How can crossing over produce new allele combinations?

Crossing over can rearrange existing allele combinations during meiosis. Alleles are versions of genes, and a locus is a gene’s position on a chromosome. Two loci are linked when they lie on the same chromosome. Whether a combination is recombinant depends on the allele arrangement present before the exchange.

Here the starting arrangement is AB on one homolog and ab on its partner, written AB/ab. This arrangement is called linkage phase. DNA replication gives each homolog two sister chromatids, producing four chromatids in the paired set. The two sisters begin with the same allele combination in this model.

A single specified crossover exchanges matching segments between two nonsister chromatids. If the exchange lies between the A and B loci, one participating chromatid becomes Ab and the other aB. The two chromatids not participating remain AB and ab. No new A, a, B or b allele has been invented; existing versions have been combined differently.

After meiotic separation, the four modeled haploid products are AB, Ab, aB and ab. Two are recombinant relative to AB/ab. An exchange beyond both marked loci leaves their combinations unchanged, while a starting phase of Ab/aB changes which combinations count as parental.

The two-out-of-four result belongs to this one stipulated exchange. It does not establish a 50% recombination rate across a population or describe a guaranteed four-offspring family. Exchange locations and additional events require a broader model.