Chapter05_SSM - 65781_CH05_087_119.qxd 8/1/08 12:56 PM Page...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
87 Chapter 5: Genetic Linkage and Chromosome Mapping Chapter Summary Nonallelic genes located in the same chromosome tend to remain together in meiosis rather than to undergo independent assortment. This phenomenon is called linkage. The indication of linkage is a significant devia- tion from the 1 : 1 : 1 : 1 ratio of phenotypes in the progeny of a cross of the form Aa Bb × aa bb. When alleles of two linked genes segregate, more than 50 percent of the gametes produced have parental combina- tions of the segregating alleles, and fewer than 50 percent have nonparental (recombinant) combinations of the alleles. The recombination of linked genes results from crossing-over, a process in which nonsister chro- matids of the homologous chromosomes exchange corresponding segments in the first meiotic prophase. The frequency of recombination between different genes can be used to determine the relative order and locations of the genes in chromosomes. This type of analysis is called genetic mapping. The distance between adjacent genes in such a map (a genetic or linkage map) is defined to be proportional to the fre- quency of recombination between them; the unit of map distance (the map unit or centimorgan) is defined as 1 percent recombination. One map unit corresponds to a physical length of the chromosome in which a crossover event takes place, on the average, once in every 50 meiotic divisions. For short distances, map units are additive. (For example, for three genes with order a b c , if the map distances a to b and b to c are 2 and 3 map units, respectively, then the map distance a to c is 2 + 3 = 5 map units.) The recombination frequency underestimates actual genetic distance if the region between the genes being considered is too great. This discrepancy results from multiple crossover events, which yield either no recombinants or the same number produced by a single event. For example, two crossovers in the region between two genes may yield no recombinants, and three crossover events may yield recombinants of the same type as that from a single crossover. When many genes are mapped in a particular species, they form linkage groups equal in number to the haploid chromosome number of the species. The maximum frequency of recombination between any two genes in a cross is 50 percent; this happens when the genes are in nonhomologous chromosomes and assort independently or when the genes are sufficiently far apart in the same chromosome that at least one crossover is formed between them in every meiosis. The map distance between two genes may be
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
88 ascospore ascus attached-X chromosome centimorgan chromatid interference chromosome interference chromosome map cis configuration coefficient of coincidence compound-X chromosome coupling configuration euchromatin first-division segregation frequency of recombination genetic map considerably greater than 50 centimorgans, because the map distance is equal to half of the average number of crossovers per chromosome times 100. A mapping function is the mathematical relation between the
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

Page1 / 33

Chapter05_SSM - 65781_CH05_087_119.qxd 8/1/08 12:56 PM Page...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online