Presentation on theme: "LINKAGE AND GENETIC MAPPING IN EUKARYOTES. In eukaryotic species, each linear chromosome contains a long piece of DNA A typical chromosome contains many."— Presentation transcript:
In eukaryotic species, each linear chromosome contains a long piece of DNA A typical chromosome contains many hundred or even a few thousand different genes The term linkage has two related meanings 1. Two or more genes can be located on the same chromosome 2. Genes that are close together tend to be transmitted as a unit 5-3 LINKAGE AND CROSSING OVER
Chromosomes are called linkage groups They contain a group of genes that are linked together The number of linkage groups is the number of types of chromosomes of the species For example, in humans 22 autosomal linkage groups An X chromosome linkage group A Y chromosome linkage group Genes that are far apart on the same chromosome may independently assort from each other This is due to crossing-over 5-4
5-7 Figure 5.1 These haploid cells contain a combination of alleles NOT found in the original chromosomes These are termed parental or non- recombinant cells This new combination of alleles is a result of genetic recombination These are termed nonparental or recombinant cells
yy ww mm y + y w + w m + m F 1 generation x y w m Y x y + w + m + Y F 1 generation contains wild-type females and yellow-bodied, white-eyed, miniature-winged males.
5-13 Morgan observed a much higher proportion of the combinations of traits found in the parental generation P Males P Females Morgan’s explanation: All three genes are located on the X chromosome Therefore, they tend to be transmitted together as a unit
Morgan made three important hypotheses to explain his results 1. The genes for body color, eye color and wing length are all located on the X- chromosome They tend to be inherited together 2. Due to crossing over, the homologous X chromosomes (in the female) can exchange pieces of chromosomes This created new combination of alleles 3. The likelihood of crossing over depends on the distance between the two genes Crossing over is more likely to occur between two genes that are far apart from each other 5-17
Parental chromosomes Nonparental chromosomes CWx cwx cWx Cwx Crossing over (b) Crossing over between normal and abnormal chromosome 9 Normal chromosome 9 Abnormal chromosome 9 Knob (a) Normal and abnormal chromosome 9 Interchanged piece from chromosome 8 Creighton and McClintock Experiment They demonstrated physical evidence of cross-overs. C = Colored c = colorless Wx = Starchy endosperm wx = waxy endosperm
5-36 These observations were consistent with the idea that a cross over occurred between the C and wx genes As stated by Creighton and McClintock: “Pairing chromosomes, heteromorphic in two regions, have been shown to exchange parts at the same time they exchange genes assigned to these regions.”
Map Distance pr/vg = 124/1005 x 100 = 12.3 b/pr = 61/1005 x 100 = 6 b and vg = 179/1005 x 100 = 17.8 _____6____________12.3____________ b pr vg The distance between b and vg was found to be 17.8. The actual distance is 18.3 mu.
Interference The slightly smaller lower value was a small underestimate because we did not consider the double crossovers in the calculation between b and vg. The lower than expected value is due to a common genetic phenomenon, termed positive interference. The first crossover decreases the probability that a second crossover will occur nearby.