Biology · Principles of Inheritance and Variation · NEET
It is the percentage of offspring that are recombinant (non-parental) type out of all offspring. Formula: recombination frequency = (number of recombinant offspring / total offspring) x 100. If a testcross gives 100 offspring and 15 are the new, non-parental combination, the recombination frequency is 15%. A higher number means the two genes recombine more often.
Morgan coined the terms 'linkage' and 'recombination' and showed that genes on the same chromosome do not assort independently. His student Alfred Sturtevant took this one step further: he used the recombination frequency as a ruler to measure the distance between genes and 'mapped' their positions on the chromosome. So if NEET asks who mapped gene positions using recombination frequency, the answer is Sturtevant, not Morgan.
Crossing over happens at random points along the chromosome during meiosis. If two genes are far apart, there is more space between them, so a crossover is more likely to happen in that gap and separate them. If they are very close, a crossover rarely falls between them, so they stay together (tight linkage, low recombination). That is why recombination frequency goes UP as distance goes UP.
A map unit measures distance between genes. 1 map unit = 1% recombination frequency. It is also called a centiMorgan (cM), named after Morgan. So if two genes show 9% recombination, they are 9 map units apart on the chromosome.
Recombination frequencies add up along a straight line. Find the two genes with the LARGEST frequency — they sit at the two ends. Then place the other genes in between so that the small distances add up to the big ones. Example: if a-d = 29% and it equals a-c (5) + c-b (15) + b-d (9), the order is a - c - b - d. Always check that the pieces add to the total.
Morgan found white and yellow genes showed only 1.3% recombination (very tightly linked, so they are close together). White and miniature wing showed 37.2% recombination (loosely linked, so they are far apart). This proves the rule: small frequency = genes close together; large frequency = genes far apart.
For gene mapping, the observed value is capped at about 50%. When two genes are very far apart on the same chromosome, crossovers happen so often that they behave almost like genes on different chromosomes and assort nearly independently (about 50% recombinants). That is why very distant genes can look 'unlinked' even though they are on the same chromosome.
Frequency of recombination between gene pairs on some chromosome as a measure of the distance between genes to map their position on chromosome, was used for the first time by
The recombination frequency between the genes a & c is 5%, b & c is 15%, b & d is 9%, a & b is 20%, c & d is 24% and a & d is 29%. What will be the sequence of these genes on a linear chromosome?
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.
Recombination frequency = (number of recombinant offspring / total number of offspring) x 100, expressed as a percentage. A value of 1% equals 1 map unit (1 centiMorgan).
Yes, they are directly proportional. 1% recombination frequency = 1 map unit of distance. So recombination frequency is used as a ruler to measure how far apart two genes lie on a chromosome.
Low. Closely linked genes are near each other, so crossovers rarely fall between them. They stay together and show a low recombination frequency (Morgan's white-yellow example was only 1.3%).
Morgan used the fruit fly Drosophila melanogaster. It grows on simple medium, completes its life cycle in about two weeks, gives many offspring per mating, and has easily distinguished sexes.
A testcross (F1 dihybrid crossed with a fully recessive parent) lets you read the offspring genotypes directly from their phenotypes. You can then count parental-type versus recombinant-type offspring and calculate the recombination frequency.
