Biology · Principles of Inheritance and Variation · NEET
They are linked but not the same. Crossing over is the ACTION - two non-sister chromatids of homologous chromosomes physically break and swap equal segments. Recombination is the RESULT - the new, non-parental mix of genes (alleles) that this swap creates in the gametes. Simple rule: crossing over causes recombination. Every crossing over gives recombinant products, so NEET often uses the two words for the same event, but crossing over is the mechanism and recombination is the outcome.
Crossing over happens in the PACHYTENE stage of prophase I of meiosis. Order of prophase I: Leptotene, Zygotene (pairing/synapsis), Pachytene (crossing over), Diplotene, Diakinesis. A common trap is to pick Zygotene - that is only where the homologous chromosomes PAIR up (synapsis). The actual exchange of segments happens one step later, at Pachytene, helped by an enzyme called recombinase.
It occurs between NON-SISTER chromatids of HOMOLOGOUS chromosomes. Remember two conditions: (1) the two chromosomes must be homologous (a pair - one from mother, one from father), and (2) the two chromatids that swap must be non-sister (one chromatid from each homologue). Sister chromatids are identical copies, so swapping them would create nothing new. Non-homologous chromosomes do not cross over - if a gene moves to a non-homologous chromosome, that is translocation, not crossing over.
A parental type offspring has the SAME gene combination as its parent's chromosomes (no swap). A recombinant type offspring has a NEW combination made by crossing over. Example: if a parent's chromosome carries genes AB and ab, the parental gametes are AB and ab; after crossing over, recombinant gametes Ab and aB appear. In a test cross, MORE parental types than recombinant types means the genes are linked (close on the same chromosome).
Because it shuffles alleles into brand-new combinations that neither parent had. Genes that were sitting together on one chromosome get separated and mixed with new partners. This is a major source of the genetic variation produced by sexual reproduction. For NEET, link this idea to Morgan's work on Drosophila - he showed that the closer two genes are, the LESS often crossing over separates them (low recombination frequency), and this distance is used to map genes on a chromosome.
Recombination frequency = (number of recombinant offspring / total offspring) x 100. Tightly linked genes have LOW recombination frequency (they rarely separate); genes far apart have HIGH recombination frequency. Alfred Sturtevant, Morgan's student, used recombination frequency as a measure of the DISTANCE between genes and 'mapped' their positions on the chromosome. Remember: Morgan coined the terms linkage and recombination; STURTEVANT did the gene mapping.
Crossing over takes place between which chromatids and in which stage of the cell cycle?
In a testcross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-type offspring. This indicates:
The frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes was explained by:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not exactly. Crossing over is the physical swap of chromosome segments; recombination is the new gene combination that results from it. Crossing over is the cause, recombination is the effect. NEET often uses them together, but know the difference.
At the pachytene stage of prophase I of meiosis. Pairing (synapsis) happens earlier at zygotene; the actual exchange happens at pachytene with the help of the enzyme recombinase.
Between non-sister chromatids of homologous chromosomes. It never happens between sister chromatids (identical copies) or between non-homologous chromosomes.
To measure the distance between two genes on a chromosome. Higher frequency means the genes are farther apart. Sturtevant used it to build the first gene maps.
Parental types have the same gene combination as the parents (no crossing over). Recombinant types have a new combination created by crossing over. More parental types in a test cross means the genes are linked.
