# Gene Linkage and Crossing Over ## Gene Linkage The tendency for genes located on the same chromosome to be inherited together during meiosis is called **gene linkage**, and the genes are referred to as *linked genes*. ### Linkage Groups All the genes located on a single homologous pair of chromosomes form a **linkage group**. The number of linkage groups in an organism is equal to its number of homologous chromosome pairs. For example, in humans, there are 23 linkage groups. ### Types of Linkage - **Autosomal Linkage:** Linkage of genes on autosomes (non-sex chromosomes). - **Sex Linkage:** Linkage of genes on sex chromosomes. ### Effect on Inheritance Linked genes do not assort independently, which is a deviation from Mendel's Law of Independent Assortment. This results in a higher proportion of parental phenotypes and a lower proportion of recombinant (new combination) phenotypes in the offspring. ### Detection of Gene Linkage Gene linkage is typically detected using a **dihybrid test cross**, where an individual heterozygous for two traits (F₁) is crossed with a homozygous recessive individual. | Linkage Type | Offspring Phenotypic Ratio (Test Cross) | Dihybrid F₁ Self-Cross Ratio | Explanation | | :--- | :--- | :--- | :--- | | **No Linkage** | 1 : 1 : 1 : 1 | 9 : 3 : 3 : 1 | Genes are on different chromosomes and assort independently. Parental and recombinant types are produced in equal numbers. | | **Incomplete Linkage** | Deviated from 1:1:1:1 | Deviated from 9:3:3:1 | More parental types and fewer recombinant types are produced due to crossing over occurring some of the time. | | **Complete Linkage** | 1 : 1 (Parental types only) | 3 : 1 | Genes are very close together on the same chromosome and are always inherited together. No recombinant types are produced. | ## Morgan's Experiment on Drosophila T. H. Morgan's experiments with the fruit fly, *Drosophila melanogaster*, provided key evidence for gene linkage. ### Traits Studied - Wing Length: Long wings (**Vg**) is dominant over vestigial wings (**vg**). - Abdomen Width: Broad abdomen (**A**) is dominant over narrow abdomen (**a**). ### The Cross 1. **Parental (P):** A homozygous dominant fly (Long wings, Broad abdomen) was crossed with a homozygous recessive fly (vestigial wings, narrow abdomen). 2. **F₁ Generation:** All offspring had long wings and broad abdomens (heterozygous). 3. **F₁ Cross:** Two F₁ flies were mated. ### Expected vs. Actual Results - **Expected Result (Independent Assortment):** A 9:3:3:1 phenotypic ratio in the F₂ generation. - **Actual Result:** A ratio of approximately **3:1** was observed. The vast majority of F₂ flies showed the parental phenotypes (Long/Broad and vestigial/narrow). Recombinant phenotypes (Long/narrow and vestigial/Broad) were absent or rare. **Conclusion:** Morgan concluded that the genes for wing length and abdomen width were located on the same chromosome and were therefore *linked*. They did not assort independently and were inherited together. ## Crossing Over Crossing over is the process responsible for the recombination of linked genes. It involves the exchange of genetic material (chromatid segments) between non-sister chromatids of homologous chromosomes. ### Mechanism - Occurs during **Prophase I of meiosis** (specifically the pachytene stage) when homologous chromosomes pair up. - Maternal and paternal chromatids physically overlap at points called **chiasmata** and exchange parts. - This exchange creates **recombinant chromatids**, which carry new combinations of alleles. ### Significance Crossing over breaks the linkage between genes on the same chromosome. It is a major source of **genetic variation** in sexually reproducing organisms, as it allows for the creation of new combinations of alleles in gametes, leading to offspring with different traits than their parents (recombinant types). ### Recombination Frequency The frequency of crossing over between two linked genes depends on the distance between them. The further apart two genes are on a chromosome, the higher the probability that crossing over will occur between them.