Genes located on the same chromosome are linked and are inherited in a way that deviates from the law of independent assortment. However, during crossing over in meiosis, linked genes can separate, causing recombination. The recombination value (recombination frequency) reflects the distance between genes and is used to create chromosome maps (genetic maps).
Basic Knowledge
Concept of linkage: Genes on the same chromosome tend to be inherited together. In complete linkage (without crossing over), the result of a test cross between two loci, F₁(AaBb) × aabb, is not 1:1 (without recombination); only parental types appear.
Crossing Over and Recombination: During prophase I of meiosis, homologous chromosomes undergo synapsis (pairing), and chiasmata (crossing points) are formed. Parts of chromatids are exchanged through the chiasmata (crossing over), producing recombinant gametes with new combinations.
Recombination value (recombination frequency): = (number of recombinant gametes) / (number of all gametes) × 100 [%]. The farther apart the genes are, the more likely crossing over is to occur, and the greater the recombination value becomes. The maximum recombination value is 50% (the same as independent assortment). 1 cM (centimorgan) = a distance with a recombination value of 1%.
Three-point test cross and genetic maps: The recombination value between each pair of genes is calculated from the results of a test cross involving three linked loci, and the map distances are determined. The least numerous recombinant types are double-crossover offspring and are used to determine gene order.
A test cross was performed between F₁(Ab/aB), in which two loci A and B were linked, and an aabb individual. The results were AB: Ab: aB: ab = 40: 460: 450: 50 (a total of 1000 individuals). Find the recombination value and the genetic map distance.
Answer: The parental types are Ab and aB (450 each), and the recombinant types are AB and ab (40 + 50 = 90). Recombination value = 90/1000 × 100 = 9%. Therefore, the map distance between A and B is 9 cM.
- Linkage = genes on the same chromosome tend to be inherited together
- Crossing over = chiasmata form between homologous chromosomes during prophase I of meiosis
- Recombination value = recombinant types/(all gametes) × 100%
- Maximum recombination value = 50% (indistinguishable from independent assortment)
- 1 cM = a genetic map distance corresponding to a recombination value of 1%
- Three-point test cross = determines the order and distances of three genes
- Double-crossover offspring = the least numerous recombinant types and the key to determining gene order
Points to Note
① The greater the recombination value (the closer it is to 50%), the greater the physical distance between genes, but it never exceeds 50%. ② Even for linked genes, a recombination value of 50% cannot be distinguished from independent assortment. ③ Crossing-over frequency is high at specific chromosome regions (hot spots) and is not uniform.
Practice
- Explain when crossing over occurs and what types of gametes are produced as a result.
- Give the formula for calculating recombination value and describe the relationship between recombination value and the distance between genes.
- Explain why the “least numerous recombinant types” are important for determining gene order in a three-point test cross.