Aberrations+++06

Aberrations+++06 - Chromosomal Aberrations D137 J.L Marsh...

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Chromosomal Aberrations D137 ©J.L. Marsh 2006 page 1 CHROMOSOMAL ABBERATIONS Refer to changes in chromosome structure [vs. changes in number previously discussed] This is an important meeting point between transmission and molecular genetics because mutations that affect function are also physically detectable. Introduction So far most alleles discussed = point mutations ie give altered phenotype but not visible. ie chromosomes look normal. =base changes Chromosome breaks and replication errors can occur in any cell at any time (i.e. in somatic or germline cells). Broken ends of DNA without a telomere are rapidly repaired although not always correctly. They occur naturally or can be induced. e.g. radiation can cause breaks that are resolved Replication mistakes can also cause the equivalent of breaks Aberrations detected by 1] genetic - change in linkage relationships apparent dominace altered recombination frequency between gene pairs 2] Cytological change in c'some number change in length change in pairing (best seen meiotic prophase) Structural Modifications resulting from repair of 2 breaks 0= centromere 1] Df deficiency/deletion 0--A--B--C--D-- >> 0--A-|-C--D-- 2] Dp duplication 0--A--B--C--B--C--D Tandem or reverse 3] In inversion 0--A--B--C--D-- >> 0--A--C--B--D-- 4] T translocation 0--A--B--C--D & 0--E--F >> 0--A--F & 0--E--B--C— D
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Chromosomal Aberrations D137 ©J.L. Marsh 2006 page 2 5] - rings A--B--0--C--D --> A-- ring B--0--C --D note. All = 2 breaks followed by reunion [dp is a bit different – it usually results from a pairing or replication error] When studying, consider the consequences of rearrangements relative to: i] properties of synapsis and exchange ii] centromere function iii] accuracy of genetic maps iv] origin of genetic variation v] gene dosage NOTE; Be mindful of minimum number of breaks (events) needed to generate each type of aberration. In trying to determine sequence of events remember THE PRINCIPLE OF MAXIMUM PARSIMONY. DEFICIENCIES Cytological detection Problem; how do you explain a loop when 2 homologs are paired such as in Drosophila salivary glands or meiosis? Answ: Df. But which is normal? Note: From the appearance of such figures, we can conclude that chromosomes pair by homology, not by zippering!! Genetic effects: 1] Problem: X ray a Male fly and cross to a female homozygous for a number of recessive genes on e.g. the second chromosome +/+male X female al dp b rdo hk pr cn recover: 99 +/+ progeny and 1 rdo hk pr What could it be? Triple mutant - unlikely Df - explains it nicely - e.g. DNA spanning all 3 genes and others is deleted thus uncovering the recessive alleles and making them hemizygous. al dp b rdo hk pr cn + + +| ............ |+ -Dfs usually lethal as homozygotes because several genes are missing Says most genes necessary and essential genes are scattered
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Chromosomal Aberrations D137 ©J.L. Marsh 2006 page 3 2] Dfs will reduce the apparent map distance between flanking loci normal b-cn map distance = 57 to 104 mu = 47 mu the crossover frequency in Df heterozygote = b-cn/Df = ca. 1mu.
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Aberrations+++06 - Chromosomal Aberrations D137 J.L Marsh...

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