- When she did different analysis of different corn with different phenotypes in
corn kernel, she saw two types of observations. The first type is shown in a)
and there is a second type down a bit.
- We’re looking at cells with the chromosomes so two copies of the
chromosome you would expect to see. On these particular chromosomes, you
see 2 different alleles, one which has a big C and one with a small c. The big C
in this example means wild type and the small c means recessive or the
nonfunctional. The C gene happens to code for colour so C determines a
purple colour, if you have a big C then you have a purple colour in the cell and
for Sh, the wild type, the big S with the h is shrunken so if you have a big Sh
then it is nice and plump like a regular corn you see, so that is Sh so that is a
nice plump kernel and the Wx is for shiny. If you have the wild type then you
have a purple colour that is nice and plump and shiny in appearance.
- As the kernel grows, in some of the cells, what happens is, because in this
region the Ds region, if it was present (located in the slide), as the kernel
grows, some of the cells get a break at the Ds so wherever that Ds is located
you lose that part of the chromosome. That part becomes nonfunctional and
gets destroyed. Cells with this particular genotype, they take on a different
phenotype – they are colourless so the cells that have this DNA will be
colourless, shrunken and not shiny (dull).
- This would be happening during the development of the kernel so you have a
region that is colourless, shrunken and not shiny and then you have the normal
pigmented plump and shiny.
- In other cases, there was another type of allele at the C locus so in other
words, at the position where the colour gene was, that was a different allele
(different sequence). In this case, the cells would inherit a different
combination of genes & alleles so you would have dominant ones & you have
the recessive ones. The deal with this c-Ds is that the Ds is inserted into that C
gene, the big C gene and therefore notice how it has a small c there, it means it
would be colorless so not normally allowing the purple colour to show. The
reason is because the DS has been inserted into the normal C gene. What
happens is the cells initially have this genotype and as they develop, some of
the cells have a situation where the Ds actually moves out of that region where
the C gene is and when that Ds chunk moves away, you now return to the
normal wild type gene. Therefore what you end up with are regions where the
cell has gone back to the regular phenotype with purple & spots.
- This is what Barbara McClintock figured out, that there were parts of the
DNA that were moving around, that this Ds had the ability to move out of a
region of a chromosome and therefore change the phenotype.
- Over time with experiments, people realized that the C gene would look like
this, and it would code for purple pigment. Or if you had a situation where you
had a copy of this, you would have a purple pigment.
- In the case of a mutant with a cm mutant, or the c-Ds mutant shown on the
other slide. In this case the C gene is interrupted by this Ds element it is called
a transposable element. This interrupts the coding sequence of the gene &
therefore in this case providing this stays sable (?), you get a colourless
- If people figured out that you had the Ac gene which is a completely
different gene and it was present with the cm-DS gene, so the same one as up
there then in certain circumstances in certain cells, the Ds could move out so it
would move out of this locus where the C gene was and then you’d have these
spotted kernels. But you need the Ac present, if you don’t have the Ac present
then you always have colourless and as well there was another mutant that had
Ac present in the same region in the C gene and Ac on its own was able to