12.5 Gene Linkage and Crossing Over

Gene Linkage

Mendel's Law of Independent Assortment holds true only when genes are located on different (non-homologous) chromosomes. When two or more genes are located on the same chromosome, they tend to be inherited together and do not assort independently. This phenomenon is called gene linkage.

Linked genes violate the Law of Independent Assortment because they travel together on the same chromosome during meiosis (unless separated by crossing over).

Linkage Groups

All genes present on a single chromosome form a linkage group. The number of linkage groups in an organism equals its haploid chromosome number (n).

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Crossing Over

Although linked genes tend to stay together, they can be separated by a process called crossing over.

When Does Crossing Over Occur?

Crossing over occurs during Prophase I of meiosis, specifically at the Pachytene stage, when homologous chromosomes are paired (synapsed) as bivalents.

Mechanism of Crossing Over

1. During Prophase I, homologous chromosomes pair up (synapsis) to form bivalents (tetrads of 4 chromatids).
2. Non-sister chromatids of the two homologues break at corresponding points.
3. The broken ends rejoin with the opposite chromatid, exchanging segments.
4. The X-shaped points where this exchange occurs are called chiasmata (singular: chiasma).

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Parental Types vs. Recombinant Types

After crossing over, two categories of offspring are produced:

Because crossing over does not occur in every meiotic division, parental types always outnumber recombinant types when genes are linked.

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Recombination Frequency and Gene Distance

The recombination frequency (also called crossover frequency) is the proportion of recombinant offspring produced in a cross:

$\text{Recombination Frequency}=\frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}}\times 100\%$

Key Principle

• Genes far apart → higher recombination frequency → more likely to be separated by crossing over
• Genes close together → lower recombination frequency → tightly linked, rarely separated
• Maximum recombination frequency = 50% (genes so far apart they behave as if unlinked)

Genetic Maps (Chromosome Maps)

Recombination frequencies are used to construct genetic maps (linkage maps). The unit of genetic distance is the centimorgan (cM) or map unit:

$1\text{ map unit}=1\%\text{ recombination frequency}$

Example: If genes A and B show 15% recombination frequency, they are 15 map units (15 cM) apart on the chromosome.

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Significance of Crossing Over

1. Genetic variation: Crossing over produces new combinations of alleles, increasing genetic diversity in offspring.
2. Gene mapping: Recombination frequencies allow scientists to determine the relative positions of genes on chromosomes.
3. Evolution: New gene combinations provide raw material for natural selection.
4. Breaks complete linkage: Prevents all genes on a chromosome from always being inherited as a single unit.