Linked Genes & Recombination
Adjust the physical map units between two genes to observe the dynamic probability of Prophase I crossing over and mathematically predict offspring Recombination Frequency.
GENETIC LINKAGE AND RECOMBINATION
Genes located on the same chromosome are said to be **linked**. Because they are part of the same physical DNA molecule, they do not follow Mendel's Law of Independent Assortment. However, they can be separated during Prophase I of meiosis through a process called **crossing over**.
MAPPING CHROMOSOMES
The frequency of crossing over between two genes is proportional to the distance between them. The further apart two genes are, the more likely they are to be separated by a crossover event. This is measured in **map units** (centimorgans). A recombination frequency of 1% is equal to 1 map unit.
HOW TO USE THIS VISUALIZATION
1. **Place the Genes**: Drag two gene loci to different positions on a virtual chromosome. 2. **Run Meiosis**: Watch as crossing over occurs during Prophase I. 3. **Calculate Frequency**: Count the number of recombinant offspring vs. parental offspring to find the recombination frequency. **Try This**: Move the two genes as close together as possible. Run 1,000 trials. Notice how the recombinant count drops to near zero. What does this tell you about the physical relationship between the genes?
AP EXAM CONNECTION
Unit: Unit 5: Heredity (Topic 5.4)
Learning Objective: IST-1.G
COMMON MISCONCEPTIONS
- Thinking all genes on one chromosome always stay together.
- Confusing recombination with mutation.
- Believing crossing over occurs between sister chromatids (it occurs between non-sister homologous chromatids).
KEY TAKEAWAYS
- Linked genes are on the same chromosome.
- Crossing over creates recombinants.
- Distance determines crossover frequency.
- Max recombination frequency is 50%.
PRACTICE QUESTIONS
Q1 (QUANTITATIVE): Two genes have a recombination frequency of 15%. How many map units apart are they?
Show Answer & Explanation
Answer: 15 map units (centimorgans).
Explanation: By definition, one percent recombination is equal to one map unit on a chromosome.
Q2 (CONCEPTUAL): If two genes have a recombination frequency of 50%, are they linked?
Show Answer & Explanation
Answer: No, they are behaving as if they are on different chromosomes.
Explanation: A 50% frequency is the maximum possible, representing the random assortment seen in unlinked genes.
