13.1 Chromosomal Theory and Genetic Linkage

Chromosomal Theory and Genetic Linkage

Chromosomes are the physical carriers of genes, and their behavior during cell division directly explains the inheritance patterns Mendel first described. Understanding how chromosomes segregate, assort, and occasionally swap segments during meiosis is essential for predicting how traits pass from parents to offspring.

Principles of Chromosomal Inheritance

The chromosomal theory of inheritance states that genes are located on chromosomes, and it's the behavior of chromosomes during meiosis that produces the inheritance patterns Mendel observed. This theory was confirmed largely through the work of Thomas Hunt Morgan using fruit flies (Drosophila melanogaster).

Here are the core principles:

• Chromosomes carry genes arranged along their length. A single chromosome contains many genes.
• Chromosomes exist in homologous pairs. You inherit one homolog from each parent. Both carry the same genes at the same loci, but they may carry different alleles (versions of a gene). For example, one homolog might carry an allele for brown eyes while the other carries an allele for blue eyes.
• Chromosomes segregate during meiosis. Each gamete receives just one chromosome from each homologous pair. This is the physical basis for Mendel's law of segregation.
• Chromosomes assort independently during meiosis. The way one homologous pair separates has no effect on how another pair separates. This explains Mendel's law of independent assortment. For instance, the chromosome carrying a gene for eye color sorts independently from the chromosome carrying a gene for hair color.

Genetic Linkage and Inheritance Patterns

Mendel's law of independent assortment works perfectly when genes are on different chromosomes. But what happens when two genes sit on the same chromosome?

Genetic linkage is the tendency of genes located close together on the same chromosome to be inherited as a unit. Linked genes do not assort independently because they physically travel together during meiosis.

• The closer two genes are on a chromosome, the stronger their linkage and the less likely they are to be separated. Morgan first demonstrated this using genes for body color and wing shape in Drosophila, which did not sort independently the way Mendel's law predicted.
• Linkage reduces the frequency of recombinant offspring (offspring with new allele combinations not seen in either parent).
• However, linkage can be broken by crossing over during meiosis. When crossing over occurs between two linked genes, their alleles end up on different chromatids and can be separated into different gametes.
• Linkage disequilibrium refers to the non-random association of alleles at different loci in a population. It can result from physical linkage, but also from other factors like natural selection or recent mutations.

Crossing Over and Recombination

Crossing over is the physical mechanism that breaks linkage and generates new allele combinations. It occurs during prophase I of meiosis:

1. Homologous chromosomes pair up tightly in a process called synapsis.
2. Non-sister chromatids (one from each homolog) exchange segments of DNA at points called chiasmata (singular: chiasma).
3. The result is recombinant chromatids that carry a mix of alleles from both parents.

The frequency of crossing over between two genes depends on how far apart they are:

• Genes far apart on the same chromosome have more opportunities for a crossover to occur between them, so they recombine more often.
• Genes very close together rarely have a crossover between them, so they stay linked.
• Genes at opposite ends of a long chromosome may recombine so frequently that they behave almost as if they're on separate chromosomes.

This distance-dependent recombination frequency is what makes gene mapping possible.

Chromosome Behavior During Cell Division

To understand linkage and crossing over, you need to know how chromosomes behave through the stages of cell division:

• Interphase: DNA is decondensed (spread out) and not visible as distinct chromosomes. DNA replication occurs during S phase, so each chromosome becomes two identical sister chromatids.
• Prophase: Chromosomes condense and become visible. Each chromosome consists of two sister chromatids joined at the centromere. In meiosis I, this is also when synapsis and crossing over occur.
• Metaphase: Chromosomes line up along the cell's equatorial plate. Spindle fibers attach to the centromeres.
• Anaphase: Sister chromatids (in mitosis and meiosis II) or homologous chromosomes (in meiosis I) separate and move toward opposite poles.
• Telophase: Chromosomes decondense, nuclear envelopes reform, and cytokinesis divides the cytoplasm.

The key difference: mitosis produces two genetically identical daughter cells, while meiosis produces four genetically unique haploid gametes.

Gene Mapping Through Test Crosses

Because recombination frequency reflects physical distance between genes, you can use cross data to build a genetic map showing gene order and spacing.

Map units: One centiMorgan (cM) equals a 1% recombination frequency between two genes.

$\text{Map distance (cM)} = \frac{\text{number of recombinants}}{\text{total offspring}} \times 100$

For example, if you observe 20 recombinant offspring out of 1,000 total, the map distance is $\frac{20}{1000} \times 100 = 2 \text{ cM}$.

Three-point test cross procedure:

1. Cross an organism heterozygous for three linked genes ($ABC/abc$) with one that is homozygous recessive ($abc/abc$). Because the recessive parent contributes only recessive alleles, the offspring's phenotype directly reveals which alleles the heterozygous parent passed on.
2. Classify the offspring phenotypes and count each class.
3. Calculate the recombination frequency between each pair of genes (A–B, B–C, and A–C).
4. Determine gene order: the arrangement that makes the individual distances add up correctly is the correct order. For example, if A–B recombination is 10%, B–C is 25%, and A–C is 35%, the gene order is A–B–C (because 10 + 25 = 35).

Key Terminology

• Locus: The specific physical location of a gene on a chromosome.
• Allele: A variant form of a gene at a given locus.
• Haploid (n): Having a single set of chromosomes, as in gametes.
• Diploid (2n): Having two sets of chromosomes (one from each parent), as in somatic cells.
• Genotype: The combination of alleles an organism carries at a particular locus.
• Phenotype: The observable traits of an organism, produced by the interaction of genotype and environment.