Chromosome Theory & Morgan's Experiments (PDF)

CHROMOSOME THEORY Mendel identified particles of inheritance, but he did not know about chromosomes, meiosis or DNA Morgan- the first to associate a specific gene with a specific chromosome Used fruit fly, Drosophila melanogaster Short generation time Lots of offspring to...

CHROMOSOME THEORY Mendel identified particles of inheritance, but he did not know about chromosomes, meiosis or DNA Morgan- the first to associate a specific gene with a specific chromosome Used fruit fly, Drosophila melanogaster Short generation time Lots of offspring to score MORGAN’S EXPERIMENT Isolated white-eyed mutant fly, fig. 15.2 In breeding experiments, found that white eyes were inherited as Mendelian recessive BUT, white eyes could only be found in male offspring in F2 generation, fig. 15.3 LINKAGE Genes present on same chromosome tend to be inherited together Morgan found the allele for white eyes was linked to the sex chromosomes, sex- linked gene Defective gene present on X chromosome, no copy on shorter Y chromosome, hemizygous, Fig. 15.3 SEX DETERMINATION Scheme varies among organisms, fig. 15.6 In humans, Y chromosome carries few genes In humans, sex-linked genes are usually found on the X chromosome (X-linked) Females only express rec. traits if homozygous Heterozygous females are carriers of that trait SEX-LINKED TRAITS Humans have a number of sex-linked traits Color blindness Hemophilia Immune system defects MORGAN’S EXPERIMENTS Cross flies differing at two loci, body color and wing shape, Fig. 15.9 Results violate typical Mendelian prediction Body color and wing shape usually inherited together, so must be on same chromosome, linked But some intermediate types were seen RECOMBINATION A new combination of the traits inherited from parents Independent assortment allows recombination of unlinked genes Crossing over allows recombination of linked genes, fig. 15.10 MENDEL RULES WOULD PREDICT b+ vg+ b vg b+ vg b vg+ b vg b+b vg+vg bb vgvg b+b vgvg bb vg+vg ¼ of offspring b+bvg+vg (gray body normal wings) ¼ of offspring bbvgvg (black body vestigial) ¼ of offspring b+bvgvg (gray body vestigial) ¼ of offspring bbvg+vg (black body, normal wings) BUT GENETIC MAPS Can use recombination frequency to get approximate order and distance of genes along a chromosome, fig. 15.11, 15.12 The further apart genes are, the more likely a cross over will occur between them STRUCTURAL ANOMALIES Deletion- a portion of the chromosome is missing Duplication- a portion of the chromosome has been duplicated, gene dose effects Inversion- portion of chromosome is in reverse orientation Reciprocal translocation- joining fragments of non-homologous chromosomes, fig 15.16 These result from errors in the crossing over process during prophase 1 IMPRINTING In some cases, expression of trait depends on which parent passed on the allele, fig.15.17 SUMMARY Mendel and Morgan laid the foundation for our understanding of inheritance Together with developments in cytology this established the role of chromosomes as carriers of genetic information But still did not define the molecules of inheritance or the means by which genotype becomes phenotype Chromosomes are composed of DNA and Protein