Priscilla Lane and Margaret Green studied the linkage...

Priscilla Lane and Margaret Green studied the linkage relations of three genes affecting coat color in mice: mahogany $(m g),$ agouti $(a),$ and ragged $(R g) .$ They carried out a series of three-point crosses, mating mice that were heterozygous at all three loci with mice that were homozygous for the recessive alleles at these loci (P W. Lane and M. C. Green. 1960. Journal of Heredity $51: 228-230) .$ The following table lists the results of the testcrosses. (TABLE CANNOT COPY) a. Determine the order of the loci that encode mahogany, agouti, and ragged on the chromosome, the map distances between them, and the interference and coefficient of coincidence for these genes. b. Draw a picture of the two chromosomes in the triply heterozygous mice used in the testcrosses, indicating which of the alleles are present on each aosome

Priscilla Lane and Margaret Green studied the linkage relations of three genes affecting coat color in mice: mahogany $(m g),$ agouti $(a),$ and ragged $(R g) .$ They carried out a series of three-point crosses, mating mice that were heterozygous at all three loci with mice that were homozygous for the recessive alleles at these loci (P W. Lane and M. C. Green. 1960. Journal of Heredity $51: 228-230) .$ The following table lists the results of the testcrosses.
(TABLE CANNOT COPY)
a. Determine the order of the loci that encode mahogany, agouti, and ragged on the chromosome, the map distances between them, and the interference and coefficient of coincidence for these genes.
b. Draw a picture of the two chromosomes in the triply heterozygous mice used in the testcrosses, indicating which of the alleles are present on each aosome

Key Concepts

Gene Linkage

Gene linkage refers to the phenomenon where genes located on the same chromosome tend to be inherited together because they are physically close to each other. This concept challenges the idea of independent assortment, laying the foundation for analyzing how recombination events affect gene combinations in offspring.

Three-Point Testcross

A three-point testcross is a genetic cross involving an individual heterozygous for three linked genes crossed with an individual homozygous recessive for those genes. This method is used to determine the order of the genes on the chromosome and to calculate the genetic distances between them by analyzing the frequency of parental, single crossover, and double crossover phenotypes.

Recombination Frequency and Map Distance

Recombination frequency is the proportion of recombinant progeny produced in a cross and directly reflects the physical distance between genes on a chromosome. This frequency is converted into a map distance measured in centiMorgans (cM), where 1% recombination is equivalent to 1 map unit, aiding in the construction of genetic maps.

Double Crossovers, Coefficient of Coincidence, and Interference

Double crossovers occur when two crossover events happen between three linked loci. The coefficient of coincidence is defined as the ratio of the observed number of double crossovers to the number expected if crossovers occurred independently. Interference, calculated as one minus the coefficient of coincidence, quantifies the extent to which one crossover event inhibits another nearby crossover.

Chromosome Mapping

Chromosome mapping involves arranging genes in order along a chromosome and determining the distances between them based on recombination data. This process uses the outcomes of genetic crosses, such as the three-point testcross, to infer the relative positions and linkage relationships among genes.

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