Question

"A key assumption is that crossing over is random. In which case, would you have an equal number of recombinants?"

Does this mean that each of the totals for the recombinant groups should all be relatively similar to each other if crossing over is actually random?

Answer 1

There is a strong relationship between gene segregation and meiosis. Crossing over and recombination occurs during meiosis yield recombinant chromosomes. Crossing over is random. In a chromosome the location of the break points on the DNA sequence is random but the frequency of recombination is relatively constant between homologous chromosomes.

The probability of crossing over between genes on a chromosome is dependent on the distances between the genes. Genes on the same chromosomes tend to be inherited together called linked genes because the DNA sequence with genes inherited as a unit during meiosis. The genes that are located closer in the break point on a particular chromosome have the higher the probability that they will be inherited as a unit. But the linked genes do not follow the inheritance patterns predicted by Mendel's Theory of Independent Assortment. In non-sister chromatids the crossing occurs between genes when they are relatively far apart on the homologous chromosomes and produce an equal number of recombinant and non-recombinant. As a result the test cross produce offsprings in a ratio 1:1:1:1 ie., paternal, recombinant (paternal-maternal), recombinant (maternal-paternal) and maternal). Half of the offspring in the test cross have recombinant chromosomes, i.e, 50% frequency of recombination is observed. The unlinked genes may either reside on different chromosomes or reside far apart on the same chromosome. When two genes in homologous chromosomes are very close to break points for crossing over, fewer recombinant chromosomes are produced.