
|---|---|
| Horse | 32 |
| Donkey | 31 |
| Mule | 32 + 31 = 63 (2n) |
Part 3 of 5Polyploidy in Plants
Polyploidy in Plants
Polyploid
- Polyploid A cell or organism with more than two complete sets of chromosomes, common in plants.
How it forms
- How it forms A sterile hybrid may double its chromosome number, so every chromosome then has a partner.
Result
- Result The doubled plant becomes fertile and is reproductively isolated from its parents, so is a new species.
Examples
- Examples Bread wheat is a hexaploid, formed from the hybridisation of several ancestral grass species.
Why animals differ
- 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.
- A horse gamete has 32 chromosomes and a donkey gamete has 31.
- The mule zygote has 32 + 31 = 63 chromosomes.
- The odd number means chromosomes cannot all form homologous pairs.
- 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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