4.8.10Additional lessons
Gene Linkage — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 77 of 101
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.
Section 1 of 5What Linkage Means
Linked genes
- Linked genes Genes whose loci are on the same chromosome, so they tend to be inherited as one unit through meiosis.
Autosomal linkage
- Autosomal linkage The linked genes are on an autosome, which differs from sex linkage where the genes are on a sex chromosome.
Parental combination
- Parental combination The alleles that were together on one chromosome in a parent usually stay together in the gametes.
Effect on ratios
- Effect on ratios Linkage reduces the number of different gamete types, so the usual Mendelian ratios are not seen.
Section 2 of 5Writing Linked Crosses
The chromosomes are drawn, not the genes alone.
1
Show each chromosome of the pair with its alleles in brackets, for example (TG) for the chromosome carrying T and G, and (tg) for the other.
2
Write the genotype of a double heterozygote as (TG)(tg) to show which alleles lie on the same chromosome.
3
State that, with no crossing over, the double heterozygote makes only two gamete types, (TG) and (tg).
4
Combine them with the gametes of a double recessive (tg)(tg), which makes only (tg) gametes.
5
Read the offspring as 1 : 1 and compare this with the 1 : 1 : 1 : 1 ratio expected for unlinked genes.
| Gene arrangement | Gamete types from the heterozygote | Test cross ratio |
|---|---|---|
| Unlinked genes | TG, Tg, tG, tg | 1 : 1 : 1 : 1 |
| Fully linked genes | TG and tg only | 1 : 1 |
Section 3 of 5Linkage in Real Organisms
Linkage in Real Organisms
Number of genes
- Number of genes Each chromosome carries many genes, so many genes are linked to others on the same chromosome.
Gene closeness
- Gene closeness Genes that are close together are separated by crossing over only rarely, so linkage between them is strong.
Gene distance
- Gene distance Genes far apart on a chromosome are often separated, so they behave almost as if unlinked.
Evidence
- Evidence Offspring showing ratios far from 9 : 3 : 3 : 1 or 1 : 1 : 1 : 1 suggest that the genes are linked.
Section 4 of 5Worked Example
Question
- A scaly lizard with normal tail and green scales (TtGg, with T and G on the same chromosome) is crossed with one with a short tail and white scales (ttgg).
- Assuming no crossing over, what offspring are expected?
- The heterozygote has chromosomes (TG) and (tg), so its gametes are (TG) and (tg) in equal numbers.
- The double recessive parent produces only (tg) gametes.
- Offspring are (TG)(tg) and (tg)(tg) in equal numbers.
- Phenotypes(TG)(tg) has normal tail and green scales, and (tg)(tg) has short tail and white scales.
Answer
- 1 normal tail, green scales : 1 short tail, white scales;
- there are no other phenotypes if linkage is complete.
Section 5 of 5Exam Checklist & Tips
4.8.10 – Key Points for the Exam
- Linked genes Genes found on the same chromosome.
- Autosomal linkage Linkage of genes on a chromosome other than the sex chromosomes.
- Parental type An offspring phenotype that matches one of the parents.
- Double heterozygote An individual heterozygous at two gene loci.
Common Mistakes
- Writing the gametes of a linked heterozygote as four types when crossing over is absent.
- Using the 9 : 3 : 3 : 1 ratio for linked genes.
- Confusing autosomal linkage with sex linkage.
- Writing alleles side by side without brackets, which hides which alleles are on the same chromosome.
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