Genes and Chromosomes lecture

In today's lecture what I want to do is to look at a little bit of detail at how the connection between Genes and chromosomes was forged.

Chromosomes had not been described when Mendel lived and worked, it was only later with the advent of improved microscopes in the second part of the 19th century of biologists began to describe the structure of cells in some detail, and one of the things they noticed was the formation of dark bodies in the nucleus of cells that would appear just before cells divide and furthermore they notice that as cells were dividing these dark bodies would fall peculiar movement. What they were seeing were chromosomes and movements of these chromosomes in mitosis and meiosis.

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The significance of the movements of these chromosomes wasn't appreciated at first but then in the early 20th century two-cell biologist independently had an insight Walter Sutton and colleague observed that the highly choreographed movements of chromosomes during meiosis reduced by half number of chromosomes that would be found in gametes. When gametes joined the total number of chromosomes would come back up to its full complement. They realize that this reduction in chromosome number in gametes formation and subsequent restoration zygote formation could explain the patterns of trait transmission that Mendel had described with his laws of segregation and independent assortment.


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In 1903 Sutton and Bovary both independently publish their ideas, which become generally known as the chromosomal theory of inheritance. in that sense they provided a hypothetical mechanism whereby chromosome movements could completely explain these two fundamental principles that Mendel had suggested.

The chromosomal theory of inheritance should seem obvious to us at this point in the course because we are to know so much about DNA learn that before but it wasn't clear then that you could establish this relationship specifically it wasn't clear how during the early part of the 20th century you could prove that genes were on chromosomes and thus prove the chromosome theory of inheritance, sutten & Bovary had suggested this connection but it was just a hypothesis. Confirmation of this hypothesis actually can be attributed to one particular scientist and a remarkable lab group and also to the particular organism, the scientist was Thomas Hunt Morgan who was a embryologist studying patterns of development working at Columbia University.










Like most biologist at the time Morgan to became interested in mechanisms of inheritance that as people began to talk again about Mendel's work .Now Morgan was particularly interested as he was studying development in mutations, and he was interested in how new mutations arose in organisms. Many geneticists at the time had begun working on organisms that had more complex patterns of trait transmission than for example the garden peas Mendel worked on including for example small mammals such as guinea pigs and mice because the way that her color patterns of these mammals would be transmitted from parent to offspring sometimes corresponded to what Mendel observed that also led to a lot of interesting exceptions that these geneticists wanted to understand .so Morgan when he got interested in genetics he set out to work on the genetics of coat color in mammals but mammals are expensive.Morgan couldn't actually raise the money to do this work Morgan's inability to get funded to work on coat color in mammals was probably one of the most fortunate grants turndowns in the history of science because it led Morgan by necessity to start working on a different model organism the Fruit fly a small little fly its scientific name is Drosophila Melagoster and commonly known as Drosophila.

As it turns out Drosophila very quickly became and remains to this day the single most important model organism used in both classical and molecular studies of genetics.From Morgan's point of view there are a lot of advantages to working on fruit flies (i) first of all their cheap and (ii)Fruit flies are also very easy to raise in the laboratory . most important in one of the reasons that supplies remain such an important model organism today(iii) they have a very short generation time adult fruit flies will develop from eggs in only a matter of days and what this means is that it's possible to observe the results of genetic crosses in a very short period time you can do a lot of process he didn't have to wait for those garden peas to growup over a matter of months within a few days you know the answer.

There were some serious problems working with fruit flies that fruit flies that you collect from the wild don't have obvious phenotypic variance. if you put out your pineapple and collect fruit flies, to a first approximation they all look the same ,that is the fruit fly didn't offer traits that Morgan could use in particular establish crosses. This seems like the major problem How you going to understand the genetics of trait transmission if there are obvious traits that you can follow in her crosses. but remember that Morgan was interested in mutation so his first goal really when he started working with Drosophila was to see if and how a mutant phenotype might emerge in a natural wild population .

Morgan and his students and the legion of people who followed him studying fruit flies refer to the phenotypes of these fruit flies in particular ways. they referred to the characteristic that you would observe in a wild fruit fly as being the wild type phenotype. because wild fruit flies don't have a lot of visible variation. it means that basically all fruit flies that you collect our basically just composed of wild type phenotype for any particular characteristic you might be interested. now if they observed an unusual phenotype specifically phenotype that they thought was the mutation they call it a mutant phenotype. we have wild type and mutant phenotypes that are what we're really looking at when we look at fruit flies and the assumption here is that the mutant phenotype somehow must be the result of a mutation in allele for the gene responsible for the trait .another detail is that Morgan and the people who have followed up on fruit flies uses slightly different convention for labeling their alleles,the way it is Morgan designated or labeled essentially the kind of mutant alleles and genotypes he was working with was by him labeling the allele according to the phenotypic characteristic of the mutation of the mutant phenotype don't let me make this clear with an example of a well-known mutation in Drosophila which involved a reduction in the size of wings and these guys are just tiny little flies they have wings but one mutation occurs causes those wings do not develop properly that the wings are also small and scrunched up this mutation has been labeled the vestigial wing mutation or just simply vestigial wings we would label the allele responsible for this mutation VG. the interesting thing is that the mutation in are named after the mutant phenotype not the wild type phenotype.the mutant allele would be referred to as VG and the wild type allele for that same gene we would call VG + .

for any particular mutation that Morgan was studying we can safely assume that the typical wild type Drosophila the one that Morgan would just collect out on his pineapple is homozygous for the wild type allele if we were interested in the vestigial wing of trait if we just caught a wild type individual we would assume it's homozygous for VG + VG + that would be a phenotype for that particular trait.

we observe a mutant that mutant must have at least one mutant allele by for example in that case it's got to be at least VG + VG but actually more often than not the mutant alleles that we find in Drosophila are recessive alles. if we find mutation if we find a mutant phenotype of vestigial wing fly then we can be pretty sure that it's homozygous for the recessive mutant allele in other words it would be VG VG a genotype that particular trait.