Genes on the same autosome (a chromosome that is not a sex chromosome) are linked. Autosomal linkage means the alleles on one chromosome tend to be inherited together, because they do not assort independently in meiosis.In a dihybrid cross of two unlinked genes, AaBb × aabb gives a 1 : 1 : 1 : 1 ratio. If the genes are linked, with A and B on one chromosome and a and b on the other, most gametes are AB or ab (the parental combinations). The offspring are mostly like the parents, and the 1 : 1 : 1 : 1 ratio is not seen.Some offspring still have new combinations of alleles, such as Aabb and aaBb. These are recombinant offspring. They are produced by crossing over between non-sister chromatids of homologous chromosomes during prophase I of meiosis, which swaps sections of chromatid and creates Ab and aB gametes.
The closer two genes are on a chromosome, the less likely a crossover occurs between them, so there are fewer recombinants.recombination frequency = (number of recombinant offspring ÷ total offspring) × 100 Worked example: a test cross gives 44 AaBb, 42 aabb, 7 Aabb and 7 aaBb offspring. The recombinants are Aabb and aaBb: 7 + 7 = 14. Total = 100. Recombination frequency = 14 ÷ 100 × 100 = 14%.A recombination frequency of 50% means the genes are not linked (they assort independently). A frequency below 50% shows linkage, and the smaller it is, the closer the genes are.
The closer two genes are on a chromosome, the less likely a crossover occurs between them, so there are fewer recombinants.