Gene Linkage and Crossing Over: MDCAT Biology notes

Gene linkage

Each gene occupies a fixed position on a chromosome called its locus. A chromosome carries many genes, so genes whose loci lie on the same chromosome tend to be inherited together into the same gamete. These are linked genes, and the tendency is called linkage.

• Linkage was discovered by Thomas Hunt Morgan working on the fruit fly Drosophila melanogaster.
• Linked genes do not follow Mendel's law of independent assortment, which applies only to genes on different (non-homologous) chromosomes. Linkage therefore disturbs the expected 9:3:3:1 dihybrid ratio.
• The closer two loci are, the stronger the linkage.

Linkage groups

All the genes on one chromosome form one linkage group. The number of linkage groups equals the haploid number of chromosomes: 23 in humans (4 in Drosophila).

Crossing over

During prophase I of meiosis, homologous chromosomes pair up side by side. This pairing is called synapsis, and each pair is a bivalent (tetrad, four chromatids). Non-sister chromatids then break at matching points and exchange segments. The points of contact are chiasmata.

• Exchange is between non-sister chromatids of homologous chromosomes, i.e. between a maternal and a paternal chromatid; not between sister chromatids, and not between non-homologous chromosomes.
• The result is recombinant chromatids carrying new combinations of alleles. Crossing over thus increases genetic variation; it does not create new genes or reduce chromosome number.
• Crossing over partly breaks linkage; linkage reduces the frequency of recombinants.

Feature	Linkage	Crossing over
Effect on genes	Keeps genes together	Separates linked genes
Offspring produced	Mostly parental types	Recombinant types
Effect on variation	Reduces it	Increases it

Detecting linkage: Morgan's approach

Linkage is detected by a test cross: a double heterozygote is crossed with a double homozygous recessive. With independent assortment the four offspring classes would be equal (1:1:1:1). With linkage, the parental types are in excess and recombinants form well under 50%. Because recombinants are a small fraction, a large number of progeny must be counted to measure the frequency accurately.

Recombination frequency and gene mapping

$$\text{Recombination frequency}=\frac{\text{number of recombinants}}{\text{total offspring}}\times100\%$$

The chance of crossing over between two loci is directly proportional to the distance between them. Morgan's student Alfred Sturtevant used this to build the first chromosome map. 1% recombination = 1 map unit = 1 centimorgan (cM). Working out the locus of genes this way is called gene mapping. Recombination frequency never exceeds 50%, since genes that far apart behave as if unlinked.

Common MDCAT traps

• Linkage contradicts the law of independent assortment, not the law of segregation.
• Crossing over occurs in prophase I (pachytene), not metaphase I or anaphase.
• Synapsis is the pairing; crossing over is the exchange that follows.
• Human linkage groups = 23, not 46.
• Linked genes cross over sometimes, not every time.

Quick revision

• Locus = position of a gene on a chromosome.
• Linked genes lie close together on the same chromosome.
• Crossing over: non-sister chromatids of homologues, prophase I.
• More distance between genes, more crossing over.
• 1 map unit = 1% recombination.