Have you ever wished to go back in time to see the greatest moments in history? Welcome
aboard the magic university bus, which is able to take you anywhere back in time to do just that.
However, finding these monumental moments may not be an easy task. How about going back to meet
Shakespeare or Napoleon? Oh I know, what about the time that experimental genetics was being founded?
Set a course for Columbia University, New York in 1910,
we are off to witness Thomas Hunt Morgan discover the origins of sex inheritance.
This does not look like 1910 or New York. The landscape resembles the Czech Republic.
Also, is that Gregor Mendel? This must be 1865.
The simple pea plant crosses conducted by Mendel originally facilitated critical
thinking in the predictability of inherited characteristics.
The Mendelian idea that chromosome pairs separate and then are transferred to either an egg
or sperm cell, as independent factors, was not accepted by Morgan as a process by which
heredity and chromosomes were associated. This inspired his own research on the matter.
Ladies and gentlemen, all aboard for the next stop!
We have arrived in… 1902? Is this not the time period that Walter Sutton and Theodor
Boveri began to confirm Mendel’s theory? These two geneticists hypothesised that
chromosomes were the basis of the transfer of heritable genetic material by segregation,
independent assortment and possibly linkage. This formed the Boveri-Sutton chromosome theory
and filled a sizeable gap in the understanding of heredity.
The bus is moving again, everyone hop on!
I can visualise Morgan. However, he is attempting to amount evidence against Boveri and Sutton’s
theory? Of course! Morgan believed that scientific evidence could not be obtained
simply by passive observation and combining parental traits could not form an offspring.
Determined to fault the chromosome theory, Morgan would publish an
article in 1909 which called for an improved experimental approach to understand inheritance.
Off we go for the last time!
In 1910, Morgan was utilising Drosophila melanogaster to test heritability hypotheses.
Then, one day, when analysing his population,
he observed a distinctive white-eyed mutant amongst other red-eyed flies.
Interested in what was peering back at him, Morgan crossed a white-eyed male and
purebred red-eyed female. The F1 generation all possessed red eyes, however, Morgan hypothesised
that the white eye trait was still present in the F1 generation as a recessive trait,
so a F1 red-eyed female and red-eyed male were crossed. This gave a 3:1 ratio of
red-eyed to white-eyed flies. However, all F2 white-eyed offspring were males,
therefore, Morgan questioned; is the white-eyed mutation then lethal to females?
A further test cross was performed between heterozygous red-eyed females and white-eyed
males. This gave an equal ratio of white eyes to red eyes and males to females, as it was
assumed that females can be heterozygous, RW XX, or homozygous, RR XX, for red eyes
whereas males can only be heterozygous, RWX, forming the basis of sex inheritance.
Following four verifications to explain this hypothesis, many conclusions were made from such
experiments. Wild males were confirmed as possibly being heterozygous for red eyes and homozygous for
white eyes and Morgan proposed this occurred by the dropping out or mutation of an R factor to W
in a single egg, and when fertilised by a male sperm (W) gives a white eyed mutant,
WX W. This indicated a strong coupling of the R allele and X
character as the gene for eye colour is exclusively located on the X chromosome.
The significance of such a discovery sent shockwaves through the scientific community
as irrefutable evidence now existed suggesting that sex chromosomes carry genes that have
specific inheritance patterns. Morgan continued to experimentally provide evidence on concepts such
as genes existing in long lines on chromosomes, linkage and the ‘cross over’ phenomenon.
This knowledge was then utilised to explain the sex inheritance patterns in more
complex organisms, such as humans, from simply understanding the inheritance of specific traits
to the risk of many sex inherited diseases, such as red-green colour blindness and haemophilia.
Morgan won the Nobel prize in Physiology or Medicine in 1933 for his genetics work.
From then on, his findings are constantly utilised by geneticists today as a platform
for further knowledge on the evolution of traits driven by sex inherited mutations.
Back home we go! What an adventure, and what knowledge we have gained!