# Genetic mapping near 0 recombination frequency means

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Genetic Mapping Near 0% recombination frequency means the genes are very tightly linked 50% recombination frequency means the genes are entirely unlinked The recombination frequency can never be exactly 0% that would require an infinite number of progeny If no recombinants are observed in n progeny, you can only conclude that the recombination frequency is < 1/n In humans, 1 cM is roughly = 1 million nucleotide pairs (bp) of DNA
Linkage Problems - Example In a plant, leaf color and leaf shape are controlled by two linked genes. Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed A recessive mutation in this gene causes them to be smooth.
Linkage Problem (Cont.) Pure breeding white, smooth is crossed with pure breeding wild type F1: all red, pointed Test cross red, pointed x pure breeding white, smooth Gives F2: 40 white, smooth 36 red, pointed 10 white, pointed 14 red, smooth
Try It! What is the recombination frequency between the gene for color and for shape?
Strategies First, assign genotype symbols. Since the mutations are recessive to wild-type, use + for the wild type allele & lower case letters for mutant alleles: w = recessive white color s = recessive smooth shape So, the first cross is: (ws/ws) x (++/+ +) giving red, pointed progeny, (ws/+ +)
The Test Cross The second cross is (ws/++) x (ws/ws) The progeny genotypes can be determined from their phenotypes Determine which gamete types are parental and which are recombinant (ws) white, smooth and (++) red, pointy are parental (w+) white, pointy and (+s) red, smooth are recombinant
Solving Therefore, recombination frequency = (10 + 14) (10 + 14) + (40 + 36) = .24 % or 24 cM